Rf-sources for proton linacs

In existing proton linacs, the choice of the RF frequency used at the beginning of the accelerator is the result of a difficult compromise between the injection energy (that is, the dimensions of the electrostatic injector), and the diameter of the accelerating structure (chosen in such a way as to allow the first quadrupole lens to be contained in the first drift tube). With few exceptions, the RF frequency used is 200 MHz. At this frequency, triodes can supply a few MW of peak power, but only with a duty cycle of a few percent or less. Klytrons would be able to improve this situation but at this frequency the problem of klystron dimensions is encountered. Recent developments of RFQ technology allow some of these difficulties to be overcome by forming a bridge between the injector (which thus can operate at a few tens of kV) and the beginning of the accelerator (which then use an RF frequency at least twice as high). Klystrons are now perfectly adapted for operation in this frequency range (350-500 MHz). Very high-performance klystrons have been developed for electron accelerators. Klystrons can cover the entire frequency range necessary for multi-GeV proton accelerators. The present-day possibilities are discussed and improvements which can be anticipated in the near future are described