Speaker
Description
In the vacuum transmission lines of terawatt pulsed-power accelerators such as Sandia National Laboratories’ Z machine, high electric fields (MV/m) result in field emission of electrons from cathodes. Left unchecked, these electrons are lost to and heat anode surfaces, leading to the emission of positive ions, and the generation of expanding electrode plasmas ($10^{16}-10^{18}$ cm${}^{-3}$). Altogether, mega-amperes of current can be lost from a nominal $26$ MA current pulse. High self-magnetic fields ($1-100$ T) have been leveraged successfully for decades to mitigate these issues in pulsed-power accelerators by dynamically insulating anodes from emitted electrons; however, these accelerators have only employed constant-geometric-impedance transmission lines. Recent studies [1] have suggested there could be significant advantages using magnetically-insulated transmission lines (MITLs) with a variable geometric impedance.
In this talk, we present simulation-based studies assessing the viability of using variable-geometric-impedance transmission lines as an enabling technology for future pulsed-power accelerator designs; this includes characterizing power flow and magnetic insulation properties. Particle-in-cell simulations were performed using EMPIRE: a performance portable, massively parallel, three-dimensional electromagnetic plasma simulation code developed at Sandia National Laboratories. We showcase results from small-scale simulations of surrogate geometries under Z-relevant conditions and report on differences revealed therein compared to conventional (constant impedance) MITL theory. Finally, we present results from at-scale simulations of the Z accelerator.
[1] R. B. Spielman, D. B. Reisman; On the design of magnetically insulated transmission lines for z-pinch loads. Matter Radiat. Extremes 1 March 2019; 4 (2): 027402. https://doi.org/10.1063/1.5089765
*SNL is managed and operated by NTESS under DOE NNSA contract DE-NA0003525
