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The 13th harmonic of a Ti:sapphire (Ti:S) laser in the plateau region was injected as a seeding source to a 250-MeV free-electron-laser (FEL) amplifier. When the amplification conditions were fulfilled, strong enhancement of the radiation intensity by a factor of 650 was observed. The random and uncontrollable spikes, which appeared in the spectra of the Self-Amplified Spontaneous Emission (SASE) based FEL radiation without the seeding source, were found to be suppressed drastically to form to a narrow-band, single peak profile at 61.2 nm. The properties of the seeded FEL radiation were well reproduced by numerical simulations. We discuss the future precept of the seeded FEL scheme to the shorter wavelength region.

A high-voltage pulsed electron gun has been developed for the low-emittance injector system of the x-ray free electron laser (FEL) project at SPring-8. A single-crystal CeB 6 cathode was chosen as a thermionic emitter because of its excellent emission properties, i.e., smooth surface, high emission density, uniform emission density, and high resistance to contamination. A gun voltage of ÿ500 kV was determined as a compromise between the need for suppressing emittance growth and reducing the risks of high-voltage arcing. We have succeeded in producing a 500 keV beam with 1 A peak current and 3 s width. We have also measured the beam emittance by means of the double-slit method and obtained an extremely low emittance of 1:1 mm mrad (normalized, rms). These results are very promising for the x-ray FEL. In this paper, we describe the design and basic performance of the CeB 6 electron gun, and report the emittance measurement experiments.

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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.

A new simple RF structure is proposed for damped accelerating cavities in linear and circular particle accelerators. Almost all of the beam-induced higher-order modes are heavily damped through a radial transmission line terminated with a matched load. The accelerating mode is established by means of an imaginary short due to a choke structure. Preliminary measurements showed good damping properties, while a high Q accelerating mode was maintained. Because the structure has axial-symmetry, it is easy to fabricate, and is especially suitable for mass production of the disk-loaded accelerating structures for future e+e- linear colliders.