Spontaneous coherent cyclotron THz super-radiation from a short dense electron bunch
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 relatively simple and compact terahertz sources operating in the regime of spontaneous coherent radiation. This type of radiation is realized, when the effective phase size of the electron bunch with respect to the wave is small enough, so that the wave packets emitted by each of the electrons add up basically in phase. THz sources based on the spontaneous emission have a number of advantages as compared to the more traditional free-electron laser (FEL) based on the undulator emission induced due to bunching of a long electron beam by the radiated wave. First of all, an evident advantage is a relatively high efficiency of the energy extraction from electrons, which can be achieved in a simple microwave system based on the “ready-for-radiation” bunch. Actually, such an oscillator does not require either a wave feedback system or an input wave signal to provide the high-efficiency stimulated character of the radiation process. A high-efficiency together with a narrow frequency band of the radiated rf signal is provided in a relatively short and simple system (namely, just amplifier-like waveguide system). One more important advantage is that the phase of the radiated rf signal is fixed by the electron bunch phase. A key problem in realization of THz source based on the spontaneous coherent emission from a short electron bunch is a strong Coulomb repulsion in dense bunches, which leads to the increase of the axial bunch size. If the “operating” radiation mechanism of the THz source is based on the longitudinal electron bunching (either ubitron or cherenkov masers), then axial expansion of the bunch leads automatically to an increase in the bunch phase size with respect to the radiated wave and to the stopping of the spontaneous emission process. Naturally, in the cyclotron maser, the Coulomb repulsion also provides an increase in the axial size of the bunch. However, we show that in cyclotron masers this problem can be solved by the use of the group resonance regime, where the group velocity of the radiated wave is close to the axial electron velocity. In this case, the effect of compensation of the Coulomb repulsion in the phase space takes place. This means that the increase in the axial size of the bunch caused by the Coulomb repulsion does not lead to an increase in the phase size of the bunch, so that the cyclotron phases of particles with respect to the radiated wave stay almost constant. This effect is useful also because the group resonance regime is very attractive from the viewpoint of organizing the wave emission process. This is due to the fact, that in this situation an effective super-radiation regime is realized. In this regime, the radiated wave doesn’t “run away” from the electron bunch, which leads to accumulation of the radiation in the region close to the bunch and, as a result, to formation of a powerful short wave pulse propagating together with the bunch.
The work is supported by Russian Foundation for Basic Research Project 18-32-00351