Experimental and Theoretical Investigation of Power Flow in a Transmission Line Modification to HERMES-III consisting of Tapers applied in Various Geometries to Inner and Outer MITL Conductors

HERMES-III (18 MV peak, 650 kA peak, 30 ns radiation pulse) delivers power down a constant-diameter MITL to an electron beam-converter combination to generate intense bremsstrahlung pulses. Electron dynamics in this configuration has been extensively characterized and found to be quite stable. We have recently investigated power flow where the MITL sections have been narrowed through various tapered geometries to a reduced-diameter cathode and converter, an option which can allow for more flexibility in machine output. An array of TLDs/P-I-N diodes in both near- and far-field plus an axial x-ray pinhole camera are the principal diagnostics fielded. Simulations with PIC codes (Empire and CHICAGO®) were undertaken for each taper geometry investigated.



Initial experiments featuring short (42-cm) simultaneous tapers for both inner (cathode) and outer (anode) conductors resulted in unstable beam current with pinching producing converter damage after a single shot. The inner taper was replaced by a much longer (4.6 m) taper. This produced a stable beam, but with current losses suggesting that losses are proportional to the length of the taper. The most successful configuration tested to date had dual 42-cm tapers like the first experiments, but with a 60 cm stagger between inner and outer conductors. The result was a) a stable beam, b) lowest current losses, and c) operation without damage to the converter. Experiments are planned to reduce the inner conductor size further, and latest results will be reported.