Investigation of plasma evolution in a high power electron-beam diode using a two-frame laser shearing interferometer

Abstract

Summary form only given. A two-frame laser shearing interferometer (LSI) was used to study the plasma evolution in a 140-kV, 220-kA electron-beam diode with a current pulse rise time of 40 ns. LSI used a 150-ps, 100-mJ, 532-nm, 10-cm-diameter, linearly-polarized laser beam. The p- and s-polarized components of the laser were split and re-combined with the s-component delayed by a 7.5-ns optical path length. The LSI imaged a ~0.5 cm by ~10 cm diode region using two cylindrical and two spherical lenses, so that the diode diameter was de-magnified 1:3 and the diode gap was magnified 2:1. The overall spatial resolution of the LSI images across the anode-cathode (AK) gap was about 10 mum. The cathode consisted of six 2.5-cm hexagonal segmented rods evenly spaced on a 10-cm diameter circle. The diode electrodes were assembled using a systematic procedure to assure the reproducibility of the 2.54plusmn0.05 mm AK gap spacing. The LSI images showed that dense plasmas were generated from both anode and cathode surface when the current reached its peak. Expansion of the electrode plasmas into the AK gap (gap closure) resulted in a collapse of the diode impedance. From the two-frame LSI images (7.5 ns apart) the typical plasma expansion velocity is measured to be ap2 cm/mus. Two kinds of plasmas are observed during the current pulse: an arc discharge plasma and a dense jet plasma. The arc plasma typically initiates over relatively large areas on both cathode and anode surfaces. When the arc plasma is present, the anode plate typically shows significant melting at the diode location. However, the 1-mm-diameter dense jet plasma (>10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">18</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> ) is produced only from the cathode and is not reproducible. There is no melting pattern at the corresponding surface of the anode plate and much less discharge erosion on the cathode, indicating less current and/or plasma at the jet location. Plasma formation from different anode materials such as aluminum, stainless steel, tantalum, and thin electroplated gold layer were also studied. We found that the Al and Ta anodes produced the least and most plasma respectively. The ap1 mum gold layer was not effective in reducing the electrode plasma production.