THz undulator radiation of stabilized dense electron bunches
- Prof. Andrei SAVILOV
- Prof. Andrei SAVILOV (Institute of Applied Physics)
- Dr. Ilya BANDURKIN (Institute of Applied Physics, Nizhny Novgorod, Russia)
- Mr. Ilya KURAKIN (Institute of Applied Physics, Nizhny Novgorod, Russia)
- Yulia OPARINA (Institute of Applied Physics of RAS)
- Prof. Vladimir BRATMAN (Institute of Applied Physics, Nizhny Novgorod, Russia; Ariel University, Ariel, Israel)
- Dr. Nezah BALAL (Ariel University, Ariel, Israel)
- Dr. Yury LURIE (Ariel University, Ariel, Israel)
Laser-driven photo-injectors are capable to form very compact and dense electron bunches with a particles energy of 3-6 MeV, picosecond and subpicosecond duration, and charge of the order of 1 nC. When moving in the periodic field of the undulator with a period of a few centimeters, such bunches can generate coherent radiation in the terahertz range. The power and duration of such a generation is limited by spreading of the electron bunch under the action of the Coulomb repulsion, which under normal conditions does not allow for the effective implementation of such a scheme. However, it is shown in  that providing of a special configuration of the magnetic field, when a so called regime of negative electron mass is realized in the system, can significantly slow down the Coulomb repulsion.
The negative-mass regime of the electron motion is realized, when the electron moves in a combination of periodic undulator field and relatively strong homogeneous longitudinal magnetic field, and the cyclotron frequency corresponding to the longitudinal field is slightly higher than the undulator bounce-frequency of the particle. Then increase of the electron energy will lead to a reduction of its cyclotron frequency, since the latter is inversely proportional to the relativistic mass-factor of the particle. The electron approaches the undulator-cyclotron resonance, which is accompanied by a resonant increase in its transverse velocity. When it is close enough to the resonance, such a transverse velocity pumping occurs due to decrease of the longitudinal electron velocity. Thus, increasing the energy of the particle causes it to slow down in the longitudinal direction, which can be regarded as a consequence of its effective mass being negative. With regard to the dynamics of the electron bunch, it means that the Coulomb force leads not to the repulsion but to the attraction of electrons.
According to simulations, in the negative-mass regime the radiation power is greatly increased and its frequency band is significantly narrowed. For example, one can propose a rather prospective design in which the electron bunch with the particles energy of 5.5 MeV (mass factor of 12) scatters the undulator field having a period of 2.5 cm into the wave with a frequency of about 2 THz. In this case, the resonant cyclotron frequency corresponds to the longitudinal field of about 5 T, and effective negative mass regime is realized at the longitudinal field from the range near 8 T and the undulator field amplitude of about 0.2 T. Simulations predict formation of a ~10 MW / ~20 ps pulse with a very high electron efficiency (up to 20%), and with a possibility for the frequencies ranging from 1 to 3 THz.
Effective coherent undulator radiation is possible only when the longitudinal size of the bunch does not exceed the radiation wavelength. If the duration of the initial bunch exceeds this wave period, it should be pre-compressed. The simulation shows that the electron attraction in the Coulomb field of the bunch in the negative mass regime can be used to provide the self-compression of the bunch .
An evident disadvantage of the negative-mass undulator is the requirement of a strong axial magnetic field. An alternative method for the bunch compression can be the undulator super-radiation of a long-wavelength wave in additional (auxiliary) long-period undulator . If the bunch is short enough, then the bunch front is placed close to the maximum of the decelerating phase of the radiated wave, whereas the tail is placed close to the “zero” wave field. This effect can be used for creation of a “bicolor” THz source based on the spontaneous emission from a short bunch, so that the super-radiation of the auxiliary long-wavelength wave is used to compress the bunch down to a size shorter than the wavelength of the operation short-wavelength wave.
The work is supported by IAP RAS Project 0035-2014-0012, and Russian Foundation for Basic Research Projects 16-02-00794 and 18-32-00351, and Israeli Ministry of Science, Technology and Space.
 N. Balal, I. V. Bandurkin, V. L. Bratman, E. Magory, and A. V. Savilov. Negative-mass mitigation of Coulomb repulsion for terahertz undulator radiation of electron bunches. Applied Physics Letters, vol. 107, p. 163505, 2015.
 I.V. Bandurkin, I.S.Kurakin, A.V. Savilov, Effective compression of photo-injector electron bunches in the negative-mass undulators. Physical Review AB, 20, 020704 (2017).
 I.V. Bandurkin, Y.S. Oparina, A.V. Savilov. Super-Radiative Self-Compression of Photo-Injector Electron Bunches. Applied Physics Letters, 110, 263508 (2017).