Plasma evolution and dynamics in high-power vacuum-transmission-line post-hole convolutes

Abstract

Vacuum-post-hole convolutes are used in pulsed high-power generators to join several magnetically insulated transmission lines (MITL) in parallel. Such convolutes add the output currents of the MITLs, and deliver the combined current to a single MITL that, in turn, delivers the current to a load. Magnetic insulation of electron flow, established upstream of the convolute region, is lost at the convolute due to symmetry breaking and the formation of magnetic nulls, resulting in some current losses. At very highpower operating levels and long pulse durations, the expansion of electrode plasmas into the MITL of such devices is considered likely. This work examines the evolution and dynamics of cathode plasmas in the double-post-hole convolutes used on the Z accelerator [R. B. Spielman et al., Phys. Plasmas 5, 2105]. Three-dimensional particle-in-cell (PIC) simulations that model the entire radial extent of the Z accelerator convolute -from the parallel-plate transmission-line power feeds to the z-pinch load regionare used to determine electron losses in the convolute. The results of the simulations demonstrate that significant current losses (1.5 MA out of a total system current of 18.5 MA), which are comparable to the losses observed experimentally, could be caused by the expansion of cathode plasmas in the convolute regions.

Computational analysis of current-loss mechanisms in a post-hole convolute driven by magnetically insulated transmission lines

“…[12,16,23]). Dynamic load models, for example wire-arrays, produce load impedances that vary in time, generating backward-traveling waves that modify the electron sheath properties, and therefore the loss currents.…”

“…The cathode plasma formation model [12] assumes the explosive emission of electrons and desorption of a neutral, thermal particle layer from a conducting surface at a prescribed rate. Plasma desorption is initiated on a cellby-cell basis once the local electric field exceeds the threshold set for SCL electron emission.…”

“…These simulations examine the scaling of current losses in the presence of different charged particle sources as described above. To further simplify the analysis, all simulations use a fixed load impedance, rather than a time-dependent load impedance model, such as those used to model specific shots [12]. This paper is organized as follows.…”

“…(For examples of numerical simulations of specific Z shots, see Refs. [12,[16][17][18][19][20].) The model presented here does enable the study of the complex interaction between magnetically insulated sheath currents generated upstream of the convolute with the post-hole convolute.…”

“…[4] is expanded in the present work to include (i) a time-dependent electricalpower-pulse, (ii) a self-consistent model for Ohmic heating of the anode surfaces, and (iii) a cathode-plasma-formation model [12]. Using this expanded simulation model, some of the geometric design variations explored in Ref.…”