It is proposed to utilize the effect of negative mass for stabilization of the effective axial size of very dense and short electron bunches produced by photo-injector guns by using combined undulator and strong uniform magnetic fields. It has been shown that in the “abnormal” regime, an increase in the electron energy leads to a decrease in the axial velocity of the electron; due to the negative-mass effect, the Coulomb repulsion of electrons leads to their attraction and formation of a fairly stable and compact bunch “nucleus.” An undulator with a strong uniform magnetic field providing the negative-mass effect is designed for an experimental source of terahertz radiation. The use of the negative-mass regime in this experiment should result in a long-pulse coherent spontaneous undulator emission from a short dense moderately relativistic (5.5 MeV) photo-injector electron bunch with a high (up to 20%) efficiency and a narrow frequency spectrum.

We propose methods of fast cooling of an electron beam, which are based on wiggling of particles in an undulator in the presence of an axial magnetic field. We use a strong dependence of the axial electron velocity on the oscillatory velocity, when the electron cyclotron frequency is close to the frequency of electron wiggling in the undulator field. The abnormal character of this dependence (when the oscillatory velocity increases with the increase of the input axial velocity) can be a basis of various methods for fast cooling of moderately-relativistic (several MeV) electron beams. Such cooling may open a way for creating a compact X-ray free-electron laser based on the stimulated scattering of a powerful laser pulse on a moderately-relativistic (several MeV) electron beam.

It is shown that the spontaneous undulator super-radiation from a short (shorter than the radiation wavelength) electron bunch can result in a significant axial compression of the bunch under the effect of the rf field of the radiated wave. This “self-compression” can be used to realize a terahertz source of electromagnetic radiation based on the bicolor spontaneous coherent radiation from short dense electron bunches.

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Simultaneous generation at the second and fourth cyclotron harmonics has been obtained from a single-cavity self-excited gyromultiplier. Output power of the short-wavelength radiation amounts to 100 W at a frequency of 75 GHz. The proposed scheme seems to be promising for the terahertz frequency range.

The use of the "negative mass" regime provides stabilization of longitudinal size of dense photoinjector electron bunches moving through a long undulator. This allows one to increase significantly the power capabilities of a terahertz source based on coherent spontaneous emission from a short bunch. However, such type of emission is produced if the bunch length is comparable with the radiation wavelength. This work discusses the use of the negative mass regime to provide effective compression of dense bunches down to "terahertz" lengths.

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