Speaker
Description
Nb/Cu coated SRF cavities present several advantages with respect to bulk Nb cavities. However, a systematic degradation of the performance at high accelerating gradients is observed.
At CERN, a vast R&D program has been conceived to find an optimized recipe for manufacturing Nb/Cu coated cavities, from the production and treatments of the copper substrates to the thin film deposition. In the context of this research campaign, 19 1.3 GHz single cell elliptical cavities have been tested since 2021.
On the one hand, the research has been focused on pushing the performance of the cavities at 4.2K to use this technology in the FCC accelerator. The goal is to minimize the BCS resistance so that the operational requirements of the FCC SRF system are met. Besides, in view of the large scale of the FCC project, the copper substrates have been manufactured with different techniques in order to select the most suitable one in terms of reliable quality and economy of scale. The results at this temperature are encouraging, showing repeatable and optimized RF performance.
On the other hand, the research has been directed towards a more fundamental understanding of the mechanisms behind the Q slope. This would allow to mitigate this phenomenon and ultimately to extend the application of this technology to high energy, high gradient accelerators. For that, RF tests have been done at 1.85 K as well, as it is not the BCS resistance that dominates at this temperature, but the residual resistance, and the Q-slope appears more clearly. Emphasis has been made to study of the influence of the thermal cycles on the performance, equipping the cavities with flux gates to correlate the trapped flux and their performance. A systematic improvement has been observed of both the Q slope and the residual resistance with slow thermal cycles. The results of these studies are presented.
