DAMPING RESONANT CURRENT IN A SPARK-GAP TRIGGER CIRCUIT TO REDUCE NOISE

Radio frequency (RF) interference (noise) caused by the triggering of spark-gap switched pulsed-power circuits is a significant problem for electronic diagnostics. Traditional approaches to mitigation involve using RF-tight enclosures, noise resistant diagnostic designs, power filtering, and/or time-integration of signals. These approaches can be costly, and not always successful. A detailed study of the source of this noise is undertaken motivated by an experiment using a charged coaxial cable triggered rail-gap (multichannel linear spark-gap) switched system based on a quarter-module of AFRL's Shiva Star Capacitor Bank. For this, the noise interferes with an unintegrated measurement needed of the load's local electric field. The noise source is identified as resonant current oscillation after rail-gap closure in the circuit triggering the rail-gaps. The solution in this case is to replace the 50 Omega trigger cable with one with half that impedance, and install a carbon composite resistor in series with the cable output with a resistance equal to the new cable's impedance. The output impedance of the trigger circuit and, therefore, the rail-gap current behavior during breakdown is thereby preserved, but reflections back into the cable after switch closure are minimized by impedance matched resistance termination. In practice, termination is compromised by lead inductance and blocking capacitors, but near-critical damping of subsequent resonant behavior is nonetheless observed. A circuit model of this behavior is validated to help adapt and optimize the technique for other systems.