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.

I. Introduction

The Mini-G explosively driven two-stage flux compression generator system, shown in Fig. 1, is a nearly one-half scale version of LLNL's FFT system [1]. The first stage helical generator is about 100 cm in length and 26 cm in diameter with an initial inductance of about 10 uH. The armature is an annealed aluminum tube, 15 cm in diameter and 0.953 cm thick, filled with 16 kg of PBXN110 and initiated at one end by a single detonator and plane wave HE lens. The stator is comprised of four sections starting with two wires and bifurcating to 4, 8, and then 16 windings over the first 35 cm of its length. At the output end of the stator the windings attach to a conical section that couples to the input of the second stage coaxial generator. An insulator, shaped like a top hat, separates the inner and outer conductors of the two generators. This insulator is designed to hold off voltage of about 20 kV. The coaxial generator is about 100 cm in length and 33 cm in diameter, with an initial inductance of about 70 nH. This type of coaxial generator has the armature on the outside and the stator in the inside. The 25-cm diameter armature is surrounded by 31 kg of pressed and machined LX-14 explosive. On the outside of the LX-14 is a steel tamper with a ring of 51 detonators installed that simultaneously initiate the explosive. As the HE burns and expands the armature is squeezed inward at the waist, collapsing on the contoured stator in a controlled zippering action to compress the magnetic flux from the input end toward the vacuum power flow channel (VPFC). Both the helical generator and the coaxial generator are filled with one atmosphere of SF6 to better withstand the electrical breakdowns due to internal electric fields at the armature/stator contact point region, as well as at the input and output insulator triple point regions. Figure 1:

(Left) photo of a mini-g prior to shot with one of the authors, dave goerz. (right) cross-section drawing of mini-g with major components identified.