New reflex triode configuration for improved moderate-energy x-ray production

Summary form only given. A reflex triode consists of two grounded cathodes on either side of a positive-high-voltage, tantalum anode. Emitted electrons interact with the tantalum to produce bremsstrahlung and line radiation. By making the tantalum much thinner than the electron range, the self absorption of the x-rays by the tantalum can be reduced. The reflex triode configuration makes the electrons pass through the tantalum many times, improving the coupling of the electrons to the tantalum. Recent experiments on the Saturn generator at Sandia National Laboratories, Albuquerque, NM, used a triode configuration with cylindrical cathodes on either side of a flat, annular anode. For typical beam parameters (250 kV, 1.2 MA per side) and triode dimensions (19 cm radius cathodes, 3 mm anode-cathode gaps) the beam does not pinch rapidly toward the axis. As a result, the x-ray source is partially shadowed by the cathode located between the tantalum and the observation plane outside the vacuum chamber. The external x-ray signal starts about 15 ns later than expected from the current and voltage waveforms. At the time of maximum dose rate, it appears that only 60% or less of the total current produces x-rays from the part of the anode inside the inner diameter of the cathode. This beam behavior also occurs in particle-in-cell simulations of the Saturn experiments. A new triode configuration has promise for eliminating the delay and shadowing of the x-rays by the cathodes. The tantalum anode is in the form of a short cylinder, extending through the gap between two flat annular cathodes that lie in the same plane, one cathode inside the anode cylinder and one outside. This triode was tested on the Gamble II generator at the Naval Research Laboratory, Washington, DC, at the same voltage and anode-cathode gap as the Saturn triode, but at half the current and half the radius. The magnetic and electric fields should therefore be the same and scale directly to Saturn. The initial- results indicate the x-ray delay is eliminated and the measured dose matches the expected value based on measured current and voltage waveforms. The near field dose distribution is also more uniform and higher than the corresponding distribution for the triode with a flat anode. These results will be presented along with future possibilities to utilize this new triode in series or parallel configurations to increase the x-ray output from Saturn by using more of its current at the same voltage and outer diameter.