Negative-mass mitigation of Coulomb repulsion for terahertz undulator radiation of electron bunches

Abstract

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.

Superradiant and stimulated-superradiant emission of bunched electron beams

“…Tuning is carried out by varying the electron energy and/or the undulator gap. Another ultra-short (5 mm period) wiggler is under construction for operating in a mode of negative mass effect [217,218].…”

Helical undulator based on partial redistribution of uniform magnetic field

“…Such bunches can be attractive for production of power THz electromagnetic pulses [6][7][8][9][10][11][12][13], if very strong mutual Coulomb longitudinal repulsion of bunch particles is prevented or mitigated. One of the recently proposed efficient methods for stabilization of the electron bunches length and for corresponding increase of their spontaneous Doppler-upshifted terahertz undulator radiation energy [12,13] is based on development of the so-called Negative-Mass Instability (NMI) [14][15][16][17] in the bunches moving in undulators. Implementation of the method requires the undulator field to be combined with a strong uniform axial guiding magnetic field [12,13].…”

Coherent spontaneous THz undulator radiation from dense electron bunches formed in laser-driven photo-injectors

“…Unlike the above situation, the space charge can even maintain the long-living electron "core" in the bunch [5,6] if conditions for NMI in undulators [7,8] are fulfilled. It can be achieved in a combined undulator and overresonance uniform longitudinal magnetic field.…”

“…It can be done, for example, using a redistribution of the uniform field of the solenoid by a ferromagnetic (steel) helix (Fig. 5) [5]. For example, a helix with a period of 2.5 cm and an inner diameter of 10 mm wound of a steel wire with a radius 3 mm and mounted into solenoid with a field 75 kOe provides sufficient amplitude of transverse helical field 2 kOe (these parameters are used in Figs.…”