Measurements of Dark Current and Breakdown Processes using Faraday Cups and Fast Digitisers at the XBand Laboratory for Accelerators and Beams (XLAB)

Two CLIC TD24 accelerating structures, manufactured by CERN, are undergoing high-power testing on the 12 GHz RF test stand, MelBOX, at the x-Band Laboratory for Accelerators and Beams (XLAB). Installed in late 2024, these are the first devices tested at the facility. The goal is to condition the structures for stable operation at gradients of 100 MV/m. The maximum gradient is limited by electrical breakdown—vacuum arc formation under high electric fields—which interrupts RF transmission and can damage the structure. To study breakdown dynamics and validate models of their initiation, detailed, time-resolved charge measurements are needed. Faraday cups upstream and downstream, combined with high-performance 5 GS/s, 12-bit, 3 GHz FEB digitisers, enable precise characterisation of both dark and breakdown current emissions. Fast digitiser readout allows continuous acquisition at the 400 Hz repetition rate, capturing breakdown events and several hundred preceding pulses. This dataset supports in-depth analysis of precursors. We present initial results from structure conditioning, including breakdown statistics, dark current trends, and preliminary analysis of breakdown behaviour.