Development of the vacuum power flow channel for the Mini-G

The Mini-G explosive pulsed power system is a two-stage helical-coaxial FCG that is geometrically a half-scale version of LLNL's FFT device. The generator is capable of delivering 60 MA currents and 10 MJ of energy to suitable inductive loads. The Mini-G is presently used in high-energy-density physics experiments that require efficient current delivery through a vacuum power flow region to the load. As with the FFT device, the Mini-G system requires a compact, high-voltage gas-to-vacuum insulator and low-inductance vacuum power flow channel to achieve high performance and maximum energy delivery. In designing the Mini-G system, we followed the successful approach used in developing the FFT device. This included shaping the electrodes and insulators to manage electric field enhancements, applying coatings to cathode surfaces to suppress electron field emission, introducing baffles to the power flow channel to block UV, and applying coatings to electrode surfaces to absorb UV. This paper describes the design of the Mini-G vacuum interface and power flow region, and results of modeling and simulations that were done to evaluate and optimize performance. Appropriate codes were used to examine electric field enhancements, magnetic insulation, flashover inhibition and UV ray tracing in the channel. In this paper, we also present results of laboratory testing on HV vacuum insulator materials and shapes, UV induced insulator flashover, along with measurements of HV thresholds for electron emission. We also report on UV reflectance data for some of the coatings considered. To date, there have been eight experiments performed using the Mini-G system. For the first two tests, the power flow channel had an extremely low vacuum inductance of 0.9 nH. On the second Mini-G test it appeared that a partial shorting occurred in the power flow channel, limiting full energy delivery to the load. The design was modified to reduce electrical stress, improve UV attenuation, and incorporate additional diagnostics. This increased the inductance of the power flow channel to 1.5 nH. On the third Mini-G test the partial shorting reoccurred and the new diagnostics (inner Bdot probe) helped to identify the location at the vacuum insulator surface - about 10% of total current of 41 MA was diverted into the short. Further design modifications were incorporated to decrease electrical stress across the insulator and reduce UV illumination of the insulator surface. This increased the inductance of the power flow channel to 1.9 nH. On subsequent Mini-G experiments full current delivery to the load has been achieved with no occurrence of shorting.