Three parallel algorithms for classical molecular dynamics are presented. The first assigns each processor a fixed subset of atoms; the second assigns each a fixed subset of inter-atomic forces to compute; the third assigns each a fixed spatial region. The algorithms are suitable for molecular dynamics models which can be difficult to parallelize efficiently—those with short-range forces where the neighbors of each atom change rapidly. They can be implemented on any distributed-memory parallel machine which allows for message-passing of data between independently executing processors. The algorithms are tested on a standard Lennard-Jones benchmark problem for system sizes ranging from 500 to 100,000,000 atoms on several parallel supercomputers--the nCUBE 2, Intel iPSC/860 and Paragon, and Cray T3D. Comparing the results to the fastest reported vectorized Cray Y-MP and C90 algorithm shows that the current generation of parallel machines is competitive with conventional vector supercomputers even for small problems. For large problems, the spatial algorithm achieves parallel efficiencies of 90% and a 1840-node Intel Paragon performs up to 165 faster than a single Cray C9O processor. Trade-offs between the three algorithms and guidelines for adapting them to more complex molecular dynamics simulations are also discussed.

Fast parallel algorithms for short-range molecular dynamics

Many-particle charged-particle plasma simulations using spatial meshes for the electromagnetic field solutions, particle-in-cell (PIC) merged with Monte Carlo collision (MCC) calculations, are coming into wide use for application to partially ionized gases. The author emphasizes the development of PIC computer experiments since the 1950s starting with one-dimensional (1-D) charged-sheet models, the addition of the mesh, and fast direct Poisson equation solvers for 2-D and 3-D. Details are provided for adding the collisions between the charged particles and neutral atoms. The result is many-particle simulations with many of the features met in low-temperature collision plasmas; for example, with applications to plasma-assisted materials processing, but also related to warmer plasmas at the edges of magnetized fusion plasmas. >

Particle-in-Cell Charged Particle Simulations, plus Monte Carlo Collisions with Neutral Atoms, PIC-MCC

A simple electrical model for biological cells predicts an increasing probability for electric field interactions with cell substructures of prokaryotic and eukaryotic cells when the electric pulse duration is reduced into the sub-microsecond range. The validity of this hypothesis was verified experimentally by applying electrical pulses with electric field intensities of up to 5.3 MV/m to human eosinophils in vitro. When 3-5 pulses of 60 ns duration were applied to human eosinophils, intracellular granules were modified without permanent disruption of the plasma membrane. In spite of the extreme electrical power levels applied to the cells thermal effects could be neglected because of the ultrashort pulse duration. The intracellular effect extends conventional electroporation to cellular substructures and opens the potential for new applications in apoptosis induction, gene delivery to the nucleus, or altered cell functions, depending on the electrical pulse conditions.

Intracellular effect of ultrashort electrical pulses.

Electric phenomena play an important role in biophysics. Bioelectric processes control the ion transport processes across membranes, and are the basis for information transfer along neurons. These electrical effects are generally triggered by chemical processes. However, it is also possible to control such cell functions and transport processes by applying pulsed electric fields. This area of bioengineering, bioelectrics, offers new applications for pulsed power technology. One such application is prevention of biofouling, an effect that is based on reversible electroporation of cell membranes. Pulsed electric fields of several kV/cm amplitude and submicrosecond duration have been found effective in preventing the growth of aquatic nuisance species on surfaces. Reversible electroporation is also used for medical applications, e.g. for delivery of chemotherapeutic drugs into tumor cells, for gene therapy, and for transdermal drug delivery. Higher electric fields cause irreversible membrane damage. Pulses in the microsecond range with electric field intensities in the tens of kV/cm are being used for bacterial decontamination of water and liquid food. A new type of field-cell interaction, "Intracellular Electromanipulation", by means of nanosecond pulses at electric fields exceeding 50 kV/cm has been recently added to known bioelectric effects. It is based on capacitive coupling to cell substructures, has therefore the potential to affect transport processes across subcellular membranes, and may be used for gene transfer into cell nuclei. There are also indications that it triggers intracellular processes, such as programmed cell death, an effect, which can be used for cancer treatment. In order to generate the required electric fields for these processes, high voltage, high current sources are required. The pulse duration needs to be short to prevent thermal effects. Pulse power technology is the enabling technology for bioelectrics. The field of bioelectrics, therefore opens up a new research area for pulse power engineers, with fascinating applications in biology and medicine.

Bioelectrics-new applications for pulsed power technology

A review of mainly the past two years is undertaken of the industrial applications of pulsed power. Repetitively operated pulsed power generators with a moderate peak power have been developed for industrial applications. These generators are reliable and have low maintenance. Development of the pulsed power generators helps promote industrial applications of pulsed power for such things as food processing, medical treatment, water treatment, exhaust gas treatment, ozone generation, engine ignition, ion implantation and others. Here, industrial applications of pulsed power are classified by application for biological effects, for pulsed streamer discharges in gases, for pulsed discharges in liquid or liquid- mixture, and for material processing.

