Pulsed power accelerators compress electrical energy in space and time to provide versatile experimental platforms for high energy density and inertial confinement fusion science. The 80-TW “Z” pulsed power facility at Sandia National Laboratories is the largest pulsed power device in the world today. Z discharges up to 22 MJ of energy stored in its capacitor banks into a current pulse that rises in 100 ns and peaks at a current as high as 30 MA in low-inductance cylindrical targets. Considerable progress has been made over the past 15 years in the use of pulsed power as a precision scientific tool. This paper reviews developments at Sandia in inertial confinement fusion, dynamic materials science, x-ray radiation science, and pulsed power engineering, with an emphasis on progress since a previous review of research on Z in Physics of Plasmas in 2005.

This study investigates the axisymmetric plasma flow field near a fixed-potential surface, representing the conditions near a Faraday cup. A hybrid computational code simulates electrons with a fluid model using the Boltzmann relation, and heavy particles with a Particle In Cell method. The planar Bohm sheath solution is recovered accurately for plasma conditions n i = 1.1×10 14 m-3 , T i = 300 K, T e = 1eV. Conditions representative of a lowpower Hall thruster plume (n i = 1.1×10 14 m-3 , v i = 2,380 m/s, T i = 1 eV, T e = 1eV) also show good agreement with Bohm sheath theory. Simulated measurements of ion current on a 2D probe geometry are 10-20% higher than expected from elementary theory, suggesting a focusing effect of electric fields in the sheath.

This study investigates the axisymmetric plasma flow field near a Faraday probe. A hybrid fluid PIC computational code simulates heavy particles with a Particle In Cell (PIC) model, and electrons with a detailed model derived from conservation of mass, momentum, and energy. These simulations show significant departures from the planar Bohm sheath solution and from previous simulations using the Boltzmann model electron fluid. In the current work, the sheaths extend three to six Debye lengths from the probe depending on the properties of the ion distribution. This is significantly more compact than seen in the Bohm sheath solution and previous simulations, which show sheaths that extend twice as far from the probe. Simulated measurements of ion current at the probe surface do not change as much, only increasing by 1% over Bohm sheath solution values. Variation in the conductivity of the plasma is believed to be the main source of the discrepancies.

No abstract

Kinetic simulations of plasma phenomena during and after formation of the conductive plasma channel of a nanosecond pulse discharge are analyzed and compared to existing experimental measurements. Particle-in-cell with direct simulation Monte Carlo collisions (PIC-DSMC) modeling is used to analyze a discharge in helium at 200 Torr and 300 K over a 1 cm gap. The analysis focuses on physics that would not be reproduced by fluid models commonly used at this high number density and collisionality, specifically non-local and stochastic phenomena. Similar analysis could be used to improve the predictive capability of lower fidelity or reduced order models. First, the modeling results compare favorably with experimental measurements of electron number density, temperature, and 1D electron energy distribution function at the same conditions. Second, it is shown that the ionization wave propagates in a stochastic, stepwise manner, dependent on rare, random ionization events ahead of the ionization wave when the ionization fraction in front of the ionization wave is very low, analagous to the stochastic branching of streamers in 3D. Third, analysis shows high-energy runaway electrons accelerated in the cathode layer produce electron densities in the negative glow region over an order of magnitude above those in the positive column. Future work to develop reduced order models of these two phenomena would improve the accuracy of fluid plasma models without the cost of PIC-DSMC simulations.