Measuring the RF critical field of Pb, Nb, and Nb3Sn

High peak pulsed power is used to raise the cavity fields well above the cw limits. By raising the fields fast enough, high magnetic fields are reache d at the superconducting surface before local defects create large normal c onducting regions. In this way the fundamental rf critical magnetic field i s measured. By measuring the Q(0) of the cavity during the pulse, one is su re that the cavity is still superconducting at a given rf field level. The rf critical magnetic field is measured as a function of temperature up to T -c for 1.3 GHz cavities of lead on copper, niobium, and Nb3Sn on niobium. A 3 GHz measurement of Nb3Sn on niobium is also presented. Niobium and lead measurements were consistent with the superheating critical field model whe reas the Nb3Sn results fall short of that prediction.