Beam Momentum Changes due to Discharges in High-gradient Accelerator Structures

The key questions left unanswered by the Standard Model, and the recent discovery of a Standard Model-like Higgs boson, demand an extension of the research on particle physics to the TeV energy scale. The Compact Linear Collider, CLIC, is a candidate project to achieve such goal. It is a linear lepton collider based on a novel two-beam acceleration scheme capable of high-gradient acceleration in X-band accelerator structures. The high electric fields required, however, entail the occurrence of vacuum discharges, or rf breakdowns, a phenomenon whose microscopic dynamics is not yet completely understood, and whose impact on the beam can lead to a severe degradation of the collider luminosity. The understanding of the physics of rf breakdowns has therefore become a significant issue in the design of a reliable accelerator based on CLIC technology. That is addressed experimentally through the study of accelerator structures performance during high-power operations. We report on such a study carried out on a CLIC prototype structure assembled in a resonant ring at SLAC. We characterise the experimental set-up through a complex least square analysis, and we show how breakdowns can be localised in the structure on the basis of rf measurements. The same methodology lays the ground for the study of the impact of rf breakdowns on the beam. We addressed that issue at the Two-beam Test Stand, an experimental area built within the CLIC Test Facility CTF3 at CERN, where we tested a CLIC prototype accelerator structure in the presence of an electron beam. There, we found that rf breakdowns can affect both the longitudinal and the transverse beam momentum, causing a reduction of accelerating gradient and transverse kicks to the beam trajectory. In view of the CLIC design, we finally discuss what is the impact of such effects on the collider luminosity, which can be drastically reduced.