Effect of in-situ pulse-heating of Ta anodes on the radiation from bremsstrahlung diodes

Summary form only given. Tantalum anodes in high-power electron-beam diodes are copious sources of ions because that Ta readily absorbs gas, even at elevated temperatures (maximum absorption at 1300 K). When an e-beam heats the Ta, these low-z impurities (hydrogen, carbon, etc.) are released and ionized thus creating a space charge near the anode and an ion beam that can become a substantial fraction of the diode current reducing the bremsstrahlung production efficiency. Surface impurities can be removed by moderate heating prior to a shot, but temperatures 2300 K or greater are required to remove gases entrained in the interior of the Ta. When 50 /spl mu/m-thick foils were resistively heated to these temperatures using a capacitor bank driver while serving as anodes on Gamble II (1.5 MV, 0.7 MA, 60 ns), ion currents were reduced and pinching was inhibited. To heat thicker Ta X-ray converters, the capacitor bank was replaced with a battery bank. Ta foils used are 16.5/spl times/17.8 and 102 /spl mu/m-thick. The foils are held taut between brass clamps which are thermally- and electrically-isolated from Gamble II during the heating time. The heating pulse lasted 0.3-1 seconds. The Ta foil temperature increased to almost 3000 K during the pulse with a current ranging from 2.5-4.0 kA. Electrical parameters, including the ion current history, the instantaneous pressure, as well as X-ray measurements were obtained on each shot.