Energy enhancement and spectrum narrowing in terahertz electron sources due to negative mass instability

Abstract

Simulations of coherent spontaneous undulator radiation in a waveguide demonstrate that the use of negative mass instability (NMI) for retaining longitudinal sizes of dense electron bunches, which are formed in laser-driven photoinjectors, allows one to increase power capabilities of a terahertz radiation source by many times. The NMI is realized in an undulator with combined helical and over-resonance uniform longitudinal magnetic fields due to nonisochronous longitudinal oscillations of electrons, whose frequencies increase/decrease with increasing/decreasing particle energy. In such conditions, an effective longitudinal size of the bunches can be preserved at long distance even at an extremely high electron density. Correspondingly, an energy extraction efficiency of more than 20% is revealed at a narrow frequency radiation spectrum, suggesting realization of a compact and powerful THz source.

Spontaneous superradiant sub-THz coherent cyclotron emission from a short dense electron bunch

“…For the normalized electron axial momentum p k ¼ γβ k , coming from the results for the Lorentz-factor changes (14), taking into account the relation between the axial moment and energy, caused by Coulomb interaction, dp k;j dðω 0 tÞ ¼ − F c;j β k;j : ð15Þ…”

“…Modern sources of dense electron beams (including laser-driven photoinjectors) allow the 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 [1][2][3][4][5][6]. These bunches can be used for the realization of relatively simple and compact terahertz (THz) sources operating in the regime of spontaneous coherent radiation [7][8][9][10][11][12][13][14]. 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 ( Fig.…”

“…In other words, in this paper, spontaneous emission means a situation where the electronic bunch is so short that it is ideally (or almost ideally) bunched with respect to the emitted wave already at the entrance to the radiation region and remains so during the whole process of radiation. In the case of undulator radiation, this situation is provided when the bunch stays significantly shorter than the wavelength of the radiated wave [13,14]. In the case of cyclotron radiation, the situation is more complicated due to the 2D character of the electron phase with respect to the wave.…”

“…This results in the saturation of the process of spontaneous emission. Thus, in the case of undulator and Cherenkov masers, special methods providing control of the axial length of the bunch [13,14] (and, possibly, the compression of the bunch [16][17][18]) are required. This paper is devoted to studying possibilities for organizing spontaneous coherent emission from a short electron bunch in cyclotron masers.…”

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

“…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 %.…”