RF pulsed tests on 3 GHz niobium cavities

The achievable limiting RF field for S-Band and L-Band superconducting cavities is still an open question today. Previous studies on Sn and In have shown that a surface magnetic field B/sub s/ higher than the thermodynamical critical field B/sub c/ might be reached. The ultimate limiting field is then the superheating field B/sub sh/ (B/sub sh/=240 mT or E/sub acc/=60 MV/m for Nb at T=0 K). However, the maximum accelerating field observed so far is in the range E/sub acc/=37-40 MV/m for the best 1.3 GHz Nb cavities. A dedicated facility (NEPAL Supra Test Facility) is currently used at LAL for measuring B/sub sh/ on bulk Nb 3 GHz cavities supplied by INFN-Genova. High power pulses (4.5 /spl mu/s, up to 5 MW) are used to reach B/sub sh/ before cavity thermal breakdown occurs. A method for analyzing the response of a SRF cavity when subjected to pulsed high RF power was developed and the corresponding numerical simulation results were validated by comparison to experimental data. This technique is successfully applied to detect E/sub acc/ and B/sub sh/ at which the cavity magnetic breakdown occurs. Magnetic penetration depth (/spl lambda/) measurements were also performed with a low RF level test bed and the corresponding data analyzed then compared to theoretical predictions.