Vacuum arc localization in CLIC prototype radio frequency accelerating structures

A future linear collider capable of reaching TeV collision energies should support accelerating gradients beyond 100 MV/m. At such high fields, the occurrence of vacuum arcs have to be mitigated through conditioning, during which an accelerating structure’s resilience against breakdowns is slowly increased through repeated radio frequency pulsing. Conditioning is very time and resource consuming, which is why developing more efficient procedures is desirable. At CERN, conditioning related research is conducted at the CLIC high-power X-band test stands. Breakdown localization is an important diagnostic tool of accelerating structure tests. Abnormal position distributions highlight issues in structure design, manufacturing or operation and may consequently help improve these processes. Additionally, positioning can provide insight into the physics of vacuum arcs. In this work, two established positioning methods based on the time-difference-ofarrival of radio frequency waves are extended. The first method is based on signal edge detection and the second on cross-correlation. The methods are parametrized and a bias model for the edge method is developed. The localization precision of the methods is also quantified. Under certain conditions, the correlation method is demonstrated to achieve a precision of less than one accelerating cell. The methods are applied to data collected from four CLIC prototype structures: three constant gradient accelerating structures, the T24, T24 open and TD26CC, and one constant impedance deflecting structure, the CLIC Crab Cavity. The TD26CC and Crab Cavity operated as expected, whereas the T24 and T24 open developed hot cells close to the RF input. The T24 open continued conditioning despite the hot cell. Furthermore, evidence of breakdown migration was found when comparing the two positioning methods. It was also discovered that consecutive breakdowns occurring close to each other in time also occur close to each other in space.