Super-radiative self-compression of photo-injector electron bunches and the use of this effect for realization of a THz source based on spontaneous coherent emission from a short dense electron bunch

Abstract

Laser-driven photo-injectors allow formation of compact and accessible sources of dense electron bunches with a moderate energy of 3-6 MeV, picosecond pulse durations, and charges of up to 1 nC and even greater. These bunches can be used for realization of comparatively simple and compact terahertz sources operating in the regime of spontaneous coherent undulator radiation. This type of radiation is realized, when the effective axial length of bunches is shorter than the radiation wavelength, so that the wave packets emitted by each of the electrons add up basically in phase. The THz source based on the spontaneous emission has a number of advantages as compared to the more traditional freeelectron laser (FEL) based on the undulator emission induced due to bunching of a long electron beam by the radiated wave. These are a relatively high efficiency of the emission from the "ready-for-radiation" bunch, a narrow frequency band of the radiated rf signal in a system without a feedback system and an input rf signal, and the phase of the radiated rf signal fixed by the electron bunch phase. These advantages make the THz source based on the spontaneous emission attractive for a number of important applications, including the use of synchronization of such an oscillator with the X-ray FEL in pump-probe experiments.An example of the THz source based on the spontaneous emission is described in [1, 2]; a 0.3 nC 5 MeV e-bunch with the 0.1 mm initial length propagating in an undulator with a period of 2.5 cm can generate a 20 ps narrow-band rf pulse at a frequency close to 2 THz with a power of tens MW; the efficiency of this process can be over 10 %.The spontaneous emission occurs while the bunch phase size is smaller than 2S, so that wave packets emitted by each of the electrons add up basically in phase. In the case of the ubitron (as well as Cherenkov) source, this means that the bunch length must stay shorter than the radiation wavelength during the whole radiation region. Therefore, realization of a THz source based on the spontaneous coherent radiation requires (1) creation of short enough (0.3 mm or shorter) dense electron bunches and (2) the presence of a mechanism of stabilization of the phase size of the bunch with respect to the radiated wave. The latter is especially important, taking into account the axial Coulomb repulsion, which is very strong in short dense photo-injector bunches and lead to a significant spreading of the bunch during its motion through a path being as short as few decimeters.In this work, we propose and study a radiative mechanism for compression and control the axial size of the bunch. This is the axial compression under effect of the spontaneous super-radiation of a long-wavelength wave. In fact, we propose a bicolor (two-wave) rf source operating in the regime of spontaneous coherent undulator radiation (Fig.