Virtual coupling, a spring-damper system between the haptic probe and its virtual representation, the proxy, is one of the most common approaches for haptic rendering. We have extended the virtual coupling by updating the spring stiffness, sometimes used to simulate compliance of a material, depending on the direction between the proxy and the probe. This anisotropic variation of the stiffness is used in exploring inhomogeneities beneath the surface allowing detection of rigid structures even when they are obscured by another structure beneath the surface. In addition, we also compensate for the energy variation of the spring to maintain passivity and increase realism. User studies were performed to survey the success rate in the detection of obscured rigid bodies beneath the surface with the modified virtual coupling algorithm and the improvement of shape perception for sub-surface objects with the additional energy compensation term providing gradient information. We also discuss potential benefits of the proposed methods as basic extensions to well-known haptic rendering algorithms which are both simpler and yield improved performance over traditional deformation simulationtechniques.

Nonrelativistic electrostatic unmagnetized shocks are frequently observed in laboratory plasmas and they are likely to exist in astrophysical plasmas. Their maximum speed, expressed in units of the ion acoustic speed far upstream of the shock, depends only on the electron-to-ion temperature ratio if binary collisions are absent. The formation and evolution of such shocks is examined here for a wide range of shock speeds with particle-in-cell simulations. The initial temperatures of the electrons and the 400 times heavier ions are equal. Shocks form on electron time scales at Mach numbers between 1.7 and 2.2. Shocks with Mach numbers up to 2.5 form after tens of inverse ion plasma frequencies. The density of the shock-reflected ion beam increases and the number of ions crossing the shock thus decreases with an increasing Mach number, causing a slower expansion of the downstream region in its rest frame. The interval occupied by this ion beam is on a positive potential relative to the far upstream. This potential pre-heats the electrons ahead of the shock even in the absence of beam instabilities and decouples the electron temperature in the foreshock ahead of the shock from the one in the far upstream plasma. The effective Mach number of the shock is reduced by this electron heating. This effect can potentially stabilize nonrelativistic electrostatic shocks moving as fast as supernova remnant shocks.

Finite element (FE) models are frequently used in engineering and life sciences within time-consuming simulations. In contrast with the regular grid structure facilitated by volumetric data sets, as used in medicine or geosciences, FE models are defined over a non-uniform grid. Elements can have curved faces and their interior can be defined through high-order basis functions, which pose additional challenges when visualizing these models. During ray-casting, the uniformly distributed sample points along each viewing ray must be transformed into the material space defined within each element. The computational complexity of this transformation makes a straightforward approach inadequate for interactive data exploration. In this paper, we introduce a novel coherency-based method which supports the interactive exploration of FE models by decoupling the expensive world-to-material space transformation from the rendering stage, thereby allowing it to be performed within a precomputation stage. Therefore, our approach computes view-independent proxy rays in material space, which are clustered to facilitate data reduction. During rendering, these proxy rays are accessed, and it becomes possible to visually analyze high-order FE models at interactive frame rates, even when they are time-varying or consist of multiple modalities. Within this paper, we provide the necessary background about the FE data, describe our decoupling method, and introduce our interactive rendering algorithm. Furthermore, we provide visual results and analyze the error introduced by the presented approach.

The investigation of sustainable enterprise interoperability requires an explicit position of what constitutes the enterprise. In particular, human aspects like agency, sense-making, interpretation, common understanding, etc., must be taken into account. To this end, the purpose of this contribution is to make an inquiry into interoperability from the perspective of the Activity Domain Theory (ADT). In this theory, the enterprise is considered as a constellation of activity domains. The activity domain is characterized by the activity modalities - objectivation, contextualization, spatialization, temporalization stabilization, transition - which represent human, innate predispositions for coordinating actions. Interoperability is focused to one particular modality: the transition between activity domains. An example from the telecom industry is used to illustrate the approach. Implications for various aspects of interoperability are discussed. In conclusion, it is argued that the ADT is an alternative framework for advancing the understanding of sustainable interoperability between enterprises and enterprise systems.

The plasma dynamics resulting from the simultaneous impact, of two equal, ultra-intense laser pulses, in two spatially separated spots, onto a dense target is studied via particle-in-cell simulations. The simulations show that electrons accelerated to relativistic speeds cross the target and exit at its rear surface. Most energetic electrons are bound to the rear surface by the ambipolar electric field and expand along it. Their current is closed by a return current in the target, and this current configuration generates strong surface magnetic fields. The two electron sheaths collide at the midplane between the laser impact points. The magnetic repulsion between the counter-streaming electron beams separates them along the surface normal direction, before they can thermalize through other beam instabilities. This magnetic repulsion is also the driving mechanism for the beam-Weibel (filamentation) instability, which is thought to be responsible for magnetic field growth close to the internal shocks of gamma-ray burst jets. The relative strength of this repulsion compared to the competing electrostatic interactions, which is evidenced by the simulations, suggests that the filamentation instability can be examined in an experimental setting.

Visualizing molecular properties is often crucial for constructing conceptual understanding in chemistry. However, research has revealed numerous challenges surrounding students' meaningful interpretation of the relationship between the geometry and electrostatic properties of molecules. This study explored students' (n = 18) use of three visual representations of electrostatic potential to interpret whether molecules are polar or nonpolar. The representations consisted of red and blue 'lobes' (termed RB) indicating regions of negative and positive potential, a color gradient mapping electrostatic potential on a molecular surface (MAP), and a rendering of the interface between regions of positive and negative potential (ISO). Data on students' accuracy, time-on-task, and evaluation related to the three visual modes were collected via a Web-based questionnaire. ANOVA indicated that students were significantly more accurate in interpreting ISO representations, although almost half evaluated this mode as the most difficult to use. Furthermore, students took significantly longer to interpret complex molecules than simple molecules using ISO and RB. The results indicate that there may be possible pedagogical benefits in using unconventional visual representations that reduce visual complexity by making molecular relationships explicit. Hence, this has implications for future work on the role of cognitively mapping between different instructional visualizations in the development of fundamental chemical concepts.

Numerous studies have considered the ability of humans to perceive differences in forces and how this affects our ability to interpret the properties of materials. Previous research has not considered the effect of the rate of change of the material stiffness in our ability to perceive differences, however, an important factor in exploration processes such as a doctor’s palpation of the skin to examine tissues beneath. These effects are the topic of this research which attempts to quantify the effects of stiffness gradient magnitude and form on the discernment of changes in stiffness.

Instabilities behind the front of a cylindrically expanding plasma have been investigated experimentally and with a particle-in-cell simulation. Tubelike filamentary structures form behind the front of a plasma created by irradiating wire targets with a ps-duration and intense ( 1019 W cm-2) laser pulse. These filaments exhibit coherent magnetic fields with a remarkable stability ( 103 / wp: plasma frequency). PIC simulations indicate that an instability driven by a thermal anisotropy of the electron population is the cause. This instability requires a plasma density gradient and hot electrons. It can thus contribute to the generation of strongsustained magnetic fields in astrophysical jets.

The growth and saturation of Buneman-type instabilities is examined with a particle-in-cell (PIC) simulation for parameters that are representative for the foreshock region of fast supernova remnant shocks. A dense ion beam and the electrons correspond to the upstream plasma and a fast ion beam to the shock-reflected ions. The purpose of the 2D simulation is to identify the nonlinear saturation mechanisms, the electron heating and potential secondary instabilities that arise from anisotropic electron heating and result in the growth of magnetic fields. We confirm that the instabilities between both ion beams and the electrons saturate by the formation of phase space holes by the beam-aligned modes. The slower oblique modes accelerate some electrons, but they cannot heat up the electrons significantly before they are trapped by the faster beam-aligned modes. Two circular electron velocity distributions develop, which are centred around the velocity of each ion beam. They develop due to the scattering of the electrons by the electrostatic wave potentials. The growth of magnetic fields is observed, but their amplitude remains low.

