Electron dynamics in high-power extended planar-anode diodes terminating MITL's on HERMES III

Electron flow in extended planar-anode (EPA) diodes that terminate the coaxial MITL of the HERMES Ill pulsed electron accelerator is characterized as a function of time and peak diode voltage (Vp) over the range 13 to 19MV, using both measurements and a numerical model. This type of diode operates with an axial AK gap many times larger than the radial AK gap. The diode is used for generating bremsstrahlung for the study of nuclear radiation effects and for injection into electron-beam transport systems. Because the accelerator is pulsed, with the voltage across the diode ramping up to and down from Vp in a time on the order of 50 ns, the coupling of the driving electromagnetic pulse into electron flow within the diode shows a strong dependence on time. This paper provides insight into this coupling, as well as a description of how the parameters of the driving pulse can be measured. This information is important for the modeling of downstream radiation fields and for designing couplings to complex transport systems using the EPA diode as an injector. Here the authors show that the EPA diode provides an impedance match to the MITL, that parapotential flow theory and the range of H ions generated at the cathode can be used to measure the driving time-dependent voltage pulse and peak voltage (Vp), respectively, that ∼ 95% of the electrical energy transmitted to the diode can be converted to kinetic-energy of electrons incident on bremsstrahlung targets, and that the forward radiation fluence can be described by a simple power of Vp over the measured Vp range