New Method for Klystron Modeling

We have developed a new method for a realistic and more accurate simulation of klystron using the MAGIC code. MAGIC is the 2.5-D or 3-D, fully electromagnetic and relativistic particle-in-cell code for self-consistent simulation of plasma. It solves the Maxwell equations in time domain at particle presence for a given geometrical structure. It uses no model or approximation for the beamcavity interaction, and thus keeps all physical processes intact. With MAGIC, a comprehensive, full-scale simulation of klystron from cathode to collector can be carried out, unlike other codes that are specialized for simulation of only parts of klystron. It has been applied to the solenoid-focused KEK XB72K No.8 and No.9 klystrons, the SLAC XL-4 klystron, and the BINP PPM klystron. Simulation results for all of them show good agreements with measurements. We have also developed a systematic design method for high efficiency and low gradient traveling-wave (TW) output structure. All these inventions were crystallized in the design of a new solenoid-focused XB72K No.10. Its predicted performance is 126 MW output power (efficiency 48.5%) with peak surface field of about 77 MV/m, low enough to sustain a 1.5 μs long pulse. It is now in manufacturing and testing is scheduled to start from December 1998. 1 JLC KLYSTRON PROGRAM The 1-TeV JLC (Japan ee Linear Collider) project[1] requires about 3200 (/linac) klystrons operating at 75 MW output power with 1.5 μs pulse length. The main parameters of solenoid-focused klystron are tabulated in the second column of Table 1. The 120 MW-class X-band klystron program at KEK[2], originally designed for 80 MW peak power at 800 ns pulse length, has already produced 9 klystrons with solenoidal focusing system. To reduce the maximum surface field in the output cavity, the traveling-wave (TW) multi-cell structure has been adopted since the XB72K No.6. Four TW klystrons have been built and tested. All of them share the same gun (1.2 microperveance and the beam area convergence of 110:1) and the buncher (one input, two gain and one bunching cavities). Only the output structures have been redesigned each time at BINP. XB72K No.8 (5 cell TW) attained a power of 55 MW at 500 ns, but the efficiency is only 22%. XB72K No. 9 (4 cell TW) produced 72 MW at 520 kV for a short pulse of 200 ns so far. The efficiency is increased to 31% and no sign of RF instability has been observed. The limitation in the pulse length attributes a poor conditioning of the klystron. The latest tube, XB72K No.10, was designed at KEK, and is being build in Toshiba. Apart from the solenoid-focused XB72K series, KEK has also started a PPM (periodic permanent magnet) klystron development program. The design parameters are shown in the last column of Table 1. Its goal is to produce a 75MW PPM klystron with an efficiency of 60 % at 1.5 μs or longer pulse. The first PPM klystron was designed and build by BINP in the collaboration with KEK. It has a gun with beam area convergence of 400:1 for the microperveance of 0.93. The PPM focusing system with 18 poles (9 periods) produces the constant peak magnetic field of 3.8 kG. The field in the output structure is still periodic, but tapered down to 2.4 kG. There are two solenoid coils located at the beam entrance for a smooth transport of a beam to the PPM section. It achieved 77 MW at 100 ns, but there is a clear sign of RF instability at higher frequencies. The DC current monitor in the collector shows about 30 % loss of particle when RF is on. The second PPM klystron, XB PPM No.1, is being designed at KEK. Table 1: Specifications of X-band solenoid-focused and PPM-focused klystrons for JLC.