Cyclotron-undulator cooling of a free-electron-laser beam

Abstract

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.

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

“…These limitations can partially be mitigated by means of using preliminarily energy-chirped [5,6] or spatially modulated [7,8] bunches. However, significantly higher energy enhancement and spectrum narrowing can be expected [9] if a helical undulator with a very strong (over-resonance) guiding field is used and, thus, the conditions for nonisochronous longitudinal electron oscillations [10][11][12][13][14][15][16][17] are fulfilled. In this case, development of the negative mass instability can lead to formation of a long-living electron "core" with a sufficiently small longitudinal size.…”

Undulator radiation of premodulated and nonmodulated electron bunches in the negative mass instability regime

“…The NMI in undulators is possible in the presence of a strong uniform guiding field [17][18][19][20][21][22][23]. For example, it can be developed in the combined helical undulator and uniform guiding fields.…”

“…Another character of the evolution of relatively long modulated bunches and their radiation should be expected when the particles are not repulsed from areas with increased density, but are attracted to them. This "anomalous" situation occurs, in particular, in undulators with a strong (over-resonance) guiding magnetic field due to the development of the so-called undulator negative mass instability (NMI) [17][18][19] in bunches [20][21][22][23]. Due to NMI, an increase/decrease in electron energy under the action of the space charge of a dense bunch can cause approaching to/moving away from the resonance between the cyclotron and undulator oscillations.…”

Cyclotron radiation cooling of a short electron bunch kicked in an undulator with guiding magnetic field

“…An example of such a resonant system is a periodic magnetistatic undulator immersed in a uniform guiding axial magnetic field [13]. Such undulator systems are used in mm-wavelength FELs [4,14] to decrease sensitivity of these devices to the velocity spread.…”

“…It is shown in [13] that the abnormal dependence of the undulator velocity on the electron energy can be used to provide "axial cooling" of the electron bunch; this term means minimization of the spread in axial electron velocity inside the undulator due to transformation of the input spread in axial velocity into the spread in undulator velocity. It is mentioned also in [13] that the axial cooling can be, in principle, transformed into "real" cooling of the electron bunch (minimization of the spread in total electron energy) due to cyclotron emission from gyrorotating electrons, which obtain their oscillatory velocities in the axial cooling undulator (so that the undulator is used as a "kicker" imparting electrons their rotatory velocities). In this paper, we study this idea in detail.…”