Experiments with Stimulated Transition Radiation

Coherent transition radiation (CTR) in the far infrared regime can be produced from sub-picosecond electron bunches. At the SUNSHINE (Stanford University short intense electron source) facility [1] electron bunches as short as 100 fs rms can be produced routinely. Such bunches generate coherent transition radiation (TR) at frequencies from the mm regime up to the order of 100 wavenumbers. Consider an electron passing through a thin aluminum foil creating TR as it appears on the other side. While the electron is still close to the radiator it looses energy to the radiation field. That means there must be fields which decelerate the electron. If we now somehow generate externally a similar field, the electrons would be decelerated more thus stimulating the passing of more energy to the radiation field. This is the idea behind the experiment to generate stimulated transition radiation. The most appropriate external field in this case is the coherent transition radiation emitted by a previous electron bunch. This radiation is recycled in an optical cavity of appropriate length such that it arrives again at the radiator exactly when a subsequent electron bunch arrives.