Investigation of Power Flow in a Tapered Transmission Line Modification to HERMES -III using an Array of Dose- and Dose-rate Detectors in the Near- and Far-field*

HERMES-III (18 MV peak, 650 kA peak, 40 ns radiation pulse) delivers power down constant-diameter MITLs 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. An array of P-I-N diodes in the far-field with accompanying TLDs mounted within tungsten collimators has been developed to estimate variations in electron angle(s) on the converter.



We have recently investigated power flow where the constant diameter MITLs have been narrowed through a short tapered section to a reduced-diameter cathode and converter. A set of near-field radially arrayed PIN diodes was placed perpendicular to the axis, behind a set of TLDs mounted outside the vacuum. In addition, an x-ray pinhole camera (PHC) was mounted on the machine axis, several meters away. In contrast to normal HERMES operation, the resulting beam as imaged by the PHC indicated a relatively small pinch ~ 3 cm in diameter. Waveforms from the radial PIN diode array clearly show a photon sweep across the radiation axis, peaking first further away from the axis, then moving towards the axis. From this we can infer an electron beam sweep. Simulations using the CHICAGO PIC code indicate that such a tight beam pinch can only be explained by the presence of ions produced by electron strikes to the anode. Investigations are continuing, and latest results will be presented.