MITL Electrode Surface Plasma Density Measurements*

TW-class pulsed power accelerators like Sandia National Laboratories’ Z machine experience current loss within their inner Magnetically Insulated Transmission Line (MITL) and convolute regions that results from the formation and transport of 10^15 – 10^17 cm^-3 electrode plasmas [1]. However, little experimental data has been collected to characterize these plasmas. Verification experiments presented herein have been conducted on Sandia National Laboratories’ 1 MA Mykonos accelerator [2] using a diagnostically accessible parallel plate transmission line [3] which generates similar electrode current densities and field strengths to that of the Z machine’s inner MITL region [1]. A colinear Second-Harmonic Orthogonally Polarized dispersion interferometer (SHOPi) diagnostic with a sensitivity of ∼ 10^14 cm^−2 [4] temporally resolved the electrode plasma’s refractive index at a variety of probing locations across the transmission line Anode-Cathode (A-K) gap. The results provide constraining data to validate large-scale particle-in-cell simulations of transmission line power flow, plasma formation, subsequent current loss, and the scaling of current delivery.

[1] N. Bennett et al., Physical Review Accelerators and Beams 24, 060401 (2021); ibid. 26, 040401 (2023); K. Tummel et al., Phys. Plasmas, 29, 113102 (2022).

[2] J. Schwarz et al., Journal of High Energy Density Physics, in review (2024).

[3] D. Lamppa et al., IEEE Pulsed Power Conference, Invited Talk, (2023).

[4] N. R. Hines et al., Review of Scientific Instruments 93, 113505 (2022).