Laser Diagnostics of High Power Low Energy Electron Beams

Dense and cold electron beams can be of extreme interest for ion beam cooling [1] and for the generation of coherent electromagnetic radiation in the microwave range through tre Free Electron Laser process [2]. The high current needed in these applications requires a very efficent energy recovery in the electron accelerator, to save installed power, so that nondestructive diagnostics have to be taken into account for full intensity tests. A project of a high-energy electron cooling device is actually in progress as an INFN collaboration. Several prototypes have been successfully tested and the final version has been constructed in the Frascati Laboratories. The first tests will be performed in the Legnaro Laboratories [3]. The apparatus consists of a high-frequency (40 kHz) Cockcroft-Walton generator (760 kV nominal voltage) with very low ripple (10^-4) and high stability, a Pierce electron gun followed by an accelerating tube and an energy recovery system. The energy recovery system consists of a decelerating column, similar to the accelerating one, and a collector. In order to compensate the space charge effects for all the operating energies, the electron beam is guided by an axial magnetic field. A detailed description of the whole apparatus has been already published [3], while in Table I the main characteristics of the device are summarized. The transverse beam temperature will be measured by means of a pair of synchronous wave pick-up matched to the electron cyclotron frequency [4], and positioned at the two ends of the straight section. The same instrument can also give the beam position, allowing the reconstruction of the electron line of flight. The longitudinal energy spread and the transverse beam density can be measured from the spectral width and relative intensity of a laser light backward scattered from the electron beam [5].