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Industrial Applications of Pulsed Power Technology

This report employs a Vlasov–Poisson model to elucidate fundamental electron phase–space mechanics of a multipactor discharge from onset to saturation. At the onset of multipactor, the electron phase–space is primarily defined by sharp features in both the physical space and energy space. With increasing electron density, space-charge effects lead to debunching of the swarm in phase–space. The temporal evolution of the electron energy distribution is studied across a single impact cycle. The average and peak-to-peak saturation values for the entire first-order multipactor regime are presented. Comparisons between the third- and fifth-order multipactors highlight the nuanced similarities and differences in the energy distribution of the multipacting system. The Vlasov–Poisson approach, which neglects collisions, is well suited for such analysis since the multipactor phenomenon occurs under near-vacuum collisionless conditions. It also overcomes difficulties associated with kinetic schemes that require adequately sampling all of the electron phase–space, including sparely populated regions, or special treatments to model strong growths in carrier densities.

A 1D1V Continuum Vlasov–Poisson Multipactor Analysis From Onset to Saturation Across the Entire First-Order Multipactor Regime

Experimental results are presented on the degree to which electrode surface area impacts the dielectric strength of water. A water gap of 4 mm was tested under pulsed conditions with a maximum electric field in excess of 1 MV/cm and a maximum current of more than 20 kA. Stainless steel electrodes with a Bruce profile were used to generate a uniform electric field across the water gap. The profile of the electrodes enabled effective areas ranging from 0.5 cm2 to 75 cm2 while minimizing the change in capacitance of the water gap. Conclusions are made as to the effect electrode surface area has on the holdoff voltage and holdoff time of water dielectric systems

The effect of area on pulsed breakdown in water

Hall-effect thrusters scaled to power levels below 300 W are of great interest due to their compact size but still require further system optimization. A major component of these thrusters is the free electron source. The majority of the current systems utilize heated element hollow cathodes, but in the event of heater failure, the overall system becomes inoperable. For this reason, a simplistic alternate system such as an arc hollow cathode has been examined. The drawback of utilizing the arc hollow cathode is the reduction in the operational lifetime, especially when the cathode experiences multiple start-up cycles. In order to remedy this, we have developed a soft start-up and continuous operation power supply system. Utilizing this system, we were able to minimize the start-up process from the lifetime influences and examine other factors.

Diagnostics of the Start-Up Process of an Arc Hollow Cathode

Erratum: "Monte Carlo analysis of field-dependent electron avalanche coefficients in nitrogen at atmospheric pressure" [Phys. Plasmas 24, 124501 (2017)]

This study focuses on the use of St707 non-evaporable getter (NEG) material in a high power microwave (HPM) sealed-tube virtual cathode oscillator (vircator) operated at 500 Hz repetition rate. High-current pulsed operation releases gases trapped within the bulk materials and gas monolayers on material surfaces, leading to localized plasma production in the A-K gap. This can lead to gap closure, shorten the duration of microwave emission, and spoil vacuum. A single current pulse increases the chamber pressure to the low 10−6 Torr range from an initial background pressure in the low 10−9 Torr range, desorbing approximately 1014 particles. At 500 Hz operation, a sufficiently large pumping speed (∼2,500 L/s) is necessary to evacuate desorbed particles from the vircator volume during consecutive shots. Previous work has identified hydrogen (H, H 2 ) as the main outgassing species during vircator operation, with contributions from CH 4 , N 2 , CO, CO 2 , and Ar as the other primary gas constituents[1]. The St707 NEG pumps shows an affinity for pumping hydrogen, making it a suitable choice to adsorb vircator outgassing species, of which hydrogen is an order of magnitude greater than any other gas species. Previously, without the presence of NEG material, degradation of microwave output power from the vircator has been observed during subsequent shots at 1 Hz, 10 second burst mode operation. Subsequently, an increase in chamber pressure from 1.25×10−6 Torr on the first shot to 10−5 Torr on the tenth shot has been experimentally observed. This paper details the performance of the St707 NEG material (70% Zr, 24.6% V, 5.4% Fe) for maintaining UHV conditions during rep-rated vircator operation. Pumping characteristics of multiple St707 NEG pumps in the presence of rep-rated high-current pulses are presented. Diagnostic results obtained with a residual gas analyzer to observe individual gas constituents and two inverted magnetron cold cathode gauges for absolute pressure are utilized to analyze vircator and getter performance in detail.

John J. Mankowski

Papers

A 1D1V Continuum Vlasov–Poisson Multipactor Analysis From Onset to Saturation Across the Entire First-Order Multipactor Regime

The effect of area on pulsed breakdown in water

Diagnostics of the Start-Up Process of an Arc Hollow Cathode

Erratum: "Monte Carlo analysis of field-dependent electron avalanche coefficients in nitrogen at atmospheric pressure" [Phys. Plasmas 24, 124501 (2017)]

Performance of St707 getter material in a rep-rated high power microwave sealed-tube vircator under UHV conditions