SLAC - PUB - 5070 LBL - 27760 UCRL - 101688 August 1989 ( A / E ) Recent Progress in

Experimental work is now under way by collaborators at LLNL, SLAC, and LBL to invest igate re lat ivist ic k lystrons as a possible rf power source for future highgradient accelerators. We have learned how to overcome our previously reported problem of high-power rf pulse shortening and have achieved peak rf power levels of 290 MW. We have used the rf from a relativistic klystron to power a short, 11.4GHz high-gradient accelerator . The measured momentum spectrum of the accelerated electron beam corresponds to an accelerating gradient of 84 MVIm. . introduction and Background During the past year researchers at Stanford Linear Accelerator Center and Lawrence Berkeley Laboratory have continued a collaborative effort with investigators at Lawrence Livermore National Laboratory to study some basic physics issues involved in combining linear induction accelerators with relativistic klystrons. In previous papers*t2 we have reported results obtained with two experimental relativistic klystrons: a subharmonic buncher relativistic klystron (“SHARK”); and a multicavity klystron at 11.4 GHz (“SLA”). Current experiments are being performed using beams with -cl.2 MeV of kinetic energy and = 700 A of current with a -5O-ns duration. SHARK is a low-gain, two-cavity tube driven by a 4-MW, 5.7-GI-Iz source and has output power at 11.4 GHz. It provides a test bed for studying cavity performance and beam-cavity interactions. We have tested several variants of SHARK, including one with an iron magnetic shunt around the drive cavity, one with slotted noses to combat multipactor, one with a penultimate cavity to improve gain and efficiency, and one with the single standing-wave output cavity replaced by a six-cell traveling-wave output structure to reduce electric fields. The maximum rf output level obtained from SHARK is 100 MW from a l.l-MV, 400-A beam. At this current, saturation limits the output power. At higher currents, output power is limited by beam loading of the input cavity. SL4 is an 11.4-GHz, six-cavity, highgain klystron. Previously, peak rf power of 200 MW had been achieved with a standing-wave output cavity, but only with * Work supported in part by the Department of Energy, contract numbers W-7405-ENG-48 (LLNL), DE-AC03-76SFOO515 (SLAC), and DE-AC03-76SFOOO98 (LBL). Invited talk at XIV International Conference on High Energy Accelerators, Tsukuba, Japan, August 22-26,1989 an rf flat-top of much shorter duration than the beam pulse. The maximum reasonably flat rf pulse achieved in our initial test was only 70 MW. Use of a traveling-wave output structure increased the flat pulse to 170 MW. This configuration had three intermediate gain cavities, a gain of ~52 dB, and about 30% efficiency. Phase stability measurements indicated that the output pulse was satisfactory for driving a high-gradient rf accelerator. Results from a new version, in which a 290 MW output -pulse is obtained with multi-output cavities, are presented below. An approach for current modulation of a higher energy electron beam using a transverse chopper system3 is ready for testing. With the problem of pulse shortening alleviated, our main effort now is toward developing higher-energy klystrons and multi-cavity extraction devices, toward cost reduction, and toward improving the efficiency of relativistic klystrons. We have also recently performed work with ahigh-gradient accelerator, which is described below.