Measurements on a superconducting X-band structure for linear accelerator application

We report on rf tests of a superconducting disk-loaded waveguide. A three-cell 8-GHz structure, fabricated of two pieces of bulk niobium, has been excited to high-field levels at temperatures below 1.8 °K. The cavity was operated in a π/2 standing-wave mode. Peak electric fields of 37 MV/m, peak magnetic fields of 590 Oe, and effective accelerating fields of 8.0 MV/m were generated at a microwave power level of 8 W/m. The lowest surface resistance that could be obtained was 44 nΩ corresponding to a Q factor of 4×109 at 1.3 °K. These results were achieved using electropolishing and annealing at 1400 °C under moderate vacuum conditions as surface treatments. The fields are limited by fast magnetic thermal breakdown. No electron loading was observed.