Bulk niobium low-, medium- and high-β superconducting quarter wave resonators for the ALPI postaccelerator

A family of three solid niobium quarter wave superconducting resonators, cooled by direct contact with liquid helium, having resonant frequencies of 80 MHz, 160 MHz and 240 MHz and optimum velocities β ₀=0.056, β₀=0.11 and β₀=0.17 respectively, has been designed, constructed and tested. In addition to the standard chemical surface treatment also thermal treatment at 1200°C using the titanium sublimation technique was applied. The low-beta 80 MHz resonator Q₀=2.3×10⁸, and the quality factor was almost constant from low level up to the maximum field achieved of 4.2 MV/m at 5 W. The intermediate beta 160 MHz cavity had Q₀=2.2×10⁸, almost constant up to the field of 5 MV/m at 10 W. The high beta 240 MHz resonator, after deionized water high pressure rinsing, reached Q₀=3.5×10⁸ and a maximum field of 4.7 MV/m at 6 W. The 80 MHz cavity serves as a prototype for the recently started production of the low-beta section of the ALPI linac