Abstract
Hybrid-implicit particle-in-cell (PIC) algorithms permit the simulation of complex problems involving both kinetic and fluid plasma regimes over large spatial and temporal scales. Fluid electrons can be computationally fast where and when fluid assumptions are valid. Additional flexibility is obtained if discrete PIC macroparticles, with velocities advanced by either fluid or kinetic equations, are permitted to dynamically migrate between the two descriptions based on phase space criteria. Ideally, these migrations result in energetic particles treated kinetically and dense thermal plasma particles as a fluid. With an energy-conserving particle advance, resolution of the plasma Debye length is not required for numerical accuracy or stability. For pulsed-power applications, the simulation time step is usually constrained by the electron cyclotron frequency, not the more restrictive plasma frequency. A new implicit technique permits accurate particle orbits even at highly underresolved cyclotron frequencies. Thus, greater temporal and spatial scales can be accurately modeled relative to conventional PIC techniques. In this paper, we describe the hybrid PIC technique and fully electromagnetic, hybrid simulations of plasma evolution and current shunting in an idealized accelerator designed for driving a $Z$-pinch load. The dynamics of electrode heating, electron transport, and surface contaminant evolution are studied in a series of relativistic hybrid-implicit PIC simulations. These dynamics can lead to the shunting of current before reaching the $Z$-pinch load, thus degrading load performance. Examining two previously published power flow problems, we compare results from fully kinetic, multifluid, and hybrid kinetic-fluid simulations and discuss the computational performance of these three options. The key thrust of the work is to identify possible computational acceleration, through hybrid methods, required for accelerator understanding and design.
Highlights
The particle-in-cell (PIC) method [1] is the workhorse of lower-density charged particle simulation for pulsed-power systems
For relativistic particle dynamics in multimegavolt accelerators, the standard momentum-conserving, explicit algorithm adequately describes particle transport, including electron sheath flows in magnetically insulated transmission lines (MITLs), when the plasma and cyclotron frequencies and the plasma Debye length are resolved
It can be advantageous to include both descriptions and/or allow macroparticles to change their governing equations dynamically such that the computationally faster but valid description is used. We refer to this technique in which the original equations governing kinetic and fluid particles are retained as “particle migration hybrid” (PMH)
Summary
The particle-in-cell (PIC) method [1] is the workhorse of lower-density charged particle simulation for pulsed-power systems. Ohmic heating is the primary reason, but particle impact and radiation deposition contribute This heating is important, because it introduces additional physics that must be modeled including rapid desorption of surface contaminants. The resulting electrode surface plasma is significantly denser than the electron flow and has been shown to impact accelerator performance [3,4] Simulating these thermal plasmas is important but beyond the capability of traditional PIC techniques. It can be advantageous to include both descriptions and/or allow macroparticles to change their governing equations dynamically such that the computationally faster but valid description is used We refer to this technique in which the original equations governing kinetic and fluid particles are retained as “particle migration hybrid” (PMH).
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