A major challenge when designing general purpose graphics hardware is to allow efficient access to texture data. Although different rendering paradigms vary with respect to their data access patterns, there is no flexibility when it comes to data caching provided by the graphics architecture. In this paper we focus on volume ray-casting, and show the benefits of algorithm-aware data caching. Our Marching Caches method exploits inter-ray coherence and thus utilizes the memory layout of the highly parallel processors by allowing them to share data througha cache which marches along with the ray front. By exploiting Marching Caches we can apply higher-order reconstruction and enhancement filters to generate more accurate and enriched renderings with an improved rendering performance. We have tested our Marching Caches with seven different filters, e. g., Catmul-Rom, B-spline, ambient occlusion projection, and could show that a speed up of four times can be achieved compared to using the caching implicitly provided by the graphics hardware, and that the memory bandwidth to global memory can be reduced by orders of magnitude. Throughout the paper, we will introduce the Marching Cache concept, provide implementation details and discuss the performance and memory bandwidth impact when using different filters.

The development of current instabilities behind the front of a cylindrically expanding plasma has been investigated experimentally via proton probing techniques. A multitude of tubelike filamentary structures is observed to form behind the front of a plasma created by irradiating solid-density wire targets with a high-intensity (I∼10¹⁹  W/cm²), picosecond-duration laser pulse. These filaments exhibit a remarkable degree of stability, persisting for several tens of picoseconds, and appear to be magnetized over a filament length corresponding to several filament radii. Particle-in-cell simulations indicate that their formation can be attributed to a Weibel instability driven by a thermal anisotropy of the electron population. We suggest that these results may have implications in astrophysical scenarios, particularly concerning the problem of the generation of strong, spatially extended and sustained magnetic fields in astrophysical jets.

We present an algorithm that enables real-time dynamic shading in direct volume rendering using general lighting, includingdirectional lights, point lights and environment maps. real-time performance is achieved by encoding local and global volumetricvisibility using spherical harmonic (SH) basis functions stored in an efficient multi-resolution grid over the extent of the volume. Ourmethod enables high frequency shadows in the spatial domain, but is limited to a low frequency approximation of visibility and illuminationin the angular domain. In a first pass, Level Of Detail (LOD) selection in the grid is based on the current transfer function setting.This enables rapid on-line computation and SH projection of the local spherical distribution of visibility information. Using a piecewiseintegration of the SH coefficients over the local regions, the global visibility within the volume is then computed. By representing thelight sources using their SH projections, the integral over lighting, visibility and isotropic phase functions can be efficiently computedduring rendering. The utility of our method is demonstrated in several examples showing the generality and interactive performanceof the approach.

The domain of medical imaging is naturally moving towards methods that can represent, and account for, localuncertainties in the image data. Even so, fast and efficient solutions that take uncertainty into account are notreadily available even for common problems such as gradient estimation. In this work we present a CUDA imple-mentation of Normalized Convolution, an uncertainty-aware image processing technique, well established in thesignal processing domain. Our results show that up to 100X speedups are possible, which enables full resolutionCT images to be processed at interactive processing speeds, fulfilling demands of both efficiency and interactivitythat exist in the medical domain.

In recent years, many volumetric illumination models have been proposed, which have the potential to simulate advanced lighting effects and thus support improved image comprehension. Although volume ray-casting is widely accepted as the volume rendering technique which achieves the highest image quality, so far no volumetric illumination algorithm has been designed to be directly incorporated into the ray-casting process. In this paper we propose image plane sweep volume illumination (IPSVI), which allows the integration of advanced illumination effects into a GPU-based volume ray-caster by exploiting the plane sweep paradigm. Thus, we are able to reduce the problem complexity and achieve interactive frame rates, while supporting scattering as well as shadowing. Since all illumination computations are performed directly within a single rendering pass, IPSVI does not require any preprocessing nor does it need to store intermediate results within an illumination volume. It therefore has a significantly lower memory footprint than other techniques. This makes IPSVI directly applicable to large data sets. Furthermore, the integration into a GPU-based ray-caster allows for high image quality as well as improved rendering performance by exploiting early ray termination. This paper discusses the theory behind IPSVI, describes its implementation, demonstrates its visual results and provides performance measurements.

We believe that the lateral exploration of surfaces with varying stiffness, stiffness maps, using computer generated haptics is an underestimated and important procedure with impact in many application areas. Feeling the change of stiffness while sweeping the haptic probe over a surface can potentially give an understanding of the spatial distribution of this stiffness, however current algorithms lack tangential cues of stiffness changes. This introduces energy sources and sinks that potentially affects the stability of the system, apart from being physically incorrect and thus unrealistic. We discuss the forces and effects involved in the exploration of stiffness maps and propose an energy-based algorithm for tangential forces that augments the feedback from the map, in particular during lateral exploration. The algorithm is based on basic physical principles and has the potential to increase both realism and stability. A user study was conducted to analyze the effect of this algorithm on stiffness perception.

A two-dimensional numerical study of the expansion of a dense plasma through a more rarefied one is reported. The electrostatic ion-acoustic shock, which is generated during the expansion, accelerates the electrons of the rarefied plasma inducing a superthermal population which reduces electron thermal anisotropy. The Weibel instability is therefore not triggered and no self-generated magnetic fields are observed, in contrast with published theoretical results dealing with plasma expansion into vacuum. The shock front develops a filamentary structure which is interpreted as the consequence of the electrostatic ion-ion instability, consistently with published analytical models and results.

The interest in relativistic beam-plasma instabilities has been greatly rejuvenated over the past two decades by novel concepts in laboratory and space plasmas. Recent advances in this long-standing field are here reviewed from both theoretical and numerical points of view. The primary focus is on the two-dimensional spectrum of unstable electromagnetic waves growing within relativistic, unmagnetized, and uniform electron beam-plasma systems. Although the goal is to provide a unified picture of all instability classes at play, emphasis is put on the potentially dominant waves propagating obliquely to the beam direction, which have received little attention over the years. First, the basic derivation of the general dielectric function of a kinetic relativistic plasma is recalled. Next, an overview of two-dimensional unstable spectra associated with various beam-plasma distribution functions is given. Both cold-fluid and kinetic linear theory results are reported, the latter being based on waterbag and Maxwell–Jüttner model distributions. The main properties of the competing modes (developing parallel, transverse, and oblique to the beam) are given, and their respective region of dominance in the system parameter space is explained. Later sections address particle-in-cell numerical simulations and the nonlinear evolution of multidimensional beam-plasma systems. The elementary structures generated by the various instability classes are first discussed in the case of reduced-geometry systems. Validation of linear theory is then illustrated in detail for large-scale systems, as is the multistaged character of the nonlinear phase. Finally, a collection of closely related beam-plasma problems involving additional physical effects is presented, and worthwhile directions of future research are outlined.

To help in interpreting the polarity of a molecule, charge separation can be visualized by mapping the electrostatic potential at the van der Waals surface using a color gradient or by indicating positive and negative regions of the electrostatic potential using different colored isosurfaces. Although these visualizations capture the molecular charge distribution efficiently, using them to deduce overall polarity requires students to engage in the potentially demanding process of interpreting the relative positions of electron-rich and electron-poor areas. We present a visual tool that could help students assign polarity by exploiting the unique topography of the interface between negative and positive regions of electrostatic potential surrounding a molecule. Specifically, the tool renders the electrostatic potential isosurface(s) of a molecule obtained when the isovalue is set at 0. Examples of polar and nonpolar molecules are discussed.

Context. The prompt emissions of gamma-ray bursts (GRBs) are seeded by radiating ultrarelativistic electrons. Kinetic energy dominated internal shocks propagating through a jet launched by a stellar implosion, are expected to dually amplify the magnetic field and accelerate electrons.

Aims. We explore the effects of density asymmetry and of a quasi-parallel magnetic field on the collision of two plasma clouds.

Methods. A two-dimensional relativistic particle-in-cell (PIC) simulation models the collision with 0.9c of two plasma clouds, in the presence of a quasi-parallel magnetic field. The cloud density ratio is 10. The densities of ions and electrons and the temperature of 131 keV are equal in each cloud, and the mass ratio is 250. The peak Lorentz factor of the electrons is determined, along with the orientation and the strength of the magnetic field at the cloud collision boundary.

