Relaxations in electron beams and adiabatic acceleration

In order to produce electron beams with the minimum possible energy spread we have investigated the possibility of replacing the usual thermocathodes with a photoemissive source such as GaAs. A comparison carried out between these two sources indicated the latter as the best device to obtain a very-low-energy spread. Beam relaxation after emission also leads to an increase in the energy spread. Therefore an experimental study on the nature of relaxations occurring in electron beams, yielded both by a thermocathode and by a photocathode, has been performed. More specifically, we investigated the possibility of reducing the transverse-longitudinal and the longitudinal-longitudinal relaxations. With this aim, the features of adiabatic acceleration, which damp the pure longitudinal relaxation, have been examined. The experience gained during the measurement cycle demonstrated that an adiabatic structure, accelerating electrons emitted by a GaAs photocathode, leads to the best performances.