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Realistic and accurate numerical simulations of electrostimulation of tissues and full-body biomodels have been developed and implemented. Typically, whole-body systems are very complex and consist of a multitude of tissues, organs, and subcomponents with diverse properties. From an electrical standpoint, these can be characterized in terms of separate conductivities and permittivities. Accuracy demands good spatial resolution; thus, the overall tissue/animal models need to be discretized into a fine-grained mesh. This can lead to a large number of grid points (especially for a three-dimensional entity) and can place prohibitive requirements of memory storage and execution times on computing machines. Here, the authors include a simple yet fast and efficient numerical implementation. It is based on LU decomposition for execution on a cluster of computers running in parallel with distributed storage of the data in a sparse format. In this paper, the details of electrical tissue representation, the fast algorithm, the relevant biomodels, and specific applications to whole-animal studies of electrostimulation are discussed. Index Terms--Distributed currents, LU decomposition, parallel computing, tissue modeling, whole body.

CFD (computational fluid dynamics) calculations for high- and low-current arcs in an interrupter of the self-blast type have been performed. The high-current simulation provides information about the mixing process of the hot PTFE cloud with the cold [SF.sub.6] which is difficult to access for measurement. In addition, it yields the gas state inside the pressure chamber when the fault current approaches its current zero. Depending on the arcing time, fault current and interrupter geometry blow temperatures of up to 2000 K with a PTFE mass fraction of up to 10% have been found. The low-current simulation performed for this blow temperature gives detailed information about the arc behavior in the time immediately before current zero. Special attention has been paid to the flow phenomena in the expanding nozzle region. Due to the interaction of the supersonic flow with the obtuse arcing contact and the nozzle geometry, a complex flow structure with a shock is established. This has been simulated for low-current arcs in an extinguishing flow with boundary conditions as obtained from the high-current simulation. Index Terms - Ablation-controlled arc, arc modeling, CFD, gas flow phenomena, high-voltage circuit breaker, self-blast circuit breaker, switching arc.

Concurrent optical and electrical measurements have been used to characterize the physical phenomena associated with the prebreakdown and flashover behavior of high-voltage alumina ceramic insulator samples having both conventional and unconventional conical geometries. The critical parameters are the threshold voltage for initiating flashover events, the stable interelectrode current-voltage relationship, the luminescence pattern, and the breakdown rate as a function of time. These measurements have revealed that a well-conditioned sample of unconventional conical geometry can have very low, i.e., approaching zero, breakdown rate. Physical explanations of the findings are based on the current understanding of flashover mechanisms and the prediction from simulations using a field plotting technique. Index Terms - Ceramics, conical insulators, flashover, high-voltage, vacuum.

The current switching by means of a gas-filled electron switch has been studied experimentally and theoretically. The switch showed complete controllability under deep volume charge compensation conditions. The switched current density was an order of magnitude higher than the current density of a similar vacuum switch.

The formation of intense current filaments, destroying the homogeneity of the active laser volume, limits the energy extraction from XeCl-lasers. Using time-resolved spectrally integrated pictures, the morphology and temporal development of such filaments are studied. The time scale of this process is found to be controlled by HCl depletion. With strong preionization ([n.sub.e] > [10.sup.9]/[cm.sup.3]) and a fast (10 ns) rising discharge voltage applied at the end of the preionizing X-ray pulse, the filaments originate from hotspots formed in the cathode layer. The shot by shot statistics of spot formation on freshly prepared cathodes reveal that hotspots are not caused by streamers developing in the high field region of strongly reduced electron density built up by electron diffusion during formation of the cathode sheath. Unexpectedly, a large number of weak diffuse filaments are found in spotless discharges (current density 300 A/[cm.sup.2]; duration 200 ns), in spite of the strong preionization.

Insertion of internal impurities is one of the factors for the degradation of neutron output in plasma focus devices. In this context, neutron emission measurements were carried out in a 2.2 kJ (7.2 [[micro]farad], 25 kV) Mather type squirrel cage plasma focus device with various combinations of electrode and insulator materials. The neutron yield was measured by silver activation detector. The results indicate that a central electrode of low erosion rate material produces maximum neutron yield with the highest anisotropy factor. A time integrated X-ray pinhole image shows that focus corresponding to the highest neutron yield material is the most compressed. The dielectric constant of the ceramic insulators has, at best, a very weak correlation with neutron output. The nonceramic insulators like perspex, nylon, or teflon neither produce focus nor neutron.

With the experiments presented in this paper, applications of a retarding field analyzer (RFA) for the measurement of the ion energy [E.sub.i] in a vacuum arc plasma are discussed. The examined plasma was produced by a sinusoidal half-wave vacuum arc current. The experiments were concentrated on evaluating the plasma parameters at the last three milliseconds before current zero. In a current range from 300 [A.sub.rms] to 10 k[A.sub.rms], the ion energy distributions and their peak values were evaluated. With the increase of the arc current, a decrease of the ion energy was found. By additional investigations of the angular distribution of the ion energies, a transition from a collision dominated interelectrode plasma to a freely expanding plasma was observed, depending on the arc current.

