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We achieved stable operation of a free-electron laser (FEL) based on the self-amplified spontaneousemission (SASE) scheme at the SPring-8 Compact SASE Source (SCSS) test accelerator in the extremely ultraviolet region. Saturation of the SASE FEL power has been achieved at wavelengths ranging from 50 to 60 nm. The pulse energy has reached $30 J at 60 nm. The observed fluctuation of the pulse energy is about 10% (standard deviation) for several hours, which agrees with the expectation from the SASE theory showing the stable operation of the accelerator. The SASE FEL has been routinely operated to provide photon beams for user experiments over a period of a few weeks. Analysis on the experimental data gave the normalized-slice emittance at the lasing part is around 0:7 mm mrad. This result indicates that the normalized-slice emittance of the initial electron beam, 0:6 mm mrad in a 90% core part, is kept almost unchanged after the bunch compression process with a compression factor of approximately 300. The success of the SCSS test accelerator strongly encourages the realization of a compact XFEL source.

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The higher order modes (HOM's) of RF cavities at the Pohang Light Source (PLS) storage ring cause longitudinal coupled bunch mode instabilities. A longitudinal feedback system (LFS) is introduced to cure these instabilities. As a key component of the LFS, there is a single-ridged waveguide-overloaded cavity for a longitudinal bunch-by-bunch LFS kicker. Since the RF frequency is 500 MHz, the bandwidth of this kicker should be wider than 250 MHz to damp any coupled bunch mode. Also, a higher shunt impedance of the kicker is selected to use a lower power amplifier. One aluminum kicker is fabricated, and the bandwidth, HOM's, and the shunt impedance of the kicker are measured with a network analyzer. This kicker has several different features from the LFS kicker for DA8NE. First of all, there are four input/output ports to obtain a wider bandwidth. Second, there is a nose cone to obtain a higher shunt impedance. Third, the symmetric frequency response of the shunt impedance around the central frequency is provided. Finally, low beam coupling power (4 W per port at 400 mA) is obtained. According to the simulation result by the high frequency structure simulator (HFSS) code, we obtained the high shunt impedance of 621.3 (transit time factor considered value) and the wide bandwidth of 255 MHz. These are compared with the measured result.Index Terms-Coupled bunch mode instability, HOM, longitudinal feedback system kicker, wake fields.