Results. The magnetic field component orthogonal to the initial plasma flow direction is amplified to values that exceed those expected from the shock compression by over an order of magnitude. The forming shock is quasi-perpendicular due to this amplification, caused by a current sheet which develops in response to the differing deflection of the upstream electrons and ions incident on the magnetised shock transition layer. The electron deflection implies a charge separation of the upstream electrons and ions; the resulting electric field drags the electrons through the magnetic field, whereupon they acquire a relativistic mass comparable to that of the ions. We demonstrate how a magnetic field structure resembling the cross section of a flux tube grows self-consistently in the current sheet of the shock transition layer. Plasma filamentation develops behind the shock front, as well as signatures of orthogonal magnetic field striping, indicative of the filamentation instability. These magnetic fields convect away from the shock boundary and their energy density exceeds by far the thermal pressure of the plasma. Localized magnetic bubbles form.

Conclusions. Energy equipartition between the ion, electron and magnetic energy is obtained at the shock transition layer. The electronic radiation can provide a seed photon population that can be energized by secondary processes (e.g. inverse Compton).

The energetic electromagnetic eruptions observed during the prompt phase of gamma-ray bursts are attributed to synchrotron emissions. The internal shocks moving through the ultrarelativistic jet, which is ejected by an imploding supermassive star, are the likely source of this radiation. Synchrotron emissions at the observed strength require the simultaneous presence of powerful magnetic fields and highly relativistic electrons. We explore with 1-D and 3-D relativistic particle-in-cell simulations the transition layer of a shock, which evolves out of the collision of two plasma clouds at a speed 0.9$c$ and in the presence of a quasi-parallel magnetic field. The cloud densities vary by a factor of 10. The number densities of ions and electrons in each cloud, which have the mass ratio 250, are equal. The peak Lorentz factor of the electrons is determined in the 1-D simulation, as well as the orientation and the strength of the magnetic field at the boundary of the two colliding clouds. The relativistic masses of the electrons and ions close to the shock transition layer are comparable as in previous work. The 3-D simulation shows rapid and strong plasma filamentation behind the transient precursor. The magnetic field component orthogonal to the initial field direction is amplified in both simulations to values that exceed those expected from the shock compression by over an order of magnitude. The forming shock is quasi-perpendicular due to this amplification. The simultaneous presence of highly relativistic electrons and strong magnetic fields will give rise to significant synchrotron emissions.

The expansion of a dense plasma through a more rarefied ionized medium is a phenomenon of interest in various physics environments ranging from astrophysics to high energy density laser-matter laboratory experiments. Here this situation is modeled via a one-dimensional particle-in-cell simulation; a jump in the plasma density of a factor of 100 is introduced in the middle of an otherwise equally dense electron-proton plasma with an uniform proton and electron temperature of 10 eV and 1 keV, respectively. The diffusion of the dense plasma, through the rarefied one, triggers the onset of different nonlinear phenomena such as a strong ion-acoustic shock wave and a rarefaction wave. Secondary structures are detected, some of which are driven by a drift instability of the rarefaction wave. Efficient proton acceleration occurs ahead of the shock, bringing the maximum proton velocity up to 60 times the initial ion thermal speed.

An instability driven by the thermal anisotropy of a single electron species is investigated in a 2D particle-in-cell (PIC) simulation. This instability is the one considered by Weibel and it differs from the beam driven filamentation instability. A comparison of the simulation results with analytic theory provides similar exponential growth rates of the magnetic field during the linear growth phase of the instability. We observe, in accordance with previous works, the growth of electric fields during the saturation phase of the instability. Some components of this electric field are not accounted for by the linearized theory. A single-fluid-based theory is used to determine the source of this non-linear electric field. It is demonstrated that the magnetic stress tensor, which vanishes in a 1D geometry, is more important in this two-dimensional model used here. The electric field grows to an amplitude, which yields a force on the electrons that is comparable to the magnetic one. The peak energy density of each magnetic field component in the simulation plane agrees with previous estimates. Eddy currents develop, which let the amplitude of the third magnetic field component grow, which is not observed in a 1D simulation.

Plasma processes close to astrophysical shocks result in the amplification of magnetic fields and in the acceleration of electrons.We examine with PIC simulations the magnetic field amplification by the collision of two plasma clouds at a speed 0.5c, each consisting of electrons and ions. A quasi-parallel guiding magnetic field, a cloud density ratio of 10 and a plasma temperature of 25 keV are considered.We demonstrate that the magnetic energy density reaches that of the ions and that electrons are accelerated to highly relativistic speeds.

A gamma-ray burst (GRB) is triggered by a stellar implosion and the subsequent ejection of an ultrarelativistic jet. The prompt emissions are attributed to mildly relativistic collisions of plasma clouds within this jet. The resulting internal shocks are instrumental in the generation of the magnetic fields and the energetic electrons, which excite the observed electromagnetic emissions. We use particle-in-cell (PIC) simulations to examine such a shock. The colliding hot magnetized plasma clouds have densities that differ by a factor of 10. Current channels form rapidly in the 3D simulation, which resemble those in a Hammer–Rostoker equilibrium, are observed also in the 2D simulation. The 2D simulation also shows the formation of a shock, which is mediated by a strong electromagnetic wave, and the energy equipartition between electrons and ions.

Plasma expansion following the interaction of an intense laser beam with the inner surface of gold hohlraums, emulating conditions relevant to indirect drive inertial confinement fusion (ICF), has been investigated by a radiographic technique which employs a beam of laser-accelerated protons. This probing technique has made it possible to measure the electric field distribution associated with the plasma front and its propagation throughout the interior of the hohlraum with a temporal and spatial resolution of the order of a few ps and μm, respectively. The data indicate that the expanding plasma slows down approaching the opposite walls, possibly due to the interaction with x-ray heated plasma from the non-irradiated walls. The electric field at the plasma front shows a bipolar structure, suggesting the presence of ion-acoustic soliton-like structures cotraveling with the front. Data obtained using enclosed hohlraums suggest the feasibility of this type of diagnosis in gas-filled hohlraums, as currently employed in ICF experiments.

Surprisingly, very little empirical work has explored the application of students’ knowledge about electric fields to a chemistry context. In response, this paper reports a pilot study that investigated students’ conceptions about electric fields, and how interaction with a haptic virtual model impacts understanding of electric fields around molecules. Students first responded to specially-designed written free response items that probed knowledge transfer. The participants then interacted with the model while performing think-aloud tasks where different haptic modes offered by the model were activated. Qualitative induction of the data revealed that although students demonstrated a pronounced and classical understanding of electrostatic forces and electric fields, they struggled to apply this knowledge to a molecular context. Interestingly, there was a strong association between the existence of an electric field around a molecule with the notion of chemical polarity. Analysis of videotaped interaction with the model provided evidence for distinct influences on students’ understanding, which included using the model to gain unique insight into the nature of electric fields, and as a sensory tool for actively challenging existing alternative conceptions. Future work will expand the research framework presented here and also distil what specific perceptual experiences are related to any changes in knowledge.

The localized deposition of the energy of a laser pulse, as it ablates a solid target, introduces high thermal pressure gradients in the plasma. The thermal expansion of this laser-heated plasma into the ambient medium (ionized residual gas) triggers the formation of non-linear structures in the collisionless plasma. Here an electron–proton plasma is modelled with a particle-in-cell simulation to reproduce aspects of this plasma expansion. A jump is introduced in the thermal pressure of the plasma, across which the otherwise spatially uniform temperature and density change by a factor of 100. The electrons from the hot plasma expand into the cold one and the charge imbalance drags a beam of cold electrons into the hot plasma. This double layer reduces the electron temperature gradient. The presence of the low-pressure plasma modifies the proton dynamics compared with the plasma expansion into a vacuum. The jump in the thermal pressure develops into a primary shock. The fast protons, which move from the hot into the cold plasma in the form of a beam, give rise to the formation of phase space holes in the electron and proton distributions. The proton phase space holes develop into a secondary shock that thermalizes the beam.

The Weibel instability, driven by a plasma temperature anisotropy, is non-resonant with plasma particles: it is purely growing in time, and does not oscillate. The effect of a counterstreaming plasma is examined. In a counterstreaming plasma with an excess of transverse temperature, the Weibel instability arises along the streaming direction. Here it is proved that for large wave-numbers the instability becomes resonant with a finite real (oscillation) frequency, ω_r ≠ 0. When the plasma flows faster, with a bulk velocity larger than the parallel thermal velocity, the instability becomes dominantly resonant. This new feature of the Weibel instability can be relevant for astrophysical sources of non-thermal emissions and the stability of counterflowing plasma experiments.

