Suppression of Secondary Electron Yield Effect in the 650MHz/800kW Klystron for CEPC

Abstract

The Circular Electron Positron Collider (CEPC) is in pre-research, it will need more than two hundred 650 MHz/800 kW klystrons. The secondary electron yield (SEY) effect suppression is very important for the klystron working stable. The simulation uses an incident primary electron source and considers all the phases and power levels of the input microwave. Two methods are simulated for the SEY suppression. The groove cutting on the nose of cavities is much simple while the TiN coating can suppress better. The effect after groove cutting on nose is also simulated and the corresponding compensations are adopted. For simplifying the fabrication progress as well as some experience that can be referenced, the groove cutting method is adopted finally for the first klystron prototype, which is expected to be available in the summer of 2019. INTRODUCTION In July 2012 at the Large Hadron Collider, the Higgs particle was discovered, it is then a very important issue of the further research and measurement in Higgs for particle physics. For the Higgs’ energy is much lower than expected, it is very possible for building a circular collider as a Higgs factory, the CEPC at 240 GeV centre of mass for Higgs studies is then proposed by Chinese scientists in September 2012 [1]. The design scheme of CEPC after further consideration and discussion is using a circumference of 100 km with double ring, the superconducting cavities are finally adopted and will be fed by 650 MHz/800 kW Continuous Wave (CW) klystrons, with a demand of more than two hundred. The pre-research of the first klystron prototype started in June 2017, and is expected to be available by the summer of 2019. As all known, one of the main reasons why the klystron working unstable is the SEY effect [2-4], so the suppression is quite important. Two mainstream methods to suppress SEY are groove cutting and TiN coating, the former method is much simple while the latter one having better consequents. Both two methods are simulated and compared, the nose groove cutting is finally adopted for the first klystron prototype for simplifying the fabrication process. The effect on microwave performances after groove cutting is also simulated and the corresponding compensations are considered. ___________________________________________ * Work supported by National Natural Science Foundation of China (11275222) † email address: [email protected] SIMULATION OF THE SEY EFFECT SUPPRESSION The Method of Cutting Grooves on Nose The klystron is designed to be working at 650 MHz and expected an output CW power of more than 800 kW. Six cavities including a second harmonic cavity are adopted to achieve a 45 dB Power Gain (PG), for a single cavity usually provides a PG around 10 dB, and the second harmonic cavity basically has no effect on the PG. The design parameter of the klystron is listed in Table 1. Table 1: The Design Parameter of the 650 MHz/800 kW Klystron