Experimental studies of a CERN-CLIC 32.98 GHz high gradient accelerating structure driven by the MIT free electron laser amplifier

Experimental studies on a CERN-CLIC 32.98 GHz 26-cell High Gradient Accelerating (HGA) section were carried out with input powers from 0.1 MW to 50 MW. MIT´s Free Electron Laser (FEL) amplifier driven by a mildly relativistic electron beam (750 kV, 300 A, 30 ns) served as the RF power source for the structure. The maximum power in the transmitted pulse was measured to be 15 MW for an input pulse of ~35 MW. The theoretically calculated shunt impedance of 116 MΩ/m predicts a field gradient of 65 MeV/m inside the HGA. For power levels >3 MW the pulse transmitted through the HGA was observed to be shorter than the input pulse and pulse shortening became more serious with increasing power input. At the highest power levels the output pulse length (about 5 nsec) was about one quarter of the input pulse length. Various tests suggest that these undesirable effects occur in the input coupler to the HGA. Light and X-ray production inside the HGA have been observed. A feasibility study suggests that conditioning is a likely candidate for improving the performance of the HGA