Initial anode optimization for a compact sealed tube vircator

During the development and optimization of a compact sealed tube virtual cathode oscillator (vircator) at Texas Tech University, it has become apparent that processes at the anode have a significant impact on tube performance. The impact of the high energy, high current density (100-200 A/cm² or higher) beam on the anode will cause outgassing, plasma production, and anode melting and material ejection. The emitted material expands, eventually impacting the anode transparency and (combined with the plasma formed at the cathode) shorting out the anode-cathode gap. This expansion limits the maximum radiated pulse width, and can also limit the peak output power. The residual evolved gas also negatively impacts the maximum repetition rate of the tube. An effort is underway to study the thermal behavior, gases evolved, and transparency versus time for different vircator anode materials and material treatments. Several different anode materials are under investigation, including stainless steel, copper tungsten, tantalum, nickel, and molybdenum. The effect of different treatments on the anodes before tube assembly is also being studied. The gases that are evolved during operation have been characterized utilizing pressure and residual gas analyzer measurements. The pre-shot background pressure in the tube is in the ultra-high vacuum range (10^-8 to 10^-9 Torr), and the vircator is not pumped on during firing. The data collected for the different materials is presented.