Measurements of electron beam deflection and rf breakdown rate from a surface wave guided in metallic mm-wave accelerating structures

Abstract

Experiments with high gradient traveling-wave mm-wave metallic accelerating structures were performed to research the physics of vacuum rf breakdowns. The accelerating structures are open, composed of two identical halves separated by an adjustable gap. The electromagnetic fields were induced by an ultrarelativistic electron beam at the Facility for Advanced Accelerator Experimental Tests (FACET) at SLAC National Accelerator Laboratory. These accelerating structures have a fundamental beam-synchronous mode with a frequency that varies from 95 to 140 GHz depending on the gap width. When the gap is opened by more than half of a freespace wavelength, the corresponding synchronous mode remains trapped demonstrating behavior consistent with the so called "surface wave." This regime is of potential interest for femtosecond electron beam diagnostics and beam manipulation techniques. The behavior of the surface wave was characterized and the measured synchronous frequency, pulse length and beam deflection voltage are found to be in good agreement with simulations. The wave magnitude was changed by positioning the beam at different distances from the surface of a structure-half and the corresponding rf breakdown rate was recorded. The breakdown rate was 4×10-3 per pulse at a peak surface electric field of 0.3 GV/m and a pulse length of 60 ps.