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_s higher than the thermodynamical critical field B_c might be reached. The ultimate limiting field is then the superheating field B_sh (B_sh=240 mT or E_acc=60 MV/m for Nb at T=0 K). However, the maximum accelerating field observed so far is in the range E_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_sh on bulk Nb 3 GHz cavities supplied by INFN-Genova. High power pulses (4.5 μs, up to 5 MW) are used to reach B_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_acc and B _sh at which the cavity magnetic breakdown occurs. Magnetic penetration depth (λ) measurements were also performed with a low RF level test bed and the corresponding data analyzed then compared to theoretical predictions