Probing the fundamental limit of niobium in high radiofrequency fields by dual mode excitation in superconducting radiofrequency cavities

We have studied thermal breakdown in several multicell superconducting radiofrequency cavity by simultaneous excitation of two TM010 passband modes. Unlike measurements done in the past, which indicated a clear thermal nature of the breakdown, our measurements present a more complex picture with interplay of both thermal and magnetic effects. JLab LG-1 that we studied was limited at 40.5 MV/m, corresponding to Bpeak = 173 mT, in 89 mode. Dual mode measurements on this quench indicate that this quench is not purely magnetic, and so we conclude that this field is not the fundamental limit in SRF cavities