Latest research indicates that construction of a communal meaning regarding coordination of complex systems development tasks should be implemented into practice, proving that far more attention should be given to the issue of communal meaning construction in organizations.

Using Activity Domain Theory for Managing Complex Systems offers a new approach towards managing complex systems that informs the co-construction of technical support for coordination and communal meaning regarding this support. Compiling and structuring empirical observations from the Ericsson™ Company and theoretical developments from the perspective of meaning construction, this unique book combines a deep understanding of concrete, every-day conditions of the telecom industry with innovative theoretical development of the Activity Domain Theory (ADT).

The filamentation instability of counterpropagating symmetric beams of electrons is examined with 1D and 2D particle-in-cell simulations, which are oriented orthogonally to the beam velocity vector. The beams are uniform, warm and their relative speed is mildly relativistic. The dynamics of the filaments is examined in 2D and it is confirmed that their characteristic size increases linearly in time. Currents orthogonal to the beam velocity vector are driven through the magnetic and electric fields in the simulation plane. The fields are tied to the filament boundaries and the scale size of the flow aligned and the perpendicular currents are thus equal. It is confirmed that the electrostatic and the magnetic forces are equally important, when the filamentation instability saturates in 1D. Their balance is apparently the saturation mechanism of the filamentation instability for our initial conditions. The electric force is relatively weaker but not negligible in the 2D simulation, where the electron temperature is set higher to reduce the computational cost. The magnetic pressure gradient is the principal source of the electrostatic field, when and after the instability saturates in the 1D simulation and in the 2D simulation.

Two counterpropagating cool and equally dense electron beams are modeled with particle-in-cell simulations. The electron beam filamentation instability is examined in one spatial dimension, which is an approximation for a quasiplanar filament boundary. It is confirmed that the force on the electrons imposed by the electrostatic field, which develops during the nonlinear stage of the instability, oscillates around a mean value that equals the magnetic pressure gradient force. The forces acting on the electrons due to the electrostatic and the magnetic field have a similar strength. The electrostatic field reduces the confining force close to the stable equilibrium of each filament and increases it farther away, limiting the peak density. The confining time-averaged total potential permits an overlap of current filaments with an opposite flow direction.

The alignment of business and knowledge strategies necessarily includes the individual and the organizational perspectives. A major problem in this context is to reconcile these perspectives into a common framework for alignment. To this end, an intermediate level is introduced – the activity domain. The activity domain is a canonical structure comprising all kinds of organizational units, irrespective of size and organizational level. The organization is regarded as a constellation of activity domains, each having a capability to produce an outcome that the organization needs in order to fulfill its goals. Alignment is defined as the management of dependencies between capabilities so that these capabilities fit the business’s strategic intentions. As a consequence, business and knowledge strategies can be linked to the same target – the activity domain. Practical guidelines and alignment targets for these strategies are suggested.

Multiple-volume visualization is a growing field in medical imaging providing simultaneous exploration of volumes acquired from varying modalities. However, high complexity results in an increased strain on performance compared to single volume rendering as scenes may consist of volumes with arbitrary orientations and rendering is performed with varying sample densities. Expensive image order techniques such as depth peeling have previously been used to perform the necessary calculations. In. this work we present a view-independent region based scene description for multi-volume pipelines. Using Binary Space Partitioning we are able to create a simple interface providing all required information for advanced multi-volume renderings while introducing a minimal overhead for scenes with few volumes. The modularity of our solution is demonstrated by the use of visual development and performance is documented with benchmarks and real-time simulations.

In order to simulate both physically and visually realistic soft tissuedeformations, the Finite Element Method (FEM) is the mostpopular choice in the literature. However it is non-trivial to modelcomplex behaviour of soft tissue with sufficient refresh rates, especiallyfor haptic force feedback which requires an update rate ofthe order of 1 kHz. In this study the use of asynchronous regions isproposed to speed up the solution of FEM equations in real-time. Inthis way it is possible to solve the local neighborhood of the contactwith high refresh rates, while evaluating the more distant regions atlower frequencies, saving computational power to model complexbehaviour within the contact area. Solution of the different regionsusing different methods is also possible. To attain maximum efficiencythe size of the regions can be changed, in real-time, in responseto the size of the deformation.

Two charge- and current-neutral plasma beams are modeled with a one-dimensional particle-in-cell simulation. The beams are uniform and unbounded. The relative speed between both beams is 0.4c. One beam is composed of electrons and protons, and the other of protons and negatively charged oxygen (dust). All species have the temperature 9.1 keV. A Buneman instability develops between the electrons of the first beam and the protons of the second beam. The wave traps the electrons, which form plasmons. The plasmons couple energy into the ion acousticwaves, which trap the protons of the second beam. Astructure similar to a proton phase-space hole develops, which grows through its interaction with the oxygen and the heated electrons into a rarefaction pulse. This pulse drives a double layer, which accelerates a beam of electrons to about 50 MeV, which is comparable to the proton kinetic energy. The proton distribution eventually evolves into an electrostatic shock. Beams of charged particles moving at such speeds may occur in the foreshock of supernova remnant (SNR) shocks. This double layer is thus potentially relevant for the electron acceleration (injection) into the diffusive shock acceleration by SNR shocks.

Images such as Gantt, WBS, PERT, and CPM have always played an important role in project management. In recent years, new types of images have emerged in complex development projects. The purpose of this paper is to make an inquiry into how project management activities are supported by these alternative images, and suggest reasons why the more traditional images appear to be inadequate during turbulent and complex circumstances. In conclusion, we find that the alternative images are a means to managing integration activities and critical dependencies in a project. Typically, they emphasize common understanding and comprehensibility over formalism and rigour. These alternative images seem to be resonant with how our mental cognitive apparatus conceives coordination, thus making it easier to manage complex development tasks.

In continuation of a previous work, numerical results are presented, concerning relativistically counterstreaming plasmas. Here, the relativistic mixed mode instability evolves through and beyond the linear saturation, well into the nonlinear regime. Besides confirming earlier findings that wave power initially peaks on the mixed mode branch, it is observed that during late time evolution, wave power is transferred to other wave numbers. It is argued that the isotropization of power in wavenumber space may be a consequence of weak turbulence. Further, some modifications to the ideal weak turbulence limit is observed. Development of almost isotropic predominantly electrostatic-partially electromagnetic-turbulent spectra holds relevance when considering the spectral emission signatures of the plasma, namely, bremsstrahlung-partially magnetobremsstrahlung (synchrotron radiation and jitter radiation)-from relativistic shocks in astrophysical jets and from shocks in gamma-ray bursts and active galactic nuclei.

The filamentation instability between counter-propagating beams of electrons is important for the magnetic field generation in astrophysical jets. Here this instability is considered for equally dense counterpropagating electron beams. It is demonstrated that the plasma evolves into a state, in which the electric fields driven by the magnetic pressure gradient balance the magnetic forces. This system is stationary in the 1D PIC simulation. The size distribution of the current filaments closely follows a Gumbel distribution within the statistical limitations.

Supernova remnant blast shells can reach the flow speed vs = 0.1c and shocks form at its front. Instabilities driven by shock-reflected ion beams heat the plasma in the foreshock, which may inject particles into diffusive acceleration. The ion beams can have the speed vb vs. For vb vs the Buneman or upper-hybrid instabilities dominate, while for vb vs the filamentation and mixed modes grow faster. Here the relevant waves for vb vs are examined and how they interact nonlinearly with the particles. The collision of two plasma clouds at the speed vs is modelled with particle-in-cell simulations, which convect with them magnetic fields oriented perpendicular to their flow velocity vector. One simulation models equally dense clouds and the other one uses a density ratio of 2. Both simulations show upper-hybrid waves that are planar over large spatial intervals and that accelerate electrons to ~10 keV. The symmetric collision yields only short oscillatory wave pulses, while the asymmetric collision also produces large-scale electric fields, probably through a magnetic pressure gradient. The large-scale fields destroy the electron phase space holes and they accelerate the ions, which facilitates the formation of a precursor shock. 

