Extension of the operating point of the Mercury IVA from 6 to 8 MV

The output voltage of the NRL Mercury generator has been successfully increased to 8 MV. Mercury was originally designed for 6 MV, 300 kA operation in negative polarity [1]. However, new experiments required a bremsstrahlung x-ray source with a higher endpoint voltage [2],[3]. The threshold energy for photofission in targets of interest is about 5.5 MeV and, given the roughly Gaussian shape of the Mercury output pulse, very few x rays above threshold could be generated by Mercury with a 6-MV peak output voltage. Also, the cross section for fission rises sharply above the threshold energy. It was therefore decided to increase the output voltage of Mercury to the maximum possible without imposing undo cost or risk. A new center conductor was designed for Mercury that increased the effective output impedance of the MITL from about 23 to 40 Ω, thereby increasing the maximum output voltage to 8 MV. The new center conductor has carefully designed tapers (steps down in diameter) at each stage of the adder so that the adder cell voltages are equally balanced at the design limit of 1.3 MV. One limiting factor is the breakdown probability of the insulator stack in the adder cells. However, simulations have shown that the new center conductor only increases the breakdown probability a small amount. The 40 Ω output impedance was selected because it allowed us to reuse several pieces of the existing center conductor. Aluminum was used for most of the new parts. These steps allowed us to save time and keep material costs to a minimum. PIC and circuit simulations were performed to validate the design. Mercury has now been successfully operated for over 200 shots at 8 MV.