Niobium resonator development for high-brightness ion beam acceleration

Two niobium resonant cavities for high-brightness ion beam acceleration have been constructed and tested. The first was based on a coaxial quarter-wave geometry and was optimized for phase velocity β₀=0.15. The cavity, which resonates at 400 MHz in the fundamental mode, operated at an average (wall-to-wall) accelerating gradient at 12.9 MV/m under continuous-wave (CW) fields. At this gradient, a cavity Q of 1.4×10⁸ was measured. The second was based on a coaxial half-wave geometry and was optimized for β₀=0.12. This cavity, which resonates at 355 MHz in the fundamental mode, operated at an average accelerating gradient of 18.0 MV/m under CW fields. This is the highest average accelerating gradient achieved to date in low-velocity structures designed for CW operation. At this gradient, a cavity of 1.2×10⁸ was measured