Long wavelength compact-FEL with controlled energy–phase correlation

A radio-frequency modulated electron beam passing through a magnetic undulator generates coherent spontaneous emission (CSE) when the electron bunch length is comparable to the FEL resonant wavelength. At long wavelengths, CSE can be significantly enhanced by a proper ramping of the electron energy within the bunch duration, since this allows the contributions of the individual electrons to be added in phase. In this paper, the physical principles of the coherent spontaneous emission are reviewed together with its main experimental features. An accelerating structure, constructed at ENEA-Frascati for the systematic investigation of energy–phase correlation effects, is described together with an analysis of the expected performance.