The main parameters and dimensions of cathode spots have been under discussion for years. To solve these current questions, a new system was specially designed. The image converting High Speed Framing Camera (HSFC) combines a micrometer lateral resolution with a nanosecond time resolution and a very high optical sensitivity. This camera was used to study the microscopic behavior of vacuum arc cathode spots in a pulsed high current arc discharge on copper. The direct observation of these spots with high resolution revealed that one single cathode spot, as normally observed by optical means, consists of a number of simultaneously existing microscopic subspots, each with a diameter of about 10 [[micro]meter] and a mean distance of 30-50 [[micro]meter] between them. The mean existence time of these subspots on copper was found to be about 3 [[micro]seconds], where the position of a subspot remains unchanged (with an upper limit of about 5 [[micro]meter]) during its existence time. The lower limit of the current density in the cathode spots was estimated to be on the order of [10.sup.10]-[10.sup.11] A/[m.sup.2]. An upper limit of the crater surface temperature was estimated by a comparison between the brightnesses of a cathode spot and of a black body radiation lamp to about 3000 K.

General properties of current and voltage dynamics for discharges having narrow current channels are studied using a generic transmission-line model with active parameters. The line parameters for the inductance and capacitance per unit length are, except for positivity and some physically motivated monotonicity conditions, left as unspecified functions of local line current and voltage. (Specific functions would be determined by configuration-specific self-consistent variations of the current channel with fields.) The resistance per unit length is taken to vary inversely with current, thereby assuring dynamical properties common to transient electrical discharges, i.e., 1) thresholds to be exceeded by local voltage gradients for discharge current initiation, 2) locally abrupt current termination with the possibility of later self- generated current restart, and 3) residual charge gradients in the current channel at discharge termination. (This model of the resistance describes the 'glow region' of discharges.) Emphasis is placed on determining global properties of current front dynamics, i.e., the speed and direction of motion of the free boundary interfaces between instantaneously active (current-carrying) and passive (current-free) regions of the discharge current channel, and the space-time paths across which current reversal occurs. The results are applied to surface electrical discharges initiated at a charge spot.

The air breakdown times of a second spark formation, geometrically in series with a trigatron switching, have been studied and compared to that of a triggered spark gap. Despite a large breakdown jitter of greater than or equal to 1 microsecond in the trigatron, it was not observed to affect the electrical closure times of the series-connected second spark gap, which was triggered largely as a result of their direct electrical and optical couplings. While most of the multistage spark gaps were designed for fast and precision switching of high voltages, the air breakdown times of the second spark formation found a number of useful applications, particularly in achieving uniform glow discharge in pulsed gas lasers.

Under certain network conditions vacuum circuit breakers may generate high frequency currents. The excellent quenching capability of vacuum circuit breakers for line frequency currents as well as for high frequency currents plays an important role in the generation of unwanted voltage transients. This may occur when the gap distance at current zero is still too short to withstand the TRV; in practical applications that is less than a millimeter. This paper describes the results of simulation calculations regarding these phenomena in the vicinity of current zero. Simulations are based on a detailed physical model, taking into consideration the basic conservation laws, the Maxwell equations and the current continuity. The numerical solution takes into account the results of experimental streak photographs, revealing that the visible discharge covers only a small part of the contact diameter. The calculations were made by the Department of Electric Power Engineering, Rensselear Polytechnic Institute, the experiments were carried out at the Institut fur Elektrische Energieanlagen, Technische Universitat Braunschweig.

We draw the attention of filamentary structures in molecular clouds and point out the existence of subfilaments of sinusoidal shape and also of helix like structures. For two dark clouds, the Lynds 204 complex and the Sandqvist 187-188 complex (the Norma 'sword'), we make a detailed study of such shapes and in addition we find the possible existence of helices wound around the main filaments. All these features are highly reminiscent of morphologies encountered in solar ascending prominences and in experiments in plasma physics and suggest the existence of electric currents and magnetic fields in these clouds. On the basis of a generalization of the Bennett pinch model, we derive the magnitudes of the currents expected to flow in the filaments. Values of column densities, magnetic field strengths, and directions of the fields are derived from observations. Magnetic fields with both toroidal and axial components are considered. This study shows that axial currents of the order of a few times 10 to the thirteenth power A are necessary for the clouds to be in equilibrium. The corresponding mean current densities are very small and even at the very low values of the fractional abundance of electrons encountered in these clouds, the mean electron velocities are of the order of 10 to the -2 -10 to the -5 m s to the -1, much lower than the thermal velocities in the clouds. Hence the necessary currents may be carried without other observable effects on the cloud than the pinching. We suggest that helical structures may evolve as a result of various instabilities in the pinched clouds. We also call the attention to the kink instability in connection with the sinusoidal shapes. The existence of electromagnetically controlled features in the interstellar clouds can be tested by further observations.

