Scientific accelerators or colliders are powered, almost exclusively, by klystrons. Linear accelerators generally employ pulsed tubes, while storage rings require CW power. At Stanford, the original Mark III electron accelerator (1947) triggered the development of the first megawatt-level S-band klystron. Subsequently (1963) , the development at Stanford of the two-mile 25-GeV accelerator used 250 25-MW klystrons, and the upgrade of that machine to a 100-GeV center-of-mass collider (SLC-1984) necessitated replacing the original klystrons with 65-MW tubes, also at S-band. At the time, they were by far the most powerful pulsed klystrons in the world. Another production tube in the department is the BFK, a 1.25 MW CW klystron for the SLAC B-Factory, which eventually will require as many as 20 of these 15 ft-long tubes, currently the most powerful klystrons in production anywhere. SLAC physicists (together with physicists in several other countries) are now planning a new collider eventually with a center-of-mass energy of at least 1 TeV, for which a number of technical approaches have been advanced. SLAC's Next Linear Collider (NLC) version is a 25-km machine, operating at 11.4 GHz. With the present intermediate energy goal of 500 GeV center-of-mass, the NLC requires approximately 4000 klystrons, each producing 75 MW at 11.4 GHz, with pulses of 1. 6 microsecond duration and 120 Hz repetition frequency.
Two additional imperatives, have rendered the tube development task even more daunting:
- The cost of the klystrons must be a fraction of prevailing commercial prices for much lower power tubes, in small quantities.
- The klystrons must be very efficient. The electromagnets normally employed to focus and confine klystron electron beams consume too much DC power (80 MW for the NLC) and are not affordable. The NLC tubes employ permanent magnets in a periodic configuration (PPM).
klystron development program at SLAC was initiated in 1989 and has
produced good results. Both solenoid-focused and (PPM) focused
klystrons have been produced at 50 and 75 MW respectively.75-MW PPM klystron has met full specifications.
An alternative to the pencil-beam 75-MW PPM klystron is an SBK, a 75-MW sheet-beam klystron (which with a double beam could be operated at 150 MW) has been in development for the last several years. Sheet beam klystrons have beams of much lower current density, employ overmoded cavities and are also focused. Furthermore, their fabrication is much simpler than conventional klystrons and requires considerably fewer parts. Since their cathode loading is much lighter, they can be expected to have a much longer life as well. The SLAC Klystron Department will soon be completing an intensive simulation program on the SBK and proceeding to the manufacture of a prototype 75-MW tube. Manufacturing klystrons for the SLC has been the principal mission of the SLAC Klystron Department for the past 15 years. Producing them with good yield and affordable cost, has required the installation of a superior manufacturing facility and the establishment of stringent quality standards. Nevertheless, the development of an NLC 75-MW klystron, operating at a frequency four times higher than S-band represents a formidable technical challenge.