Design and development of triggering system for synchronized multi-channel operation of parallel connected Railgap switches

Railgap switches are connected to transfer energy from high energy capacitor bank to enable large coulomb transfer in distributed monde in 1.2MJ capacitor bank `RUDRA' at Energetics & Electromagnetics Division, BARC, Visakhapatnam. The total energy stored in the 1.2MJ bank is segmented in to six modules (of 200kJ each) capable of delivering 3.6MA of peak current with 5 to 7\is rise time collectively. `RUDRA' is high current test bed facility for pinch experiments like Magnetized Target Fusion (MTF) and Dense Plasma Focus (DPF) which require fast rising, high peak power and high-energy pulses for their operation. The performance of railgap switch critically relies upon multi-channel breakdown between the extended electrodes (rails) in order to ensure distributed current transfer along electrode length and to minimize the switch inductance. The initiation of several simultaneous arc channels along the electrode length in Railgap switch depends on the gap triggering technique and on the rate at which the electric field changes within the gap. In the existing Railgap switches, the consequently imposed stringent requirement on the trigger pulse is that it must have a fast rate of rise >5kV/ns and high peak voltage, largely exceeding the main gap voltage (i.e. typically in the range of 10kV-40kV). Six channels Transmission Line Transformer (TLT) based driver with input and output impedance of ~1.25 Ω and 20 Ω respectively is designed to meet aforesaid criteria of trigger pulse characteristics for demonstrating synchronized discharge of all six modules within jitter limitation of <; 5ns. The uniquely defined 20Ω output impedance of the TLT based trigger generator facilitates transport of trigger pulse to switch without any distortion in its temporal characteristics. The primary capacitance of ~51nF has been used to minimize the droop while the transmission lines are being charged. At 20 kV of primary charging, the TLT produces ~110kV output pulse of 60ns duration (FWHM) with rise time of better than 15ns (10%-90%). This corresponds to voltage gain efficiency of >70% and dV/dt of ~7kV/ns. Six channels TLT based driver is capable of efficiently driving synchronized discharge in all the six parallel connected Railgap switches within the time limit of <;15ns (i.e. transit time isolation within the parallel connected six modules) to avoid cross-flow of energy in between the parallel connected capacitors/modules.