Electron surfing acceleration (ESA) is based on the trapping of electrons by a wave and the transport of the trapped electrons across a perpendicular magnetic field. ESA can accelerate electrons to relativistic speeds and it may thus produce hot electrons in plasmas supporting fast ion beams, like close to astrophysical shocks. One-dimensional (1D) particle-in-cell (PIC) simulations have demonstrated that trapped electron structures (phase space holes) are stabilized by relativistic phase speeds of the waves, by which ESA can accelerate electrons to ultrarelativistic speeds. The 2(1/2)D electromagnetic and relativistic PIC simulations performed in the present paper model proton beam driven instabilities in the presence of a magnetic field perpendicular to the simulation plane. This configuration represents the partially electromagnetic mixed modes and the filamentation modes, in addition to the Buneman waves. The waves are found to become predominantly electromagnetic and nonplanar for beam speeds that would result in stable trapped electron structures. The relativistic boost of ESA reported previously is cancelled by this effect. For proton beam speeds of 0.6 and 0.8c, the electrons reach only million electron volt energies. The system with the slower beam is followed sufficiently long in time to reveal the development of a secondary filamentation instability. The instability forms a channel in the simulation domain that is void of any magnetic field. Proton beams may thereby cross perpendicular magnetic fields for distances beyond their gyroradius.

The impact of a flow-aligned and spatially homogeneous magnetic field on the filamentation instability (FI) is examined in a system of two equal counterstreaming non-relativistic cool electron beams. Particle-in-cell simulations that represent the plane perpendicular to the flow velocity vector confirm the reduction of the linear growth rate by the initial magnetic field. The FI is, however, not inhibited by a magnetic field with the critical strength, for which the solution of the linear dispersion relation predicts a full suppression. The saturation of the electromagnetic fields in the plasma involves a balance between the magnetic pressure gradient and the electric field resulting from the charge displacement. The simulations demonstrate that the magnetic energy gain and the field structure upon saturation do not depend on the initial magnetic field strength. This can be explained by the qualitative similarity of the spectrum of unstable wavenumbers, at least for subcritical strengths of the background magnetic field, and by the vanishing of the pressure gradient of a spatially homogeneous magnetic field. Magnetic trapping is apparently not the saturation mechanism for the considered plasma parameters. The spatial power spectrum of the saturated magnetic fields in the simulation plane can be approximated by a power-law function and the magnetic and electric spectra are similar at high wavenumbers. The final electron velocity distributions are comparable for all magnetic field strengths.

Recent particle-in-cell (PIC) simulation studies have addressed particle acceleration and magnetic field generation in relativistic astrophysical flows by plasma phase space structures. We discuss the astrophysical environments such as the jets of compact objects, and we give an overview of the global PIC simulations of shocks. These reveal several types of phase space structures, which are relevant for the energy dissipation. These structures are typically coupled in shocks, but we choose to consider them here in an isolated form. Three structures are reviewed. (1) Simulations of interpenetrating or colliding plasma clouds can trigger filamentation instabilities, while simulations of thermally anisotropic plasmas observe the Weibel instability. Both transform a spatially uniform plasma into current filaments. These filament structures cause the growth of the magnetic fields. (2) The development of a modified two-stream instability is discussed. It saturates first by the formation of electron phase space holes. The relativistic electron clouds modulate the ion beam and a secondary, spatially localized electrostatic instability grows, which saturates by forming a relativistic ion phase space hole. It accelerates electrons to ultra-relativistic speeds. ( 3) A simulation is also revised, in which two clouds of an electron-ion plasma collide at the speed 0.9c. The inequal densities of both clouds and a magnetic field that is oblique to the collision velocity vector result in waves with a mixed electrostatic and electromagnetic polarity. The waves give rise to growing corkscrew distributions in the electrons and ions that establish an equipartition between the electron, the ion and the magnetic energy. The filament-, phase space hole- and corkscrew structures are discussed with respect to electron acceleration and magnetic field generation.

Images have always played an important role in project management. Project activities are communicated through diagrams such as Gantt, WBS, PERT, and CPM.  In recent years, new types of images have emerged in large, globally distributed and complex projects exposed to changing and demanding environments. The purpose of this paper is to make an inquiry into how project management activities are supported by these alternative types of images, and suggest reasons why the more traditional types of images appear to be inadequate during turbulent and complex circumstances. As a guiding framework we employ the Activity Domain Theory and the construct of Activity Modalities. Activity modalities are suggested as basic dimensions of human activity coordination; dimensions that can ultimately be traced to the phylogenetic evolution of mankind. In conclusion we find that the alternative images focus on managing critical dependencies and the integration of system parts. Moreover, these images emphasise common understanding and comprehensibility over formalism and rigour. Finally, the alternative images appear to be well aligned with the activity modalities, thus indicating that future research into the management of complex projects needs to take this finding into account.

This paper introduces a novel technique to compute illumination for Direct Volume Rendering. By adding shadow effects to volume rendered images, the perception of shapes and tissue properties can be significantly improved and it has the potential to increase the diagnostic value of medical volume rendering. The integrated intensity of incident light for a voxel is computed using a local approximation of the ambient occlusion, thus avoiding the rendering of tissues with very low illumination. Luminous tissue effects are also introduced to enhance the illumination model, controlled through an emissive component in the transfer function. This effect allows the user to highlight specific structures and can give a better understanding of tissue density. Multiresolution volume management and GPU-based computation is used to significantly speed-up the calculations and to support large data sets. The scheme yields interactive frame rates for incrementally refined ambient and emissive illumination for arbitrary transfer function changes.

A kinetic particle-in-cell simulation and 3D point-rendering visualisation are used to investigate a two-stream plasma instability, possibly found in the accretion disc of black holes. A plasma in an oblique external magnetic field is considered. The instability gives rise to a quasi-electrostatic wave able to trap electrons and accelerate them by cross-field transport. The results of the simulation show an acceleration of the electrons to speeds similar to the bulk speed of microquasar jets.

The plasma filamentation instability or beam-Weibel instability generates magnetic fields and accelerates particles in collisionless astrophysical plasma. This instability has been examined with multi-dimensional particle-in-cell (PIC) simulations, demonstrating the formation of current flux tubes. Such simulations could not model a statistically significant number of filaments. Here, we model with a PIC simulation the filamentation instability that is driven by nonrelativistic, cool electron beams in one spatial dimension at an unprecedented resolution. We show unambiguously that the gradient of the magnetic pressure which develops during the quasi-linear evolution of the filamentation instability, gives rise to an electrostatic field component. The interplay of the magnetic and electrostatic fields results in a wavenumber spectrum of the magnetic field that is a power-law, which has been reported previously for multi-dimensional PIC simulations. The magnetic field power spectrum decreases with the exponent -5.7 and that of the electrostatic field with -3.8, yielding a ratio of 3:2. The electromagnetic fields thermalize the electrons. The electrons develop a velocity distribution in the simulation direction that decreases exponentially at low speeds and faster at high speeds. The filamentation instability can thus not efficiently accelerate electrons to high energies. The filaments develop into a stationary final state. The probability distribution of the filament sizes is a Gumbel distribution. In astrophysical settings, this implies that the long exponential tail of this distribution may lead with a reasonable probability to large current filaments, if the filamentation instability develops in a large enough volume. The coherent magnetic fields of large filaments are required to explain the synchrotron emissions of gamma ray bursts.

The filamentation instability (FI) is an aperiodically growing instability driven by counterpropagating electron beams. Its ability to generate magnetic fields is important for the energetic plasmas in gamma ray burst jets and inertial confinement fusion plasmas. The FI has been examined both analytically and with particle-in-cell (PIC) simulations. We perform PIC simulations and follow the FI through its nonlinear saturation. The power spectrum of the flow-aligned current component is self-similar during the linear phase. We show that the perpendicular current distribution is self-similar during the nonlinear evolution and that the filament size increases linearly with time. We demonstrate that, at least for warm plasmas, the current filaments can't be described by simple flux tubes. Instead, the filaments merge by magnetic reconnection to form larger, partially overlapping current sheets. In the filament overlap region the electrons are accelerated.

