Otake, Y. Aizawa, S. Asaka, T. Inagaki, T. Nagatsuka, K. Okuyama, T. Sato, K. Yamada, H. JACoW Publishing Geneva, Switzerland 2226-0366 978-3-95450-194-6 10.18429/JACoW-LINAC2018-THPO091 English 889-892 THPO091 cavity controls FEL power-supply electron Contribution to a conference proceedings 2019 2019-01 https://doi.org/10.18429/JACoW-LINAC2018-THPO091 http://jacow.org/linac2018/papers/thpo091.pdf Solid-state RF amplifiers, which have long lifetimes and small failures, are the recent trend of reliable and stable high-power rf sources for particle accelerators. Hence, we designed a 90kW solid-state amplifier with an extreme low-loss TE011 mode cavity (Q0=100, 000) power-combiner operated at 476 MHz and a 6 us pulse width. Developing this amplifier is for replacement of an IOT rf amplifier, at the X-ray free-electron laser, SACLA. In SACLA, highly RF phase and amplitude stabilities of less than 0.02 deg. and 10⁻⁴ in rms are necessary to stable lasing within a 10 % intensity fluctuation. The amplifier comprises a drive amplifier, a reentrant cavity rf power divider, 100 final amplifiers with a 1 kW output each and a TE011 mode cavity combiner. Water-cooling within 10 mK and a DC power supply with a noise of less than -100 dBV at 10 Hz for the amplifier is necessary to realize the previously mentioned stabilities. Based on the test results of the amplifier, the above-mentioned specifications with the extreme low-loss are promising. The amplifier also allows us to operate in pulsed and CW rfs for linacs and ring accelerators. We report the performance of the 90kW amplifier.