Superconducting Re-Entrant Cavities for Heavy Ion Linacs

Superconducting niobium cavities of reentrant shape have been built and tested at Stanford. The cavities have a 35cm diameter, 10cm length, and 2cm accelerating gap. They resonate at 430 MHz. A large number of tests show that the performance of the cavities is reproducible: field strengths in excess of 12 MV/m were consistently achieved at Q values in the range 2 × 109 to greater than 1010. The highest field obtained was 17 MV/m. To obtain these performance levels, our studies show that it is necessary to use special fabrication procedures, chemical treatment, and of perhaps greater importance, ultra high vacuum firing to temperatures of 1600 to 1800°C. The geometry of the accelerating gap of the cavities in relation to the resonant frequency, make the cavities suitable for accelerating heavy ions with energies upward of 1 MeV/nucleon. A sequence of cavities could thus be used to boost the energy of particles obtained from existing tandem Van de Graaff accelerators. Compared with a superconducting helix structure, the reentrant niobium cavities have a much greater mechanical stiffness, much larger surface area for cooling, comparable or better accelerating voltage gradients, and provide a single rather than a "double" accelerating gap per resonator. The latter feature makes it possible to build an accelerator as a sequence of independently controlled accelerating gaps. The phase and amplitude of each gap can be adjusted to provide essentially any velocity profile for the purpose of particle acceleration.