SUPERCONDUCTING RF TECHNOLOGY FOR PROTON AND ION ACCELERATORS

The worldwide status of superconducting RF cavities and cryomodules for low velocity ion and proton particles is reviewed, with emphasis on the construction and tests of prototypes. A number of different multi-cell structures at a range of operating frequencies have been successfully realized. This review will cover the progress of several facilities under construction or being proposed. COAXIAL CAVITIES EVOLUTION Historically the low velocity ion acceleration is the realm of superconducting quarter wave resonators (QWR) as used on ATLAS at ANL (1) or ALPI at INFN Legnaro(2). The evolution of the technology from the simple cylindrical cavity shapes to more complex RF designs has been driven by the increase in beam currents, beam apertures and a step up in the specification of accelerating voltages. In order to cope with the higher peak fields, the cavity preparation procedures used for elliptical cavities are widely adopted for the QWRs. A separate vacuum for the cavity is generally adopted to prevent contamination when cavities are installed in cryomodules. The evolution of the use of QWRs from low beam intensity to higher currents is reflected in a shift in the main efforts to improve the technology. In a low intensity application the frequency sensitivity of the cavity to external perturbation is predominant since the loaded bandwidth of the resonator is narrow. Most effort is then concentrated on how to minimize these microphonics either by mechanically damping the mechanical resonances, actively tuning the cavity using fast piezo electric tuners, or by electronically overcoming this at the expense of RF power injected in the cavity. The VCX system initially developed at ANL is an example of the later (3). Increasing the cavity bandwidth by over- coupling at the expense of RF power is a frequently adopted solution. At higher beams intensities in the mA range for recent projects like SPIRAL2 (4) or SARAF (5), the resonator bandwidth is of the order of 100 Hz. In this case it is expected that cavities can be operated without mechanical dampers of fast tuners. In the case of the IFMIF superconducting linac (6), the beam intensity is 125 mA. The microphonics are not a issue but coupling the RF power to the cavity become the driver in the cavity design.