State of the art of low energy accelerators for FEL operation

From examination of the simple gain equation, together with line broadening effects, it is apparent that the desirable properties of the electron beam for injection into a free electron laser (FEL) are (1) low emittance, (2) low energy spread, (3) high peak current, (4) adequate bunch length in the micropulse, and (5) good stability during the macropulse. This paper will give a brief description of the various sources used for FELs in the Compton regime, with comments about their performance in relation to the above requirements. Ways of improving performance which have been, or could be, used will be considered particularly with reference to linear accelerators, although microtrons and race-track microtrons will also be discussed. A particular interest will be to consider how far one might go in making use of accelerators of medium high energies (> 100 MeV) to go to shorter wavelengths (sub-μm).