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

Abstract

Short dense electron bunches produced by modern photoinjectors are attractive from the viewpoint of the realization of powerful and effective sources of subterahertz radiation based on the spontaneous coherent mechanism of emission. This type of emission is realized if the effective phase size of the bunch with respect to the radiated wave is small enough. Therefore, the repulsion of particles caused by a strong Coulomb field inside the dense electron bunch strictly limits the duration of the radiation process due to the increase in the bunch length. We show that this problem can be solved by using the cyclotron mechanism of the spontaneous radiation due to the effect of compensation of the Coulomb repulsion in the phase space.

Capabilities of Terahertz Cyclotron and Undulator Radiation from Short Ultrarelativistic Electron Bunches

“…In the study of the dynamics of bunches and radiation in this paper, two computational approaches are used. In the approximate method [13,18], the bunch is represented by a set of solid uniformly charged disks whose centers are initially placed on the waveguide axis; the disks interact with the radiation field and between each other, and the radiation field is represented by the lowest TE 11 wave with a smoothly varying amplitude. In a more general and precise spatial-frequency approach WB3D [19], macroparticles are used, which are thrown at the initial moment into the waveguide and interact with each other and with the radiation field represented by a set of waveguide modes excited at any cyclotron harmonic.…”

“…, where the coefficient 0.4 is equal to the norm of the wave. It should be mentioned that according to simulations, the model of a solid disk set [13,18] is a fairly accurate approximation for the description of Coulomb interaction inside a bunch and its radiation in a relatively narrow waveguide.…”

“…At the same time, the radiation frequency in this case is approximately Let us estimate the change in electron parameters during radiation of a dense bunch in the waveguide. The change in the electron energy can be written as (see [13] for more details):…”

“…According to the approximate Equation 16, the Coulomb interaction of the particles does not change the phase of the electron relative to the wave in the regime of group synchronism, β z,0 = β gr , despite the possible expansion of the bunch [13]. At the same time, in the high-frequency regime (H in Figure 4), where β gr > β z,0 and an electromagnetic pulse overtakes the electron bunch, the Coulomb interaction changes the phase of the electron relative to the wave and also weakens the effect of the wave on the particle.…”

“…Two efficient radiation mechanisms are considered and compared in this paper, namely, (1) cyclotron radiation of bunches moving in a uniform magnetic field [13] and (2) undulator radiation of bunches moving in the field of a helical undulator and in a strong (super-resonant) uniform longitudinal guiding field under conditions where the negative mass (NM) instability can stabilize the longitudinal size for a dense bunch core [14,15]. In these two cases, the electrons in the bunch can move near identical stationary helical trajectories, but the properties of these "electron oscillators" and the mechanisms of their emission and bunching/repulsion in the wave field and the Coulomb field of the other particles differ significantly.…”