* Corresponding author. Tel.: 81-29-282-5464; fax: 81-29-282- 6057. E-mail address: mainehara@felwu0.tokai.jaeri.go.jp, eisuke@ jfel.tokai.jaeri.go.jp (E.J. Minehara). Nuclear Instruments and Methods in Physics Research A 445 (2000) 183}186 JAERI superconducting RF linac-based free-electron laser-facility E.J. Minehara*, M. Sawamura, R. Nagai, N. Kikuzawa, M. Sugimoto, R. Hajima, T. Shizuma, T. Yamauchi, N. Nishimori Free Electron Laser Laboratory, Advanced Photon Research Center, Kansai Research Establishment, Japan Atomic Energy Research Institute, 2-4 Shirakata-shirane, Tokai-mura, Naka, Ibaraki-ken 319-1195, Japan Abstract Recently, the JAERI superconducting RF linac based FEL has been successfully lased to produce 0.36 kW of FEL light using a 100 kW electron beam in quasi-continuous wave operation. A 1 kW class laser is our present program goal, and will be achieved by improving the optical out coupling in the FEL optical resonator, the electron gun, and the electron beam optics in the JAERI FEL driver. Our next 5-year program goal is to produce a 100 kW-class FEL laser and multi-MW class electron beam in average, quasi-continuous wave operation. Conceptual and engineering design options needed for such a very high-power operation will be discussed to improve and to upgrade the existing facility. ( 2000 Elsevier Science B.V. All rights reserved. PACS: 41.60.C; 84.40; 85.25 Keywords: Free electron laser; Quasi-continuous wave; Superconducting RF linac driver 1. Introduction In a conventional laser device, there are com- monly three major components of the device like a #ash lamp, a gain medium like a crystal, and optical resonator mirrors. In the conventional laser system, heat losses and damages in the components give serious limitations to the applications and intrinsic performances since its invention in 1960. In a free electron laser (FEL) system unlike the conventional, the losses in the medium will be quickly removed from the inside because the me- dium consists of an undulator generating an alter- nating magnetic "eld and a highly energetic electron beam. Resultantly, no deterioration is ob- served in the optical quality of the medium during the high-power operation. However, a normal con- ducting RF linac as the FEL driver produces a large amount of heat, and is very ine$cient like the lamp. In order to improve drastically the e$- ciency and power extraction, and to realize very low errors of the amplitude and phase in acceler- ation, we introduce a superconducting RF linac because of a negligibly small heat loss inside the cavities. We summarize our results and three steps of the JAERI superconducting RF linac-based FEL 0168-9002/00/$ - see front matter ( 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 0 0 ) 0 0 0 6 1 - 9 SECTION II.