Experiments with cylindrical reflex triodes connected in series

Reflex triodes with cylindrical converters made from thin tantalum were developed for producing x-rays at high fluence in the warm (10-100 keV) region of the bremsstrahlung spectrum.¹ This paper presents results from the first experiment with two cylindrical reflex triodes connected in series on the NRL Gamble II pulsed power generator. The tantalum converters are concentric, with 10 and 20 cm radii, and have 4 and 6 mm radial anode-cathode gaps. The full generator current, about 1.1 MA, passes through each triode, effectively using the current twice to produce x-rays. The impedance of each triode is made equal to about 0.25 Ohms to limit the peak voltage on each triode to about 300 kV. Vacuum voltmeters record the voltage at each triode. The dose distribution is measured outside the 3-mm thick aluminum vacuum barrier using an array of CaF₂ thermoluminescent dosimeters. The dose distribution and dose-rate compare well with a model derived from ITS² calculations that assume perfect reflexing through the tantalum. On Gamble II, this approach increases the fluence-area product by about a factor of three, compared with a single triode at the same voltage. The series configuration is useful for increasing the irradiated area for generators with matched-load voltage greater than 0.6 MV.