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Boltzmann-Electron Model in Aleph

Hughes, Thomas P.; Hooper, Russell H.

We apply the Boltzmann-electron model in the electrostatic, particle-in-cell, finite- element code Aleph to a plasma sheath. By assuming a Boltzmann energy distribution for the electrons, the model eliminates the need to resolve the electron plasma fre- quency, and avoids the numerical "grid instability" that can cause unphysical heating of electrons. This allows much larger timesteps to be used than with kinetic electrons. Ions are treated with the standard PIC algorithm. The Boltzmann-electron model re- quires solution of a nonlinear Poisson equation, for which we use an iterative Newton solver (NOX) from the Trilinos Project. Results for the spatial variation of density and voltage in the plasma sheath agree well with an analytic model

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TYPE SAND Report YEAR 2014
OSTIDOI

Sheath-current retrapping in the Z MITLs

Digest of Technical Papers-IEEE International Pulsed Power Conference

Hughes, Thomas P.; Clark, Robert E.; Oliver, Bryan V.; Pointon, Timothy D.; Stygar, William A.

An important issue in designing a higher-power version of the Z machine at Sandia National Laboratories is electron current loss in the vacuum section, which consists of four radial transmission lines and a convolute (current-adder). There is evidence from experiments on Z that 1-2MA of current out of about 20MA is lost in the vacuum section before reaching the wire-array load [1]. Calculations using the LSP [2] and QUICKSILVER [3] particle-in-cell codes have shown much less current loss [4,5,6]. The current loss in the calculations is due to sheath-current loss in the region of the convolute, and is associated with the magnetic nulls which are intrinsic to the current splitting in the convolute Detailed 2-D calculations for the radial MITLs show that, in the region between the insulator stack and a radius of about 20cm (over which the radial-line vacuum impedance increases slowly from 2Ω to 3Ω), excess electron sheath current is mostly retrapped to the cathode electrode. The electron sheath current is given approximately by Mendel's force-balance expression [7] applied locally, and as a result, the sheath current decreases as Zv-2, where Zv is the vacuum impedance. Between a radius of 20cm and the convolute, where the radial-line vacuum impedance increases more sharply (to 6Ω at 10cm) there is significant "launching" of sheath current. The sheath behavior in this region is qualitatively similar to that predicted using a "constant flow impedance" model, but in the simulations the sheath is unstable and breaks up into vortices.

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TYPE Conference YEAR 2003
ScopusOSTI
23 Results
23 Results