Control of discharge in high voltage fluid insulation

An instrument to produce ionizing radiation includes a high voltage source of charge and ionizing radiation; a housing filled with insulating gas and containing the high voltage source; an insulator to which the high voltage source is mounted so that the source is spaced from the housing; one or more collector electrodes arranged in the housing such that the high voltage source preferentially discharges to the collector electrode(s); a control system which determines a level of ionization of the insulating gas through the amount of discharge to the collector electrode(s); and/or discharge rate limiting means controllable by the control system to deionize the insulating fluid at a controlled discharge rate and thereby maintain the maximum rate of discharge below a predetermined current. In this way, breakdown events can be inhibited.