Construction and Evaluation of a Fast Switching Trigger Circuit for a Cosmic Ray Detection Spark Chamber

A pulsing circuit is designed and constructed capable of raising up to +6.7kV between plates of a small (~50cm side length) spark chamber. The device is triggered by the ~2V output of a cosmic ray detection device, and its overall gain of ~1:3000 is achieved by a sequence of amplifiers and fast switching devices – namely a Bipolar Junction Transistor (BJT), Insulated Gate Bipolar Transistor (IGBT), 1:53 gain pulsed transformer and a triggered spark gap. The qualitative response of each component is assessed with respect to its response and relaxation time. The overall delay time between electronic input and this 6.7kV pulse is found to be as short as 580±100ns. The response of the electronics is investigated under a range of square input pulse amplitudes and durations, simulating input pulses expected from the cosmic ray detector. A pulse shape of 200ns and depth of 2V or more is found to minimise rise times within limits set by adjacent components. The majority (over 60%) of the overall rise time is identified with the LC charging of the step-up transformer. The 55.7±2ms RC recharge time of the capacitor driving the transformer input limits the system’s maximum repetition rate to ~10Hz. It is found that the assembled system is compatible with operation of a small spark chamber, and recommendations are made regarding further fine tuning and potential future challenges.