Design and performance of the pre-fire detection system for the FRX-L main capacitor

The main capacitor bank of the FRX-L (field-reversed configuration experiment) supplies current (1.6 Meg amp) to a single-turn coil that is used to create high-density plasmas for magnetized-target fusion (MTF) experiments. The energy stored in the main bank (200 kJ) is transferred to the load coil via an array of four Maxwell rail-gap switches in parallel. Each switch is capable of transferring approximately a quarter of the main bank charge flow without damage. Therefore, if a single switch pre-fires it is subject to much greater stress than it is able to tolerate. In this paper, we discuss the design and performance of a system that is able to discriminate between a switch pre-fire, trigger noise, and normal operation and initiate a trigger in time to safely prevent damage. This system consists of innovative electric field detectors, fiber-optic isolation of signals and high-speed solid-state circuitry that interfaces with the Maxwell trigger system without modification.