R&D Progress in High Efficiency CW Klystron for CEPC*

After the discovery of the Higgs boson at the Large Hadron Collider (LHC) in 2012, a 240 GeV Circular Electron Positron Collider (CEPC) was proposed in China. The total RF input power of the CEPC collider ring is about 60 MW. To reduce energy demands and establish a more environmentally friendly and energy-efficient scientific facility, the high efficiency klystron has been developed as a key technology for the CEPC at the Institute of High Energy Physics in China, aiming to increase klystron efficiency to above 80%. This paper reviews the recent progress of high efficiency continuous wave (CW) klystrons for the CEPC. As of August 2024, high power test results of the second prototype klystron indicate that the output power and efficiency have reached 803 kW and 78.5%, respectively. Currently, a multi-beam klystron with an expected efficiency exceeding 80% is in development, which will be tested in the first half of 2025.