Beam tests and phase locking of superconducting cavities for heavy ion linac

This paper describes our progress in two important aspects of the development of a superconducting heavy ion linear accelerator: phase locking and bunching. This accelerator is based on a 430 MHz niobium reentrant cavity developed at Stanford. A compact electronic package has been developed to perform for each cavity the function of phase locking, as well as, supplying RF power, controlling cavity fields, and controlling the frequency tuners. Frequency tuning of the cavities was accomplished by use of two tuners per cavity, a coarse, motor-driven tuner and a fine piezoelectric one. The coarse tuner (2 MHz range) was initially operated to bring the frequencies of the cavities within the range of a few hundred hertz of each other. The fine one was continuously operated to automatically correct for small frequency differences. Large frequency changes were prevented by stabilizing the pressure of the dewar. The tuning and pressure stabilization allowed us to keep the frequency differences within 20 Hz. Further stabilization and phase locking will require damping of the acoustical modes of the dewar. The bunching capability of the superconducting cavities was demonstrated using a 2 MeV proton beam from a Van de Graaff accelerator. More than 43% of the beam was compressed into 250 psec bunches which would be suitable for the proposed accelerator. The practicality of these cavities in an operating accelerator was shown by their dependable operation for a few months, on and off, during these tests, even with minor vacuum failures.