Instabilities driven by two electron beams that stream at the relativistic relative velocity vb are important as a plasma thermalization and magnetic field generation mechanism in astrophysics and in inertial confinement fusion. The time-evolution of such instabilities, which are subdivided into the two-stream, mixed mode and filamentation (beam-Weibel) instability, is multi-dimensional and nonlinear and the instabilities are usually modelled with particle-in-cell (PIC) simulations. We examine the wave spectrum we obtain, if both beams have a density ratio 9 and (1-vb^2/c^2)^(-1/2)=4. The mixed mode dominates and grows fastest for highly relativistic vb. An electromagnetic finite grid instability is shown to generate artificial magnetic fields, imposing a simulation constraint.

Colliding plasmas can form current filaments that are magnetically confined and interact through electromagnetic fields during the nonlinear evolution of this filamentation instability. A nonrelativistic and a relativistic electron flow are examined. Two-dimensional (2D) particle-in-cell (PIC) simulations evolve the instability in a plane orthogonal to the flow vector and confirm that the current filaments move, merge through magnetic reconnection and evolve into current sheets and large flux tubes. The current filaments overlap over limited spatial intervals. Electrons accelerate in the overlap region and their final energy distribution decreases faster than exponential. The spatial power spectrum of the filaments in the flow-aligned current component can be approximated by a power-law during the linear growth phase. This may reflect a phase transition. The power spectrum of the current component perpendicular to the flow direction shows a power-law also during the nonlinear phase, possibly due to preferential attachment. The power-law distributed power spectra evidence self-similarity over a limited scale size and the wavenumber of the maximum of the spatial power spectrum of the filament distribution decreases linearly in time. Power-law correlations of velocity fields, which could be connected to the current filaments, may imply super-diffusion.

Visualization is the process of making something perceptible to the mind or imagination. The techniques for producing visual imagery of volumetric data have advanced immensely during the last decades to a point where each produced image can include an overwhelming amount of information. An increasingly viable solution to the limitations of the human sense of visual perception is to make use of not only vision, but also additional senses.

This thesis presents recent work on the development of principles and algorithms for generating representations of volumetric data through the sense of touch for the purpose of visualization. The primary idea introduced in this work is the concept of yielding constraints, that can be used to provide a continuous set of shapes as a representation of features of interest in various types of volumetric data. Some of the earlier identified standard human exploratory procedures can then be used which enables natural, intuitive and effective interaction with the data. The yielding constraints concept is introduced, and an algorithm based on haptic primitives is described, which forms a powerful yet versatile implementation of the yielding constraints. These methods are also extended to handle time-varying, moving and low quality data. A framework for multimodal visualization has been built on the presented methods, and this is used to demonstrate the applicability and versatility of the work through several example applications taken from different areas.

This paper presents an approach for Distributed Software Development (DSD) that is based on two foundations. The first one is an integration centric engineering process, which aims at managing crucial dependencies in DSD projects. The second foundation is a strategy for operationalizing the coordination of the engineering process. The purpose of this strategy is to simultaneously provide global information system support for coordination and achieve common understanding about what should be coordinated and how. The approach has been successfully used at Ericsson, a major supplier of telecommunication systems worldwide, for coordinating extraordinary complex projects developing nodes in the third generation of mobile systems. Although many obstacles have to be addressed, the results indicate that the approach is a viable way to manage DSD during very demanding circumstances. © 2006 Elsevier B.V. All rights reserved.

In this paper, we assume that the social reality and the communal meaning about this reality are reflected in each other. In object-related activity two kinds of congruent elements are constructed – objectivated elements oriented internally towards the human mind and objectified elements oriented externally towards the social reality. We propose that the mutual construction of mind and social reality proceeds along certain dimensions called activity modalities. Previously, the “objectification side” of these modalities has been grounded in the practice of developing large telecom systems. The purpose of this paper is to ground the “objectivation side” of the modalities. We claim that the “ontological categories” in the conceptual structure proposed by Jackendoff are compatible with the activity modalities. Thus, the contribution of this paper is a reinforced grounding of activity modalities as basic dimensions for coordinating human activity.

The emission of energetic radiation by astrophysical sources, e.g. the accretion disks of micro-quasars and active galactic nuclei, and gamma ray bursts, requires efficient particle acceleration mechanisms. Previously, electron surfing acceleration (ESA) has been introduced as a process that may accelerate electrons to unlimited energies. The ESA mechanism requires a quasi-electrostatic wave that can, for example, be driven by a two-stream instability. The wave traps electrons and transports them across an ambient magnetic field that is oriented perpendicularly to the wave vector. For sufficiently strong waves, the electrons cannot de-trap, which is opening up the prospect of unlimited electron acceleration. Particle-in-cell simulations have, however, revealed an important limitation of ESA. The saturated wave is not stable and its collapse releases the electrons. For non-relativistic phase speeds, the electron energy is limited to keV-regimes. We have found with the help of particle-in-cell simulations that relativistic phase speeds of the wave stabilize the system. We discuss how the electrons can be trapped for a longer time while the relativistic phase speed implies a rapid electron acceleration. We show that the peak electron speed accessible to the ESA mechanism increases into relativistic regimes. Quasi-electrostatic waves with a phase speed of 0.6c, which we may find in the upstream region of mildly relativistic shocks in micro-quasar accretion disks, can accelerate electrons to gamma factors of a few. Increasing the phase speed of the waves to values between 0.9c and 0.99c, which we may find at the most energetic astrophysical objects, e.g. active galactic nuclei and gamma ray bursts, yields peak electron gamma factors above 100. We show how a new limit emerges,in form of a peak electron gamma factor that equals the proton-to electron mass ratio. Such electron flow speeds are comparable to the fastest observed plasma flow speeds associated with gamma ray burst jets and the collimated jets of active galactic nuclei. ESA may thus contribute to their generation.

Electrons that are trapped by a quasi-electrostatic wave move, on average, with the phase speed of the wave. In the presence of a magnetic field B, the trapped electrons could, in principle, be accelerated to cosmic ray energies through cross-field transport. We model this cross-field transport with a particle-in-cell (PIC) simulation for an oblique B. The electron energies at the simulation's end exceed 5 MeV for all pitch angles and they can reach GeV energies along the wavevector. We discuss environments, in which such conditions may exist and for which such an acceleration would be relevant.

Plasmas collide at relativistic speeds in many astrophysical and high-energy density laboratory environments. The boundaries that develop between such plasmas and expand at much larger speeds than the ion sound speed cs are not well understood. Here, we address two identical electron-proton plasma slabs that collide with a relativistic speed and a Mach number v/cs of over 400. The collision speed, the plasma temperature and magnetic field are such that the growth rate of the two-stream instability exceeds that of all other instabilities. We model a planar turbulent boundary (TB) with one-dimensional (1D) and 2D particle-in-cell (PIC) simulations. We show that the boundary dissipates its energy via electron phase space holes (EPSHs) that accelerate electrons at the boundary to relativistic speeds and increase significantly the speed of some protons. Our results are put into the context of a dynamic accretion disc and the jet of a microquasar. It is shown that the accelerated electrons could contribute to the disc wind and to relativistic leptonic jets, and possibly to the hard radiation component of the accretion disc.

We present analytical and numerical studies of the dynamics of relativistic electron and ion holes in a collisionless plasma. Electromagnetic radiation can be trapped in relativistic electron phase-space holes mainly due to the relativistic mass increase of the electrons that are accelerated by the potential of the phase-space hole and by the quivering component of the electromagnetic field. Relativistic ion holes may exist in plasmas where the electrons are thermalized to extremely ultra-relativistic energies. They may be responsible for the acceleration of particles to GeV energies in active galactic nuclei and supernova remnant shocks. The analytic solutions are employed as initial conditions for numerical simulations in which the dynamics and stability of the phase-space holes are investigated. The results have relevance for intense laser-plasma experiments and for astrophysical plasmas.

