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 fieldEax>2.5 MV/m). The cavity, with an anodically deposited protective film of Nb2O5, had a highQand 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.