Cathode Plasma Formation in Magnetically-Insulated Transmission Lines

We have studied cathode plasma formation in magnetically insulated transmission lines (MITLs) using time-resolved laser holography, visible light photography, and emission spectroscopy. Bare and graphitecoated aluminum cathode surfaces were tested. Results indicate that the cathode plasma density at 40 to 70 ns is typically peaked at the cathode at 3 to 7x1015. cm-3 and drops by an order of magnitude in 0.03 to 0.05 cm. A luminous plasma extends much farther into the anodecathode gap, expanding to 0.3 cm in 40 ns. Spectroscopic scans from 300 to 700 nm show that the Ha (656.3 nm) and Hß (486.1 nm) lines are the major contributors to the cathode plasma luminosity. A onedimesional simulation of cathode plasma expansion is consistent with the holographic density measurements, but indicates that some process other than direct cathode plasma expansion is responsible for the low density luminous plasma extending several millimeters into the gap.