Shocks play a key role in plasma thermalization and particle acceleration in the near Earth space plasma, in astrophysical plasma and in laser plasma interactions. An accurate understanding of the physics of plasma shocks is thus of immense importance. We give an overview over some recent developments in particle-in-cell simulations of plasma shocks and foreshock dynamics. We focus on ion reflection by shocks and on the two-stream instabilities these beams can drive, and these are placed in the context of experimental observations, e.g. by the Cluster mission. We discuss how we may expand the insight gained from the observation of proton beam driven instabilities at near Earth plasma shocks to better understand their astrophysical counterparts, such as ion beam instabilities triggered by internal and external shocks in the relativistic jets of gamma ray bursts, shocks in the accretion discs of micro-quasars and supernova remnant shocks. It is discussed how and why the peak energy that can be reached by particles that are accelerated by two-stream instabilities increases from keV energies to GeV energies and beyond, as we increase the streaming speed to relativistic values, and why the particle energy spectrum sometimes resembles power law distributions.

The ever increasing performance of supercomputers is now enabling kinetic simulations of extreme astrophysical and laser produced plasmas. Three-dimensional particle-in-cell (PIC) simulations of relativistic shocks have revealed highly filamented spatial structures and their ability to accelerate particles to ultrarelativistic speeds. However, these PIC simulations have not yet revealed mechanisms that could produce particles with tera-electron volt energies and beyond. In this work, PIC simulations in one dimension (1D) of the foreshock region of an internal shock in a gamma ray burst are performed to address this issue. The large spatiotemporal range accessible to a 1D simulation enables the self-consistent evolution of proton phase space structures that can accelerate particles to giga-electron volt energies in the jet frame of reference, and to tens of tera-electron volt in the Earth's frame of reference. One potential source of ultrahigh energy cosmic rays may thus be the thermalization of relativistically moving plasma.

Particle acceleration by means of nonlinear plasma wave interactions is of great topical interest. Accordingly, in this paper we focus on the electron surfing process. Self-consistent kinetic simulations, using both relativistic Vlasov and particle-in-cell (PIC) approaches, show here that electrons can be accelerated to highly relativistic energies (up to 100mec2) if the phase speed of the electrostatic wave is mildly relativistic (0.6c to 0.9c for the magnetic field strengths considered). The acceleration is strong because of relativistic stabilization of the nonlinearly saturated electrostatic wave, seen in both relativistic Vlasov and PIC simulations. An inverse power law momentum distribution can arise for the most strongly accelerated electrons. These results are of relevance to observed rapid changes in the radio synchrotron emission intensities from microquasars, gamma ray bursts and other astrophysical objects that require rapid acceleration mechanisms for electrons.

Theoretical and numerical studies of relativistic ion holes in a relativistically hot electron-ion plasma are presented. Previous particle-in-cell (PIC) simulations have shown that the ion holes are formed as a result of relativistic beam-plasma instabilities in the foreshock region of internal shocks of gamma-ray bursts and the relativistic jets of active galactic nuclei. In this process, the electrons are heated to ultra-relativistic temperatures so that their relativistic mass becomes comparable to the proton mass, and relativistic ion holes are formed by a secondary ion beam instability. The electrostatic potentials associated with the ion holes are large enough to accelerate particles to GeV energies. We use a semi-analytical model to construct relativistic ion holes and investigate their stability by means of fully relativistic Vlasov simulations. This investigation is relevant for astrophysical settings where the ion holes may work as efficient particle accelerators.

Gamma-ray bursts are associated with relativistic plasma flow and intense X-ray and soft gamma-ray emissions. We perform particle-in-cell simulations to explore the growth and saturation of waves driven by the electrostatic two-stream instability that may contribute to the thermalization of the relativistic plasma flows and to the electromagnetic emissions. We evolve self-consistently the instability driven by two charge-neutral and cool interpenetrating beams of electrons and protons that move at a relative Lorentz factor of 100. We perform three simulations with the beam density ratios of 1, 2 and 10. The simulations show that the electrostatic waves saturate by trapping the electrons of only one beam and that the saturated electrostatic wave fields spatially modulate the mean momentum of the second beam, while retaining its temperature. Cavities form in the charge density of the latter beam which, in turn, compress the electrostatic waves to higher intensities. A runaway process develops that terminates with the collapse of the waves and the development of an exponential electron high-energy tail. We bring forward evidence that this energetic tail interacts stochastically with the charge density fluctuations of the relativistic proton beam. In response, an electron momentum distribution develops that follows an inverse power law up to a spectral break at four times the beam Lorentz factor.

Direct volume haptics can provide both guidance and extra information during exploration of volumetric data. In this paper we present a novel approach to volume haptics enabling haptic exploration of tissue shape, borders and material properties in data despite low contrast and low signal to noise ratio, as is common in medical MRI data or low dose CT data. The method uses filtering based on implicit knowledge and addresses the problem of overlapping scalar ranges through the introduction of fuzzy classification and corresponding transfer functions for material properties as well as classification-based distance masking for haptic force direction.

In complex systems’ development, it is necessary to understand how things depend on each other in order to plan and control the development task. Ericsson, a major supplier of telecommunication systems all over the world, has successfully used a construct called the system anatomy for this purpose. The anatomy shows, in a compact way, the crucial functional dependencies in the system. Since the anatomy has had a profound practical impact, it is relevant to analyse how this construct can be grounded also theoretically. In this paper, we present such a grounding in which the anatomy and its associated plans are seen as manifestations of affordances. These affordances enable different groups of actors to reconcile their actions. Besides affordances, the theory is grounded in the Russian theory of activity and the Activity Domain Theory. The findings indicate that the suggested theory is a promising socio-technical approach that may complement existing approaches for development of complex systems.

With the deregulation of the markets and the introduction of new technologies, the need to manage diversification, change and complexity in product development has become crucial. To cope with this situation an alternative foundation for PLM is needed in which social issues are considered alongside with more traditional, technical ones. Such a major re-orientation of PLM needs to be informed by a theoretical perspective, which integrates social and technical issues into a coherent sociotechnical framework. To this end we propose the Activity Domain Theory. This theory has been used as a guide at the Ericsson telecommunication company in order to suggest an alternative foundation for PLM. Preliminary results are the enhancement of the organisational language with the construct of activity domains, a clean-up of fundamental categories in information management, a multi-domain product structure concept, an alternative business process model, an information system  managing core information elements and a strategy towards balancing centralised and decentralised control.

We present a numerical study of the surfing mechanism in which electrons are trapped in Bernstein-Greene-Kruskal (BGK) modes, and are accelerated across the magnetic field direction by the Lorentz force in magnetized space plasmas. The BGK modes are the product of an ion-beam Buneman instability that excites large-amplitude electrostatic upper-hybrid waves in the plasma. Our study, which is performed with particle-in-cell (PIC) and Vlasov codes, reveals the stability of the BGK mode as a function of the magnetic field strength and the ion beam speed. It is found that the surfing acceleration is more effective for a weaker magnetic field owing to the longer lifetime of the BGK modes. The importance of our investigation to electron acceleration in astrophysical environments has been emphasized. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.

We examine the phase space evolution of electrons under the influence of two counter-propagating electrostatic waves that move obliquely to an ambient magnetic field. The waves are driven by proton beams and their interaction with the electrons yields phase space structures that involve one spatial and three velocity components. We show how using color allows us to integrate the full four-dimensional electron data provided by a particle-in-cell (PIC) simulation into a single image. We find an initially developing antisymmetric electron phase space distribution and what appears to be a locking of the phase of the trapped electrons relative to the magnetic field. The phase space distribution at later times shows that there is no separatrix between the trapped electron islands of both waves.

We examine the saturation of electrostatic waves driven by proton beams moving at relativistic speeds through a plasma. We visualize the plasma phase space distribution after the saturation of the wave that has developed between the background electrons and the proton beams. A secondary instability grows between the background protons and a proton beam due to relativistic effects. We show how the saturated wave forms a proton phase space vortex that may generate cosmic ray electrons by its collapse. © 2005 IEEE.

Recently the constraint approach to proxy-based volume haptics was introduced which provided a stable and effective means of conveying information about volumetric data through a haptic instrument. In this paper we present a proof that the approach is incapable of handling nonorthogonal constraints and discuss the implications of this restriction in detail. We also describe how full utilization of haptics applications in which multiple properties are used to enhance the understanding of complex data requires the use of non-orthogonal constraints. We then show how proxybased volume haptics can be modified to allow for general constraints through the introduction of haptic primitives used to model the constraints. By balancing the forces exerted by the primitives on the proxy continuously, nonorthogonal constraints can be handled.

