Jeremiah J. Boerner

Fully kinetic simulation of atmospheric pressure microcavity discharge device

Summary form only given. In this talk we will present our recent work on simulating the discharge process in a microscale device using the PIC-DSMC simulation code Aleph. Microcavity discharges have been experimentally studied and computationally simulated, but to our knowledge there have been no completely kinetic simulations. The nominal breakdown process under consideration occurs due to a large field applied across a dielectric spacer between anode and electrode surfaces. Ideally, one would be able to exhibit control over the plasma chemistry (e.g., selection of specific excitations) and predict spatial and temporal evolution. In particular, pre-initiation behavior, initiation itself, and convergence to a subsequent steady state will be presented. Scaled results and comparison to existing device measurements will also be provided.

2D PIC-DSMC simulation of microscale breakdown after vacuum seal failure

Summary form only given. An electrostatic particle-in-cell code with complex boundary conditions and direct simulation Monte Carlo particle collisions performed on a separate, adaptable collision mesh is utilized to investigate DC breakdown after vacuum failure. Previously, it has been found that cold field electron emission can explain the breakdown voltage deviation from the Paschen curve measured for small gaps. Furthermore, prior 1D simulations found that breakdown was sensitive to a fixed non-uniform background neutral gas distribution across the gap and that if the gap size is of order the mean free path then gas concentrated near the anode results in smaller breakdown voltages because electrons reaching the anode have energies near the peak of the ionization cross section. In the present work the two electrodes are separated by a vacuum gap and air at atmospheric pressure is allowed to flow into the gap from either the anode or cathode due to a small “crack” in the electrode resulting in a non-uniform neutral gas distribution. The simulation includes Auger neutralization, cold field emission (CFE) of electrons, electron-neutral elastic, ionization, and excitation interactions and ion-neutral interactions including charge exchange. The simulated breakdown voltages at various electrode gap sizes are compared to the Paschen curve and breakdown is found to be sensitive to the neutral gas density distribution as it develops across the gap from either the cathode or anode leak.

PPPS-2013: Accommodating large temporal, spatial, and particle weighting demands for simulating vacuum ARC discharge

Summary form only given. We have developed novel modeling approaches for simulation breakdown of vacuum arcs. Initiating an arc in vacuum spans many orders of magnitude in temporal and spatial scales, and number densities. We have developed specific approaches for each of these challenges-some more tested than others. These approaches are implemented in Aleph, a massively parallel 3D unstructured mesh electrostatic PIC-DSMC code. Aleph includes dynamic load balancing, volume chemistry (elastic collisions, charge exchange, ionization, etc.), and a variety of surface mechanisms. Our tool chain allows us to use conformal meshes to CAD geometry, a requirement for production use.

1D PIC simulation of microscale breakdown in gaps with a non-uniform background neutral gas density

Summary form only given. An explicit, electrostatic particle-in-cell (PIC) code with complex boundary conditions and direct simulation Monte Carlo (DSMC) particle collisions is utilized to investigate one dimensional direct current breakdown. Two electrodes are separated by a microscale gap with a non-uniform neutral gas distribution. For example, there may be a higher density near the anode as a result of vacuum seal failure near the anode. The simulation model includes Auger neutralization and cold field electron emission from the cathode as well as electron-neutral elastic, ionization, and excitation interactions. The simulated breakdown voltages at various electrode gap sizes are compared to experimental data and the Paschen curve. Previously, it has been found that cold field electron emission can explain the breakdown voltage deviation from the Paschen curve measured for small gaps. Furthermore, even in large gaps, as breakdown proceeds the plasma density becomes large enough and thus the cathode sheath thin enough that cold field emission dominates and super-exponential current growth results. Breakdown was found to be sensitive to the neutral gas density distribution across the gap. Specifically, if the gap is large enough that the cold field emission is negligible then gas concentrated near the cathode results in higher breakdown voltages since electrons leaving the cathode due to Auger neutralization are not yet energetic enough to ionize the high density neutral gas at the cathode. Conversely, if the gap size is of order the mean free path then gas concentrated near the anode results in smaller breakdown voltages because the electrons reaching the anode have energies near the peak of the ionization cross section near the higher density anode region. These lower breakdown voltages should be taken into account when designing vacuum electronics for failure tolerance.

Engineered plasma interactions for geomagnetic propulsion of ultra small satellites

Previous astrophysical studies have explained the orbital dynamics of particles that acquire a high electrostatic charge. In low Earth orbit, the charge collected by a microscopic particle or an ultra-small, low-mass satellite interacts with the geomagnetic field to induce the Lorentz force which, in the ideal case, may be exploited as a form of propellantless propulsion. Efficient mechanisms for negative and positive electrostatic charging of a so-called attosatellite are proposed considering material, geometry, and emission interactions with the ionosphere’s neutral plasma with characteristic Debye length. A novel model-based plasma physics study was undertaken to optimize the positive charge mechanism quantified by the system charge-to-mass ratio. In the context of the practical system design considered, a positive charge-to-mass ratio on the order of 1.9x10-9 C/kg is possible with maximum spacecraft potential equal to the sum of the kinetic energy of electrons from active field emission (+43V) and less than +5V from passive elements. The maximum positive potential is less than what is possible with negative electrostatic charging due to differences in thermal velocity and number density of electronic and ionic species. These insights are the foundation of a practical system design.

3D vacuum ARC breakdown simulation: Many challenges and some solutions

Summary form only given. We present our current capabilities and plans targeting the simulation of 3D vacuum arc discharge in realistic geometries. Vacuum arc discharge is an incredibly challenging problem due to the enormous dynamic changes in plasma growth, collisional processes, and time scales. Our simulation model targets a co-planar Cu-Cu vacuum breakdown experiment. We will estimate the computational requirements for this physically relevant breakdown system assuming a fully kinetic description. A fully kinetic description is required to accurately capture the initial breakdown. Progress on unstructured mesh collisional PIC methodology, dynamic particle weighting, managing multiple temporal and spatial scales, electrode models, and efficient parallel scaling will be addressed.