Quick turnaround positive polarity options for mercury

Mercury, a 2-TW inductive voltage adder located at the Naval Research Laboratory in Washington, DC, had previously been converted from negative to positive polarity output by rotating each of the cells[1]. Positive polarity was needed to field an ion-beam diode.[2] However, rotating the cells takes about 2 to 3 weeks and is very labor intensive. So, when we next needed to operate in positive polarity, we considered two options to quickly turnaround Mercury to positive polarity; reverse charging the Marx and fielding a vacuum convolute. Charging the Marx with the opposite polarity is the simplest way to operate Mercury in positive polarity. However, because the breakdown and flashover strengths of the components are lowered when the polarity is reversed in this way, it is required to limit the Marx charge to 2/3 of its normal charging voltage to prevent failures. This limits the maximum output voltage, with the existing center conductor, to about 4.5 MV. An alternative approach we considered was to operate in negative polarity but to field a vacuum convolute inside our large diameter load chamber. Simulations and field calculations showed that this approach could work and the maximum output would be about 6 MV. We chose to operate Mercury in positive polarity by reverse charging the Marx and successfully operated an ion diode at a lower power level. To improve the pulse shape, we modified the self-breaking PFL water switch hardware. After this and a few other changes, Mercury can now be switched to a reliable and repeatable positive polarity mode in days instead of weeks, although at a lower than maximum power level. Details and results will be presented.