Effects of rf breakdown on the beam in the Compact Linear Collider prototype accelerator structure

Understanding the effects of rf breakdown in high-gradient accelerator structures on the acceleratedbeam is an extremely relevant aspect in the development of the Compact Linear Collider (CLIC) andis one of the main issues addressed at the Two-beam Test Stand at the CLIC Test Facility 3 at CERN.During a rf breakdown high currents are generated causing parasitic magnetic fields that interact withthe accelerated beam affecting its orbit. The beam energy is also affected because the power is partlyreflected and partly absorbed thus reducing the available energy to accelerate the beam. We discusshere measurements of such effects observed on an electron beam accelerated in a CLIC prototypestructure. Measurements of the trajectory of bunch trains on a nanosecond time scale showed fastchanges in correspondence of breakdown that we compare with measurements of the relative beamspots on a scintillating screen. We identify different breakdown scenarios for which we offer anexplanation based also on measurements of the power at the input and output ports of the acceleratorstructure. Finally we present the distribution of the magnitude of the observed changes in the beamposition and we discuss its correlation with rf power and breakdown location in the acceleratorstructure