Measurements of dark current and breakdown phenomena using Faraday cups at the Xband Laboratory for Accelerators and Beams (XLAB)

Two CLIC TD24 accelerating structures, manufactured by CERN, are being tested on the high gradient 12 GHz RF test stand at XLAB. Installed at the end of 2024, these are the first devices to be tested at XLAB. Testing aims to verify that following conditioning they can be reliably operated at accelerating gradients of 100 MV/m. The conditioning process involves slowly increasing the peak RF power and pulse length input to the structure. The peak accelerating gradient at which a structure can operate reliably is limited by electrical breakdown. As conditioning progresses the likelihood a that a breakdown will occur decreases. When a breakdown occurs significant charge is emitted by the structure. Considerable progress has been made in the development of the theory of breakdown formation and the underlying interaction mechanisms. Accurate measurements of breakdown phenomena are required to test these understandings. Faraday cups installed upstream and downstream of the structures and connected to high resolution digitisers are employed to measure the behaviour of the breakdown current emissions and dark current. The results of these measurements are presented here.