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

Review of Charged Particle Dynamics. Beam Optics and Focusing Systems Without Space Charge. Linear Beam Optics with Space Charge. Self-Consistent Theory of Beams. Emittance Variation. Beam Physics Research from 1993 to 2007. Appendices. List of Frequently Used Symbols. Bibliography. Index.

Theory and Design of Charged Particle Beams

We report a hierarchical multi-scale modeling study of implanted helium segregation on surfaces of tungsten, considered as a plasma facing component in nuclear fusion reactors. We employ a hierarchy of atomic-scale simulations based on a reliable interatomic interaction potential, including molecular-statics simulations to understand the origin of helium surface segregation, targeted molecular-dynamics (MD) simulations of near-surface cluster reactions, and large-scale MD simulations of implanted helium evolution in plasma-exposed tungsten. We find that small, mobile He n (1⩽  n  ⩽  7) clusters in the near-surface region are attracted to the surface due to an elastic interaction force that provides the thermodynamic driving force for surface segregation. This elastic interaction force induces drift fluxes of these mobile He n clusters, which increase substantially as the migrating clusters approach the surface, facilitating helium segregation on the surface. Moreover, the clusters' drift toward the surface enables cluster reactions, most importantly trap mutation, in the near-surface region at rates much higher than in the bulk material. These near-surface cluster dynamics have significant effects on the surface morphology, near-surface defect structures, and the amount of helium retained in the material upon plasma exposure. We integrate the findings of such atomic-scale simulations into a properly parameterized and validated spatially dependent, continuum-scale reaction-diffusion cluster dynamics model, capable of predicting implanted helium evolution, surface segregation, and its near-surface effects in tungsten. This cluster-dynamics model sets the stage for development of fully atomistically informed coarse-grained models for computationally efficient simulation predictions of helium surface segregation, as well as helium retention and surface morphological evolution, toward optimal design of plasma facing components.

/pdf/analysis-of-helium-segregation-on-surfaces-of-plasma-exposed-4lvgexbztz.pdf

Analysis of Helium Segregation on Surfaces of Plasma-Exposed Tungsten

We have recently proposed to combine the advantages of a pseudospark-sourced sheet electron beam (PS-SEB) with a planar slow wave structure to generate high power terahertz radiation. To verify this idea, experimental investigation of an extended interaction oscillator based on the PS-SEB has been conducted and presented. A PS-SEB of approximately 1.0 mm × 0.17 mm in size with 21.5 A peak current (1.26 × 104 A/cm2 beam current density) and 34.5 kV peak voltage was measured after propagating a distance of 10-mm without the need of an external focusing magnetic field. A radiation pulse of ∼35 ns in duration and an output power of over 10 W at a frequency of ∼0.2 THz were measured.

/pdf/experimental-demonstration-of-a-terahertz-extended-xa2hop6ojf.pdf

Experimental demonstration of a terahertz extended interaction oscillator driven by a pseudospark-sourced sheet electron beam

This paper reports the development of a sealed-off double-gap pseudospark switch and its performance analysis. The modular pseudospark geometry has two gaps that are separated by a cavity drift space region. It utilizes a single ferroelectric trigger unit in a hollow cathode region of the first gap to initiate the discharge for rapid breakdown of both the gaps. The switch has been pinched for deuterium gas and a gas reservoir has been used for maintaining optimum gas pressure in the sealed-off device. The switch design has been analyzed for higher hold-off voltage with high charge transfer capability at higher repetition rates. The developed switch holds a 50 kV voltage in vacuum, and 40 kV during gas discharge at optimum gas pressure with a peak anode current of 8 kA and pulse duration of 1.3 μs while it is operated with a resistive load of 2.9 Ω. The switch has been tested using a constant current charging power supply for pulse repetition frequency up to 50 Hz at different voltages and peak currents corresponding to 200 nF energy storage capacitors. Its performance has been analyzed in terms of pulse parameters including hold-off voltage, jitter, fall time, rise time, peak current and delay time to qualify the design.

https://iopscience.iop.org/article/10.1088/1361-6595/aaab80/pdf

A sealed-off double-gap pseudospark switch and its performance analysis

In the present paper, a pseudospark discharge based multiple gap plasma cathode electron gun is reported which has been operated separately in self and trigger breakdown modes using two different gases, namely, argon and hydrogen. The beam current and beam energy have been analyzed using a concentric ring diagnostic arrangement. Two distinct electron beams are clearly seen with hollow cathode and conductive phases. The hollow cathode phase has been observed for ∼50 ns where the obtained electron beam is having low beam current density and high energy. While in conductive phase it is high current density and low energy electron beam. It is inferred that in the hollow cathode phase the beam energy is more for the self breakdown case whereas the current density is more for the trigger breakdown case. The tailor made operation of the hollow cathode phase electron beam can play an important role in microwave generation. Up to 30% variation in the electron beam energy has been achieved keeping the same gas and by varying the breakdown mode operations. Also, up to 32% variation in the beam current density has been achieved for the trigger breakdown mode at optimized trigger position by varying the gas type.

