Estimating Plasma Sheath Inductance in the MJOLNIR Dense Plasma Focus

A dense plasma focus (DPF) is a coaxial plasma gun that completes its discharge as a z-pinch. The MJOLNIR (MegaJOuLe Neutron Imaging Radiography) DPF at LLNL is designed for flash neutron radiography. We are designing a suite of electromagnetic diagnostics to better understand current flow inside the DPF head. Our goal with these new diagnostics is to pinpoint the location and timing of parasitic current paths or restrikes that divert current from the pinch region that produces radiation. Based on correlations between current traces and neutron yields, we hypothesize that restrikes lower the neutron yield on shots. This presentation will discuss our method of voltage measurements across the DPF's transmission plate using pairs of differentially fielded Rogowski coils. The voltage measured by the Rogowski coils is used to ensure the breakdown voltage of insulators in the transmission plate is not exceeded. Voltage traces can also be correlated with neutron yields and timings as well as operating parameters like voltage and pressure. Furthermore, by combining this voltage measurement with a measurement of current flowing into the DPF head we can estimate the inductance of the plasma sheath as a function of time. Along with the plasma inductance as a function of time we have a series of photodiodes to track the plasma sheath position and estimate its rundown velocity. By combining these two diagnostics along with neutron yield measurements we aim to identify restrike locations and times inside the DPF head.