Voltage dependence of electron dynamics in high-power extended planar-anode diodes

Measurements confirm the ability of a numerical model to simulate basic features of electron flow and subsequent radiation output from EPA (extended planar-anode) diodes on HERMES III. HERMES III is a 19 MV, 700 kA, 25 ns electron accelerator. These diodes simultaneously terminate a coaxial MITL (magnetically insulated transmission line), and produce intense bursts of bremsstrahlung. The measurements and modeling show that (a) the EPA diode provides an impedance match to the MITL, (b) parapotential flow theory and the range of H− ions generated at the cathode can be used to measure the time-dependent voltage pulse and peak voltage VP, respectively, and (c) that ∼95% of the electrical energy transmitted to the diode can be converted to kinetic energy of electrons incident on bremsstrahlung targets. Further, it is demonstrated that the forward radiation fluence can be described by DA/Q=(76±11)Ṽ1.61 over the VP range 13–19 MV. DA, Q, and Ṽ refer to the forward radiation fluence measured in Mrad(CaF2) cm2, the incident charge measured in C, and the median current-weighted accelerating voltage measured in MV, respectively. The dependence on VP of the beam characteristics at the targets is shown and the detailed kinetic-energy distribution there is discussed.