Special Aspects of Cyclotron RF Systems

Abstract

Modern cyclotron rf systems benefit greatly from the technical developments in the allied fields of high power military and commercial transmitters. Special stability requirements and high Qs still require a special art in cyclotrons which is not exhibited in other systems. A survey of the problems and their solutions is given with some examples from current practise.

The basic principles of design of cyclotron rf systems that are used to provide the energy to the circulating ions have changed little since the concept was developed by E. O. Lawrence in 1930. The only development that caused a major change in design philosophy was the idea of phase focussing and frequency modulation. Present rf systems therefore group themselves into two different categories, the fixed frequency machines which are now practically all of the sector focussed variety and the fm machines of which the improved MSC at CERN¹) is the foremost example.

With increasing energies and increasing beam currents the main demand on the rf designer has been one of increasing power input and various techniques have been evolved to deliver the large powers now demanded. The TRIUMF cyclotron²) has an rf system capable of delivering 1.8 Mwatt of power.

Along with these increases in power have come demands from the beam designers for ever increasing voltage and frequency stability or reproducibility in the case of synchro-cyclotrons and the increasing beam currents have demanded wave shape tailoring or modification to increase extraction efficiencies to the level at which a machine can be operated without the tank and resonators becoming so radioactive that the only method of repair amounts to a complete demolition of an existing system and a replacement by a new piece of hardware, a lengthy and undesired procedure.

The higher power levels have had their attendant problems of resonator cooling and the easy tuning techniques of the early machines have been replaced by elaborate schemes of various kinds. One of the most ingenious of these is the cavity bending technique employed in the SIN accelerator³).