Dependency of the capture of field emitted electron on the phase velocity of a high-frequency accelerating structure

Surface electric fields within high gradient linear accelerators can exceed 200 MV/m and lead to field emitted (FE) electrons entering the structure. When the accelerating field conditions permit, these FE electrons can become captured in the RF fields and be transported through the accelerating structure as a dark current. Understanding the capture and transport of these FE currents in high frequency linear accelerators, and at accelerating gradients well above the capture threshold, is important for the operation of CERN's X-band test stands and other high gradient linear accelerators. Such dark current leads to a background radiation, which dictates shielding requirements and can damage adjacent instrumentation, as well as a background current within the structure, which can affect beam diagnostics and in the most extreme cases can cause transverse kicks on bunches. The capture of field emitted electrons is described analytically in a one dimensional approximation and is then evaluated numerically for a test structure geometry. A particular focus for the analysis is how the interaction varies with phase velocity. We demonstrate how the phase velocity varies with respect to the nominal driver frequency and structure operational temperature. Measurements on the X-band test stands at CERN demonstrate that the capture increases 12%–28% for a 1 MHz increase in the driver frequency. A three dimensional RF and particle simulation found a similar order of magnitude result for a 1 MHz increase corrborating the measurements.