Tests on superconducting helix resonators

A niobium helix-resonator cavity close to the frequency suitable for a heavy-ion accelerator has been subject to an 8-week test, including 323 h at full power (axial electric field Eax>2.5 MV/m). The cavity, with an anodically deposited protective film of Nb2O5, had a high Q and supported high fields without deterioration over the whole period during which it was subject to a moderate internally generated x-ray dose. Properties were found to be stable against considerable thermal cycling and exposure to air. Measurement of emitted x-ray maximum energy allowed an independent corroboration of the field measurement calibration.