100 kV, 100 J, pulse generator for repetitive operation, giving 20 ns risetimes into 50 Ω loads

Poynting High Voltage Ltd. have manufactured a 6 kW average power, pulse generator required for fundamental research into electrical rock fragmentation. Rock samples are resistively inhomogeneous and for steep fronted waveforms, the current flow is along grain boundaries and not through the bulk material. The electrical energy deposited in the rock sample under these circumstances produces adiabatic expansion of the grain boundary and causes cleavage and subsequent rock fracture. The power supply design was required to discharge a 20 nF capacitor charged to 100 kV (100 J) into a resistive load (50-100 Ω) at repetition frequencies up to 50 Hz. The voltage rise time at the load had to be better than 20 ns, necessitating low inductance in the output circuit. The customer also insisted that the design did not use a pulse transformer in the output circuit. The output switch would probably be a spark gap, which could not be guaranteed to de-ionise if the capacitor discharge current was immediately followed by the charging current. The systems design had therefore to address two major problems: the charger circuit had to work with a high electrical efficiency and use charge command technology to ensure a sufficient delay time between the end of the previous pulse and the start ofthe next charge cycle to allow the spark gap to de-ionise; and the output circuit had to have reliable insulation levels fix sustained operation at 100 kV consistent with a low inductance design