A high-performance Nb helical cavity

A 92-MHz superconducting-Nb helix resonator of exceptional quality has been thoroughly tested under a variety of conditions. The unit is a full-scale λ/2 structure with dimensions appropriate for heavy-ion acceleration. When operated at a temperature of 1.8K and with bare (not anodized) Nb surfaces, the low-field Q is 9.4 × 10⁹, equivalent to a surface resistance of 5 × 10^-10ohms. The maximum surface magnetic field is 1200 G and the maximum surface electric field is 37 MV/m, which corresponds to a traveling-wave axial accelerating field of 4.6 MV/m. These characteristics set new performance standards for helix resonators. A systematic study of the effects of various surface treatments, including abuses of the cavity, are described. The tests consist of 24 liquid helium cooldowns, at 4.2K and 1.8K, of the cavity with bare and anodized Nb surfaces which at various times were electropolished, oxypolished and heat treated. RF and helium conditioning are discussed as techniques to get through multipactoring barriers and extend the maximum obtainable electric field.