Klystrons for the Stanford linear accelerator center

A 2-mile-long linear accelerator has been built at Stanford University that uses 245 klystrons to produce an electron beam with an energy up to 20 GeV. Although most of the tubes used are being procured from industry, a major portion of the development work has been and is still being done at Stanford. We will review the important results of this work. In general, the klystrons are designed to produce a minimum of 21 MW peak power with extremely stringent phase and amplitude stability performance requirements. They are focused by permanent magnets and operate up to 250 kV beam voltage with a microperveance of 2. During the past few years, gradual improvements in performance have been achieved by optimizing the bunching parameter and the output cavity coupling, and we have measured efficiencies approaching 45 percent in a permanent magnet. Permanent magnet structure limitations, field shaping, and changes in the klystron design to reduce the magnetic field requirements will be discussed. The output window life has been improved by coating techniques developed to decrease single surface multipactor and reduce window operating temperature. Questions of coating stability will also be discussed. Finally, the paper will touch on the initial results of statistical analysis of tube life as a function of operating experience.