Datasets from Computational Fluid Dynamics (CFD) can be post-processed and visualized to aid understanding of the flow phenomena present. Visualization of CFD data, however, often suffers from problems such as occlusion and cluttering when methods such as glyphing and volume rendering are applied. In this paper we present a case study where new modes for haptic interaction are used to enhance the exploration of CFD data. A VR environment with interactive graphics and an integrated graphical user interface has been implemented. In contrast to previous work on haptic interaction with CFD data we employ a 'reactive' haptic scheme as opposed to direct force maping. The reactive approach not only generates more stable feedback but also provides clearer and more intuitive cues about the underlying data. Two haptic modes are used to enhance the understanding of different features in the flow data: One presents the orientation of the data and also guides the user to follow the stream as it flows around the aircraft fuselage. The other provides a haptic representation of vortex data. This mode enables the user to perceive and so follow tendencies of vorticity and vortices.

We examine the electron acceleration by a localized electrostatic potential oscillating at high frequencies by means of particle-in-cell (PIC) simulations, in which we apply oscillating electric fields to two neighboring simulation cells. We derive an analytic model for the direct electron heating by the externally driven antenna electric field, and we confirm that it approximates well the electron heating obtained in the simulations. In the simulations, transient waves accelerate electrons in a sheath surrounding the antenna. This increases the Larmor radii of the electrons close to the antenna, and more electrons can reach the antenna location to interact with the externally driven fields. The resulting hot electron sheath is dense enough to support strong waves that produce high-energy sounder-accelerated electrons (SAEs) by their nonlinear interaction with the ambient electrons. By increasing the emission amplitudes in our simulations to values that are representative for the ones of the sounder on board the OEDIPUS C (OC) satellites, we obtain electron acceleration into the energy range which is comparable to the 20 keV energies of the SAE observed by the OC mission. The emission also triggers stable electrostatic waves oscillating at frequencies close to the first harmonic of the electron cyclotron frequency. We find this to be an encouraging first step of examining SAE generation with kinetic numerical simulation codes.

In this paper we describe a method for IS design which focus on constructing the social reality in which the IS is used. This reality is structured as a particular form of work practice – the activity domain – which is the main construct in the Activity Domain Theory. The gist of this theory is to integrate coordinating elements of a practice into a coherent whole in which the IS is one of these elements.The theory originated in the Ericsson telecommunication company where it has been gradually refinedover more than a decade by the author. It has profoundly influenced the coordination of the development of the 3^rd generation of mobile systems

A better understanding of the relaxation of relativistic plasma flow is required to identify particle acceleration and radiation generation mechanisms at gamma ray bursts (GRBs). We perform particle-in-cell (PIC) simulations of the electrostatic two-stream instability for a beam speed VB with Gamma (v_b) = 100. The two p+e- beams are charge neutral and have comparable densities. The instability saturates by trapping the electrons of one beam and it modulates the electron density of the second beam. The electrostatic fields are compressed in the forming cavities to high intensities. The waves collapse, after which the electrons show an exponential tail that is eventually transformed into a power law tail.

This study is about the coordination of complex systems’ development. A Framework has been designed and deployed by the author in the development practice of Ericsson, a major supplier of telecommunication systems on the global market. The main purpose of the study is to investigate the impacts on coordination from the Framework. The development projects are very large and subject to turbulent market conditions. Moreover, they have many participants (often several thousand), have tight time constraints and are distributed to many design centres all over the world. In these projects, coordination of the development is of crucial importance. The Framework is grounded in a tentative theory called the Activity Domain Theory, which in turn is based on the praxis philosophy. In this theory the interaction between the individual and her environment is mediated by signs. Coordination is conceived as a particular activity domain which provides coordination to the development projects. The coordination domain is gradually constructed by the actors in this domain by iteratively refining a conceptual model, a process model, a transition model, a stabilizing core and information system support. In this process individual knowledge, shared meaning and organizational artefacts evolve in a dialectical manner. The Framework has been introduced in the Ericsson development practice over a period of more than ten years. Between 1999 and 2002 approximately 140 main projects and sub-projects at Ericsson have been impacted by the Framework. These projects were distributed to more than 20 different development units around the world and were carried out in a fiercely turbulent environment. The findings indicate that the Framework has had a profound impact on the coordination of the development of the most complex nodes in the 3rd generation of mobile systems. The knowledge contributions include an account for the history of the Framework at Ericsson and an identification of elements which contribute to successful outcomes of development projects.

Magnetic confinement fusion science leads many other branches of plasma physics in its capacity to predict, interpret and understand the behaviour of energetic particle populations. The range of applications of this capability should be extended, for the mutual benefit of fusion research and of other branches of science. In this paper we review progress in applying fusion-derived techniques to one of the central questions of astrophysics: the origin of the cosmic ray population that is magnetically confined within our Galaxy. While it is widely believed that supernova remnant shocks provide the main acceleration sites for cosmic ray electrons and protons, the fundamental 'injection' problem remains. Namely, how particles are initially accelerated from ambient thermal to mildly relativistic energies, beyond which Fermi-type processes take over. The cosmic ray injection environment is magnetized and has many other physical resemblances to beam-heated and deuterium-tritium tokamak plasmas, in consequence, many of the same physical processes come into play. These include, for example, collective beam-plasma instability, resonant wave-particle coupling, and the stochasticization of particle orbits. A broad range of analytical and numerical techniques familiar in the fusion context has been successfully applied to the injection problem (see, for example, Dieckmann M.E. et al 2000 Astron. Astrophys. 356 377). Ideas from magnetic fusion have also been used to help design and interpret recent magnetized plasma experiments (Woolsey N.C. et al 2001 Phys. Plasmas 8 2439) using the high-power VULCAN laser, which address the cosmic ray injection problem from a new perspective.

The surfatron offers the possibility of particle acceleration to arbitrarily high energies, given a sufficiently large system. Surfatron acceleration of electrons by waves excited by ions reflected from supernova. remnant (SNR) shocks is investigated using particle simulations. It is shown that surfatron and stochastic acceleration could provide a seed population for the acceleration of cosmic ray electrons at SNR shocks.

A sounder measures the density of plasmas in various parts of the solar system. The sounder emits wave pulses into the ambient plasma and listens to the response. Intensity peaks in the wave response are typically related to two mechanisms. One is provided by waves that are reflected off plasma inhomogeneities and propagate back to the emitting antenna, where they are then detected. The second is provided by waves propagating with the same group velocity as that of the receiving antenna. In the second case the waves stay close to the antenna and thus yield a long-lasting response. Response peaks to sounding at the upper hybrid (UH) frequency have, in most cases, been related to reflected waves. In this work we examine if accompanying waves can give rise to the UH response peak. We examine quantitatively how the plasma response to sounding at the UH frequency depends on the plasma density, on the electron temperature, and on the emission amplitude. For the first two parameters this is done by solving the linear dispersion relation. The well-known property of the UH waves to change from having a zero group velocity to propagating waves, depending on how the electron density compares to the electron cyclotron frequency, is applied to Alouette sounder data. It is discussed how the change in the group velocity may affect the spectral profile of the UH resonance. We present results from numerical particle in cell (PIC) simulations which show that in the case of nonpropagating UH waves, energy can be coupled into the plasma even though the vanishing group velocity of the UH waves should not allow this. The PIC simulations and sounder data from the Alouette mission show that in the case of propagating UH waves the response duration to sounding may be used to determine the electron temperature. Emission amplitudes that are typical for plasma sounders are also shown to suppress the generation of certain electron cyclotron harmonic waves.

Contact searching is an important and time-consuming part of computer simulation of certain deformation processes. Contact searching can be facilitated by orthogonal range searching. We have experimentally evaluated four methods for orthogonal range searching: the projection method, the cell method, the k-d tree method, and the range tree method.

The results of our experiments indicate that two of these methods, the cell and k-d tree methods, have practical significance. The cell method is in most cases faster than the k-d tree method.