The interaction of current channels has long been a topic of interest for those working in solar physics (magnetic flux rope interactions), magnetic field line reconnection (tearing of current sheets and coalescence of current fibers), and fusion physics (the behavior of currents in tokamaks and pinches). A series of experiments on the dynamic interaction of two current channels has been performed in the LAPD, a new laboratory device at UCLA. The two current channels are formed by coating the cathode source nonuniformly and then biasing it with respect to an anode 10 m away. Volume data of the plasma parameters n(sub e), V(sub p), and T(sub e), and B are taken at 5355 locations within the plasma volume, and at 10 000 time steps. The plasma produced in the current channels is high beta but the plasma ions are unmagnetized. The self-generated magnetic fields of the current channels are as large as 20% of the background magnetic field. The experiment consists of two phases, one in which a net current is drawn through the plasma, and one in which the net current is zero. The current channels are observed to twist about each other, merge, and evolve toward a force-free state when a net current is drawn through the plasma. When the net current is zero the interaction between the channels is greatly reduced. The dynamics of the system are dominated by electron pressure and associated electrostatic fields. However, as the currents twist, a small amount of magnetic helicity is generated. The helicity and its temporal rate of change are computed and found to compare within experimental uncertainty to the predicted rate.

The effect of charged dust on Hall current instabilities in the ion frequency regime is investigated for plasma parameters representative of the lower E region of the ionosphere above about 95 km. Linear kinetic theory is used, and both the longer wavelength Farley--Buneman two-stream instability and the shorter wavelength, higher frequency Hall current instability are considered. Under daytime conditions, it is found that the presence of negatively (positively) charged dust can increase (decrease) the critical drift for the Farley--Buneman instability. In addition, the presence of positively charged dust can increase the critical drift for the shorter wavelength, higher frequency instability. Possible application to dusty regions such as meteor trails is briefly discussed. Index Terms--Dusty plasma, E region, Hall current instability, two-stream instability.

The electrodynamic tether is a powerful new tool for in-space propulsion and in situ ionospheric research. To fully exploit its potential, knowledge of both its steady-state and transient electrical responses is needed. The tether's transient response is governed by the interaction of the tether and its endpoints with the surrounding ionospheric plasma. Presented here is an improved model of the plasma--tether interaction that accounts for high-induced voltages and a dynamic nonlinear sheath. In this work, the model for the plasma--tether system was developed analytically and verified via particle-in-cell simulations and through experimental data. The model is valid in the temporal regime between the ion and electron plasma periods, and for large negative applied voltages. The model is based on an ion-matrix-sheath that is a function of applied voltage. Although the investigation was geared toward electrodynamic tethers, it also has application to other areas of research that employ the dynamic nature of the plasma sheath. Index Terms--Electrodynamic tethers, high-voltage sheath, ion-matrix sheath, transient plasma sheath.

This research concentrates on the application of pulsed corona discharges to crack heavy tar components (hydrocarbons) of thermally generated biogas into lighter ones. The method has the advantage that it can operate at a high temperature and be retrofitted to existing installations. The corona discharge is energized by 100-150-ns-wide voltage pulses (100 kV) at a continuous repetition rate of 600-1000 pulses per second. The power dissipated by the corona discharges is 1.5 kW average and 50 MW peak in each pulse. To be cracked by discharges, the hydrocarbons of the tar mixture need to be gaseous, and therefore, the corona reactor must operate at a high temperature. During this first phase of the program, the reactor runs at a modest temperature of up to 200 [degrees] C. The reactor is a 3-m-long stainless steel cylinder, 0.25-m diameter with a corona wire along the axis. The pressure is 1 atmosphere. Index Terms--Biogas, discharge cleaning, electromagnetic compatibility, high voltage, hot gas cleanup, pulsed corona, pulsed power, tar removal.

The effect of including an inductance in the circuit of a glow-discharge-pulsed electron gun for materials processing is analyzed. Due to the particular nonlinear behavior of the current vs voltage in these guns, and the strong nonlinear self-focusing of the beam, the adequate shaping of the temporal profile of the discharge becomes relevant to the efficiency of the system. A critical damping value for the inductance is obtained and within the range of practical interest, analytical expressions are derived for the required inductance, capacitance and source voltage for a given peak power, pulse duration, gas pressure, and pulse shape. The efficiency increase with the improved design is also discussed. Index Terms--Electron gun, glow discharge, surface treatment.