Extremely high power klystrons for particle accelerators

Abstract

To make possible the realization of the different high-energy particle accelerator projects for physics researchers now in the design or planning stages, one fundamental necessity is the availability of extremely high power RF sources. Today, for such machines designed to operate beyond about 300 MHz, the only feasible candidate is the multicavity klystron amplifier. Furthermore, high conversion efficiency is essential, due to the high and ever-rising cost of electrical energy. Given here are the main considerations in improving klystron efficiency,emphasizing the role of computer-aided design (CAD), and a description of a 1 MW 352-MHz CW klystron from Thomson-CSF designed to meet the expressed requirements. Measured performances of 'this tube include 65-66 %efficiency, which may still be slightly improved. Also considered are the constraints of manufacturing andtesting an electron tube of this size (over 4 meters long). Secondly, a pulsed klystron capable of delivering 35 MW peak a t 3 GHz has also been developed by Thomson-CSF, for use in linear accelerators. This tube is also described and the considerations in maximizing its efficiency are discussed. Finally, mention is also made of a suitable driver for the latter tube. INTRODUCTIONPhysics researchers are proposing more and more high-energy particle accelerator projects, especially of the circular type, which are large consumers of RF power. Furthermore, it is becoming quite common for them to consider that the availability of 1 MW (CW) RF sources is now an established fact. It must nevertheless be remembered that at frequencies higher than 300 MHz, klystron amplifiers are a t present the sole possibility for delivering such power levels. Klystrons can achieve these performances because of the clear separation of functions in these tubes : electron gun, amplification section, output circuit, window, collector, etc. These tubes are also very sturdy, but they tend to be rather large and bulky.Because of the high levels of power involved and the high cost of electrical energy today, the klystron operating efficiencies (dc to RF) must be as high as possible. The Electron Tube Division of Thornson-CSF therefore has made a concentrated effort in this direction. The results are apparent in the TH 2089, for example, a new "Superklystron" developed by the Division, which delivers over 1 MW of continuous-wave (CW) output power a t 352 MHz, with a measured efficiency of 65 to 66 %. The main application of the TH 2089 will be as the source of RF power for the storage ring of the LEP accelerator, a t CERN in Geneva, Switzerland. Let us note in passing that the klystrons for this application, despite or rather because of their size, must operate in the horizontal position in narrow underground galleries adjacent to those of the accelerator.Secondly, as concerns the linear accelerators (linacs) used as injectors for such machines as the LEP or as research tools in their own right, a requirement for higher and higher peak RF powers is noted, for example 35 MW in 6 ps pulses...