Temporal characterization of the 19-MV, 700-kA HERMES III electron beam

The shot-to-shot variation of the HERMES III Accelerator parameters at the electron-beam diode, the time dependence of the electron flow in the diode, and the time dependence of the subsequent radiation produced from the bremsstrahlung target are measured and compared with model results. The measurements are made with current shunts, H^- range filters, a calorimeter, a fast-framing γ-ray camera, and Compton diodes. The measurements show that the peak voltage and energy delivered to the target are 19±0.2 (±2) MV and 327±6 (±60) kJ, respectively; the electron beam converges to a radius of 14.0±0.3 cm near peak power; and the FWHM of the radiation pulse varies from 14.5±0.9 ns at the radiation focus to 23.6±0.9 ns in the far field. (The errors not in parentheses correspond to the RMS shot-to-shot variation. Those in parentheses correspond to the estimated systematic uncertainty.) The measurements agree with the calculated shot-variability and the dynamics of the electron flow, and suggest that the model can be used to predict the response from similar multi-stage, coaxial, high-power accelerators. With over 1000 shots accumulated between the commissioning of HERMES III and the present set of measurements, no degradation of output has been observed.