Radiation safety design for the Linac Coherent Light Source (LCLS), which provides ultra fast intense X-ray FEL, was performed at SLAC national accelerator laboratory. The LCLS has specific layout, i.e, an experimental area having through holes sits in front of intense high energy electron beam. For this condition extensive simulations and experiments had been carried out during several years to find out solutions to meet requirements of radiation dose rates under normal operation, mis-steering condition and system failure. In this effort, prompt dose rate in accessible area and ground water activity were estimated for shielding structure of 17 GeV, 5 kW main dump. To obtain dose rate in the experimental area, production of muon and high energy bremsstrahlung were estimated for beam losses of a bending magnet and thin insertion devices at more than 70 m upstream of the area. Dose rates for failure cases of beam containment system and personal protection system were estimated to ensure that the dose rate in accessible area is kept below the limit under these conditions. The accuracy of the design was confirmed during commissioning phase through field measurements prior to user runs.

The progress of top-up operation technology pioneered at the SORTEC 1-GeV ring in 1990 is briefly reviewed. At the SORTEC ring for lithography use, the top-up operation has been achieved with a 200 mA storded current, which was automatically controlled within 1% fluctuation, in May 1990. The stable top-up operation with the routine current stability in the range of 10^-3 to 10^-4 was finally established after intense efforts more than a decade at Swiss Light Source (routine use in 2001), Advanced Light Source (routine use in 2001), SPring-8 (routine use in 2004), Taiwan Light Source (routine use in 2005), etc. A contribution of the top-up operation for achieving the highest luminosity 2.108x10³⁴/cm²/s in the world (in 2009) at the KEKB Factory is highly appreciated.

This paper describes present status of klystron modulators for the KEK electron-positron injector linac. The rf system of this linac consists of 60 high-power klystrons, each capable of 50 MW, 4.0 µs pulsed power at a repetition rate of 50 pps. Each klystron is pulsed by its own line-type modulator, containing a hydrogen thyratron switch tube. The modulator availability and thyratron lifetimes are presented. A 45 kV solid-state switch consisting of 15 static-induction (SI) thyristors stacked in series has been also developed and tested to replace a thyratron, which improves the reliability and maintainability of the modulators.

XFEL/SPring-8 employs 72 units of the modulator power supplies for the pulse power klystrons and the electron gun. We took following guide line for our design work: use common component for all klystron power supplies at different frequency ranging from L-band, S-band and C-band, minimize EM radiation from switching thyratron, keep steady operation against environmental temperature and moisture change, compact design, easy handling and suitable for mass-production. To meet all requests, we decided to house all the PFN circuitry and high voltage pulse transformer into a single tank, and filled with insulation oil. Currently, the mass production, and installation are in progress. We expect beam operation before April 2011.

For decades, electron storage rings have been afflicted by irregular phenomena that give rise to a sudden decrease in the beam lifetime. These phenomena are ascribed to the trapping of positively ionized macroparticles (“dust”), and are still problematic to the stable operation at the Photon Factory Advanced Ring (PF-AR). Careful observations for many years led us to a hypothesis that two kinds of electric discharges in vacuum may trigger the dust trapping, and the measures we took on the basis of this hypothesis, namely, switching distributed ion pumps off and conditioning discharge sources with higher current beams, could significantly suppress the occurrence of the phenomena. Experimental demonstration of the hypothesis was also carried out at PF-AR, and proved that the electric discharges surely triggered the dust trapping. A remarkable result was additionally obtained in this experiment; a dust particle trapped by the beam was visually observed with video cameras. Visual observation of trapped dust would play a key role in further investigation of the dust-trapping mechanism.

This article describes physical characteristics of the cesium telluride thin film as a photocathode material for an RF electron gun. The electron gun using the photocathode is also overviewed at the beginning of the article. The performance of photocathode in relation to a elemental ratio of cesium and telluride and to an electron affinity are discussed in detail referring new results obtained from recent studies in addition to accumulated knowledge, especially from the viewpoint of applications.

My effort in the last stage of wandering about accelerator was to bridge the gap between nuclear and high-energy physics at the KEK-PS. Since the TRISTAN construction started, the KEK-PS has been opened to nuclear physics users. Among various possibilities, emphasis was placed on the hypernuclear experiments, K-decay experiments, and later, the long-base-line neutrino experiment (K2K), which were successfully carried out. Although the TRISTAN ex- periment was unable to find the top quark, the CP-test experiments at TRISTAN-II (KEKB) have proven the Kobayashi-Maskawa theory successfully. During the last three years of my tenure in KEK, I served as a science adviser to Minister of Education, and I was involved in international affairs of accelerator science.