A multiple gap plasma cathode electron gun and its electron beam analysis in self and trigger breakdown modes

Molecular dynamics simulations of helium clustering and bubble growth under tungsten surfaces

In this paper, simulation studies have been performed to analyze the role of plasma for sheet-electron beam propagation inside a drift space region filled with argon gas using particle-in-cell simulation code VSim6.2. The sheet-electron beam has been propagated inside the drift space of size 100 mm $\times100$ mm $\times200$ mm. A beam emitter of size 7 mm $\times1$ mm, which is equal to size of anode aperture, has been taken at the entrance of the drift space with identical operating conditions to the experiments, i.e., beam voltage 17 kV, beam current 156 A, and argon gas pressure 11 Pa. The transversal and longitudinal distribution of the sheet-electron beam is mainly dependent on beam voltage and current and also on gas pressure inside the drift space. The simulated images of the sheet-electron beam inside the drift space region have been compared with the experimental ones under these conditions. A good agreement has been found between the experimental and simulation results. This analysis has helped in obtaining transient plasma parameters, such as ion density, space charge neutralization time, and secondary electron temperature, simultaneously. These parameters are also analyzed at different operating conditions in the drift space region for sheet-beam stability. The operating condition at which a sheet-electron beam propagation up to 80 mm without any external magnetic field and instability has been obtained and the same has been explained on the basis of charge neutralization factor.

Analysis of Experimental Results on Pseudospark Discharge-Based Electron Beams With Simulation Model

This paper presents the analysis of cylindrical electron beam and beam-impact-generated plasma using particle-in-cell simulation code VSim 6.2 at different operating conditions. The transient plasma parameters, such as, beam density, charge neutralization factor, space charge potential, ionization time, beam compression, and plasma electron temperature, have been estimated and analyzed for a 40-keV, 10–100-A electron beam propagating in argon gas-filled drift space at different operating pressures. These parameters are difficult to be measured experimentally. The evaluated charge-neutralizing factor has been found dynamic in space and time, which is maximum at the beam envelope. The voltage profile of space charge potential required for effective electron beam propagation approaches zero due to formation of the plasma in the order of ionization time. The estimated average plasma electron temperature has been found to be less than ~1 eV, which is less damaging for interaction structure of the plasma-assisted microwave sources. It has been observed that an optimum pressure limit needs to be maintained inside the drift space to prevent the loss of electron beam due to instabilities resulting in excessive collisions with the drift space walls.

Estimation of Beam and Plasma Parameters for Electron Beam Transport in Ion-Focused Regime

Oral dosage forms are the most popular way of taking medication, despite having some disadvantages compared with other methods like risk of slow absorption of the medicament, which can be overcome by administering the drug in liquid form, therefore, possibly allowing the use of a lower dosage. However, instability of many drugs in liquid dosage form limits its use. Effervescent technique can be used as alternate to develop a dosage form which can accelerate drug disintegration and dissolution, is usually applied in quick release preparations. Along with the development of new pharmaceutical technique, effervescent tablets are more and more extensively to adjust the behavior of drug release, such as in sustained and controlled release preparations, pulsatile drug delivery systems, and so on. In this review we have discussed about formulation, evaluation, advantages of effervescent tablets and also demonstrated the new applying of effervescent technique in effervescent tablets.
Keywords: Effervescent tablets. Carbon tablet, Effervescent granules.

Arvind Singh Jadon

Papers

A multiple gap plasma cathode electron gun and its electron beam analysis in self and trigger breakdown modes

Molecular dynamics simulations of helium clustering and bubble growth under tungsten surfaces

Analysis of Experimental Results on Pseudospark Discharge-Based Electron Beams With Simulation Model

Estimation of Beam and Plasma Parameters for Electron Beam Transport in Ion-Focused Regime

Effervescent Tablets: Everything You Need To Know