A collinear second-harmonic orthogonally polarized interferometer for MITL gap plasma measurements on Sandia’s Mykonos accelerator

TW-class accelerators suffer current loss within their inner Magnetically Insulated Transmission Line (MITL) regions. Fully relativistic, Monte Carlo Particle-In-Cell (PIC) and MagnetoHydroDynamic (MHD) simulations highlight electrode plasmas as the primary source of shunted current loss across the Anode-Cathode (A-K) gap [1]. A collinear Second-Harmonic Orthogonally Polarized (SHOP) interferometer has been developed which can detect free electron areal densities as low as and up to [2]. This sensitivity allows for time-resolved detection corresponding to plasma sourced from a single monolayer of desorbed contaminants, despite probing in the harsh pulse power environment. Utilization of a 1550 nm Fundamental-Harmonic (FH) and 775 nm Second-Harmonic (SH) probing beams permitted spatial focusing through the small A-K gap dimensions of ~1 mm width. An added benefit of using common communications wavelengths was the ability to fiber-couple the diagnostic. Experiments were performed on Sandia’s Mykonos accelerator [3], via several 650 kA parallel plate geometries that generated mm scale plasma lengths. Described in this presentation are the first time-resolved free electron areal density measurements inside a region comparable to the Sandia’s Z machine inner MITL [4]. By varying the plate’s electric fields and current densities, as well as sampling along differing optical paths, the collinear SHOP interferometer has provided valuable insight to pulsed power desorption physics and current loss mechanisms.

[1] N. Bennett, et al., Physical Review Accelerators and Beams, 26 (2023) 040401.

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

[3] M.G. Mazarakis, et al., IEEE transactions on plasma science, 38 (2010) 704-713.

[4] D.B. Sinars, et al., Physics of Plasmas, 27 (2020)