This produces a ~20-30kV pulse at the output of the transformer. The transformer's secondary can be connected in series with something like a flashlamp or spark gap, to trigger it by overvoltage. It's based on the TIG welder HF start and the HV trigger/capacitor dumper.
Construction is pretty simple. Two doorknob caps are mounted in series, because I wasn't sure of the voltage rating and wanted to be on the safe side. Total capacitance is around 400pF. They're charged from a HV supply to around 20kV (not shown). When the voltage rises to a suitable level, a small spark gap (little white thing with screws sticking out the end) breaks down, dumping the capacitors through the primary of the output transformer.
The spark gap is just a piece of 8mm Delrin, drilled out and fitted with stainless M3 screws for the electrodes and small tubing nipples so an aquarium bubbler can blow the gap clean.
The transformer primary is about 4 turns of 4-filar wound hookup wire (1.2mm sheath dia.). The core is an ungapped TV flyback core. The secondary is 10 turns of heavy mains wire. The step-up ratio just gives a bit extra voltage on the secondary.