The ELFA project: Guidelines for a high-gain FEL with short electron bunches

ELFA (Electron Laser Facility for Acceleration) has both a fundamental and a technological novel goal: (i) the fundamental goal is to test with short bunches the existence of three different high-gain regimes at the heart of FEL physics: the already observed steady-state regime and the two novel regimes of cooperative synchrotron radiation, i.e., weak and strong superradiance; (ii) the technological goal consists in exploring the possibilities of matching the advanced technologies of high-gain FELs and of superconductive acceleration. The experimental apparatus-photocathode injector, accelerating structure with 352 MHz superconducting cavities, wiggler - is discussed with emphasis on the problems related to the very high current, I ≅ 400 A. Alternative normal-conducting acceleration schemes at 425 MHz are also discussed. The applications of ELFA should range from high-gradient particle acceleration to plasma heating and condensed matter physics.