Performance of St707 getter material in a rep-rated high power microwave sealed-tube vircator under UHV conditions

This study focuses on the use of St707 non-evaporable getter (NEG) material in a high power microwave (HPM) sealed-tube virtual cathode oscillator (vircator) operated at repetition rates up to 200 Hz. High-current pulsed operation releases gases trapped within the bulk materials and gas monolayers on material surfaces, leading to localized plasma production between the A-K gap. This potentially leads to gap closure, shortens the duration of microwave production, and spoils low vacuum levels. A single current pulse may increase the chamber pressure to the low 10^-6 Torr range from an initial background pressure in the low 10^-9 Torr range, desorbing approximately 10¹⁴ particles. For single pulse operation, it is sufficient to maintain this background pressure (10^-9 Torr) with a small ion pump rated at 20 L/s. However, at 200 Hz operation, a pumping speed of 1,000 L/s is needed to evacuate 10¹⁴ particles from the vircator volume between pulses. Rather than adding a bulky pump system with the required high pump speed, St707 getter material (70% Zr, 24.6% V, 5.4% Fe) has been used to maintain ultra-high vacuum (UHV) conditions during rep-rate operation. Characteristics of the St707 NEG pump in a repetitive, high-current pulse environment are presented. Diagnostic results obtained with an inverted magnetron cold cathode gauge for absolute pressure are utilized to analyze NEG performance in detail.