Superconductors for pulsed RF accelerators

The choice of superconducting materials for accelerator RF cavities has been determined in the past only in part by basic properties of the superconductors, such as the critical field, and to a larger extent by criteria which include fabrication processes, surface conditions, heat transfer capabilities and so on. For CW operated cavities the trend has been toward choosing materials with higher critical temperatures and lower surface resistance, from Lead to Niobium, from Niobium to Nb3Sn. This trend has been dictated by the specific needs of storage ring CW system and by the relatively low fields which could be reached without breakdown. The work performed at SLAC on superconducting cavities using microsecond long high power RF pulses has shown that in Pb, Nb, and Nb3Sn fields close to the critical magnetic fields can be reached without magnetic breakdown. The major results of the tests on the three technical superconductors are illustrated. From these measurements comes the demonstration that for sufficiently short pulses there is no specifically predetermined critical field, and it is possible to exceed, depending on the material, even the superheated critical field