Energy-Recovery Option for a Future X-ray Free-Electron Laser

A superconducting electron linac together with an energy-recovery system, which has been demonstrated as a useful driver for high-power IR-FELs, seems to be an attractive device for a future X-ray FEL facility as well, because the energy-recovery enable one to reduce the RF generator power required for the operation and also reduce the radiation from the beam dump. Automatic cancellation of beam loading by energy-recovery makes bunch-train structure flexible, which is preferable for experimental users. We summarize critical issues in energy-recovery option for a future X-ray Free-Electron Laser. 1 FUTURE XFEL AS A USERS FACILITY Since a free-electron laser (FEL) has, in principle, no limitation in its lasing wavelength, extensive efforts have been devoted to develop FELs in the region of wavelength in which other coherent sources are not available. As a result of recent progress of high-brightness photo-cathode injectors and demonstration of SASE-FEL in visible and UV, construction of X-ray FELs becomes a realistic target for coming decade. A proof-of-principle experiment of XFEL, SLAC/LCLS, has been proposed and will be constructed soon [1]. An XFEL facility for experimental users will be considered, after the demonstration of XFEL at SLAC/LCLS. Another XFEL device DESY/TESLA-FEL is designed as a users facility, where four beam-lines of SASE-FEL and six beamlines of spontaneous radiation are installed and XFEL radiation with flexible pulse trains, from a single-shot to 10MHz repetition, is available [2]. Such flexible pulse trains with high-duty operation is intrinsic for various scientific applications, but it is only achievable by superconducting linac. A future XFEL users facility, therefore, will be based on a superconducting linac. 2 ENERGY-RECOVERY LINACS An energy-recovery superconducting linac has been developed as a driver of high-power IR-FEL at TJNAF [3], in which beam average current can be increased four times with keeping RF generator power. In JAERI, a similor system is under construction as a high-power FEL driver [4]. A synchrotron light source and a linac-on-proton collider using energy-recovery superconducting linacs are also proposed [5][6]. The principle of the energy-recovery is the conversion of electron energy into RF power by reinjecting the high∗ [email protected] energy electron beam into superconducting RF cells at decelerating phase. The advantage of the energy-recovery is, primarily, to reduce the required RF power to run the accelerator. We see, in the next section, how much RF power can be saved by energy-recovery. The energy-recovery also decreases the electron energy at the beam dump as small as the injection energy. Since heat load and radiation at the beam dump are greatly reduced by small dump energy, the design of radiation shield around the beam dump can be simplified. Thus, the capital cost and running cost of the facility can be saved by energy-recovery. Automatic cancellation of beam-loading by accelerating and decelerating beams also promises flexible operation of the bunch trains. This is a great benefit for experimental users at a future XFEL facility. injector linac undulators dump Figure 1: Energy-recovery XFEL. 3 MINIMUM RF GENERATOR POWER Since the primary advantage of the energy-recovery is saving RF generator power to drive the accelerator, we start with a quantitative discussion on the RF power. The optimal coupling coefficient between an RF cavity and an external RF source, which makes the generator power minimum is found to be [7]