Superconducting Solenoids for RF Accelerators and Electron Guns

Compact solenoids are applied at an increasing rate for beam focusing in superconducting RF accelerating structures, for instance in secondary ion beam linacs and for free electron lasers. High field solenoids with a center field of 7.9 T for the HIE-ISOLDE high-beta cryomodules at CERN are currently being designed and manufactured at Danfysik. NbTi coils and a soft iron yoke are carefully designed for operation in a LHe bath at a temperature of 4.5 K at field integral requirements of $\leq 13.5\ \hbox{T}^{2}\hbox{m}$ . The quenching characteristics of this solenoid have been studied and simulated, and a passive quench protection circuit is part of the solenoid design. The yoke outer diameter is 200 mm; the total length 312 mm. An HTS solenoid based on second-generation high-temperature superconducting technology was designed, built, tested, and delivered by Danfysik to the free electron laser project of the University of Wisconsin. The compact 136-mm-long HTS solenoid with a center field of 0.2 T operates in the temperature range between 5 and 70 K. The field integral is 3.1 $\hbox{T}^{2}\hbox{mm}$ with very small integrated field errors. Conduction cooling of the compact solenoid makes it easy to assemble and operate very close to the RF cavity.