Recently developed interferometer allows for low density plasma measurements in MITLs

Note: This article is a reprint from: Hines, N. R. (2025, February 10). Recently developed interferometer allows for low density plasma measurements in MITLs. Sandia National Laboratories. https://mykonos.sandia.gov/2025/02/10/shop-di-diagnostic/

Understanding the physics leading to current loss in Magnetically Insulated Transmission Lines (MITLs) is crucial for improving the efficiency of present and next generation TW-class pulsed power accelerators. Plasmas with areal densities ranging from 10¹⁴ to 10¹⁸ cm⁻², originating from the MITL’s stainless-steel surface desorption of neutral particles, can lead to significant current reduction at the machine’s intended load hardware [1]. Gaining insights into how these plasmas form and transport within the MITL may help reduce the presently experienced current loss and support better predictions of current loss in next-generation facilities.

To address this challenge, we are thrilled to commission the Second-Harmonic Orthogonally Polarized Dispersion Interferometer (SHOP-DI) diagnostic [2], [3]. This cutting-edge system leverages a 1550 nm fiber laser, partially frequency doubled to 775 nm, to achieve highly precise, temporally resolved measurements of refractive index changes. The SHOP-DI is specifically designed to analyze the sub-millimeter length plasmas that are expected within MITLs, providing critical data to understand current loss and optimize current delivery.

The SHOP-DI system has been successfully deployed on the University of New Mexico’s HelCat Plasma Source [4], as well as on the newly developed Parallel Plate Platform at Sandia’s 1 MA Mykonos accelerator facility [5], [6]. This diagnostic is used alongside other advanced tools, such as Avalanche PhotoDiodes (APDs), Streaked Visible Spectroscopy (SVS), fast-framing self-emission imaging, fiber coupled Photonic Doppler Velocimetry (PDV), Two-Color Triature Interferometry (TCTI), Miniature X-ray Diodes (miniXRDs), and B-dot probes. Collectively, these instruments mark a significant advancement in Sandia’s diagnostic capabilities, providing deeper insights into low density plasma formation and transport, and facilitate support for the ongoing research in the field of power flow physics.

References

[1] N. Bennett et al., “Electrode plasma formation and melt in Z-pinch accelerators,” Physical Review Accelerators and Beams, vol. 26, no. 4, p. 040401, April 2023, doi: https://doi.org/10.1103/PhysRevAccelBeams.26.040401.
[2] N. R. Hines et al., “A fiber-coupled dispersion interferometer for density measurements of pulsed power transmission line electron sheaths on Sandia’s Z machine,” Review of Scientific Instruments, vol. 93, no. 11, p. 113505, November 2022, doi: https://doi.org/10.1063/5.0101687.
[3] N. R. Hines et al., “Development of a colinear Second-Harmonic Orthogonal Polarization (SHOP) interferometer for electron areal density measurements in Magnetically Insulated Transmission Lines (MITLs),” Technical Report SAND2023-10581, 2023, doi: https://doi.org/10.2172/2430189.
[4] A. G. Lynn et al., “The HelCat dual-source plasma device,” Review of Scientific Instruments, vol. 80, no. 10, October 2009, doi: https://doi.org/10.1063/1.3233938.
[5] D. Lamppa, S. Simpson, B. Hutsel, M. Cuneo, G. Laity, and D. Rose, “Assessment of Electrode Contamination Mitigation at 0.5 MA Scale,” Technical Report SAND2021-12691, 2021, doi: https://doi.org/10.2172/1825219.
[6] J. Schwarz et al., “Mykonos: A pulsed power driver for science and innovation,” High Energy Density Physics, vol. 53, p. 101144, December 2024, doi: https://doi.org/10.1016/j.hedp.2024.101144.
Posted in: Advanced Diagnostic Development, Dispersion Interferometer