Efficient Electron Sources of Coherent Spontaneous Radiation with Combined Helical and Uniform Magnetic Fields

Two methods to mitigate repulsion of electrons in dense bunches from photo-injectors with a relatively low particle energy and to enhance the power of terahertz radiation have been studied. First method is based on using very short bunches and small undulator periods that allows a significant shortening radiation sections. According to simulations bunches with duration (50100) fs and energy 6 MeV that presumably could be formed in the constructing Israeli THz FEL [1] would fairly effectively radiate at frequencies up to (10-20) THz. The second method is based on an idea recently proposed by A.V. Savilov for longitudinal electron bunching [2, 3]. This is possible when a bunch moves in a combined magnetic field of a solenoid and of a undulator and the electron cyclotron frequency is sufficiently large in comparison with their undulator frequency. In such situation, an increase/decrease of particle energy in the repulsed Coulomb field of space charge leads to a decrease/increase in particle longitudinal momentum. Correspondingly, Coulomb repulsion can lead to an effective attraction of the particles (this effect is analogous to the known cyclotron negative-mass instability). A large value of the uniform field that is necessary in this method can be used to easily obtain a undulator field by inserting a simple steel helix inside a pulsed solenoid. Simulations confirm that the particle attraction can provide a powerful and narrowband radiation of the bunch with electron energy (5-6) MeV and duration 0.3 ps at the frequencies up to 3 THz. SHORT BUNCHES IN MICROUNDULATORS The first opportunity may be realized if bunches with duration of about of 100 fs or even shorter are formed at the entrance of a radiation section. In this case, one can use a mm-period undulator (microundulator) and produce the radiation with the frequency up to 10 THz and higher. A small undulator period provides a relatively narrowband radiation at comparably short radiation length where the longitudinal particles expansion is not too large even at very high bunch charges if the corresponding energy chirp is also used. For such situation, we propose a helical undulator in the form of a set of a helically spaced magnet block interspaced with a preliminarily non-magnetized steel helix; such a set with helically periodic elements being inserted into a solenoid redistributes its field adding the required helical component in it (Fig. 1). The magnet block should be permanently magnetized toward the solenoid field. This method was successfully developed for a plane prototype in [4]. A strong solenoid field prevents also the transverse particles expansion. According to simulations on the basis Microwave Studio code the optimized undulator systems of such a kind with the ratio of longitudinal helix thickness to the separations 1:2 can provide a strong transverse helical field with the amplitude up to (0.7-1.0) T at an acceptable gap-period ratio 1:3 (Table 1). Such fields provide, in particular, a sufficiently large undulator parameter K=0.3-0.4 at the small period of (4-5) mm. Simulations of Coherent Spontaneous Undulator Radiation of an electron bunch in a combined magnetic field of the solenoid and steel and preliminarily magnetized helical insertions was carried out on the basis of a self-consistent one-dimensional model of the bunch in the form of a charged plane layer using simple formulas for the field of an arbitrary moving charged plane [5]. Such one-dimensional simulations were used for estimations of interaction and radiation from thin disc-like electron bunches with the following parameters: charge (50-200) pC, radius 0.4 mm, duration (50-100) fs, energy 5.5 MeV and a large energy chirp (0.3-1.0) MeV moving in a waveguide mounted into the undulator with the length (5-10) cm. According to calculations one may expect to obtain in such situation narrowband picosecond pulses with the radiation frequency up to (10-20) THz and energy up to (0.10.4) J. Figure 1: Microundulator for a source of Coherent Spontaneous Radiation of a dense electron bunch with the frequency up to (10-20) THz consisting of a solenoid and insertions in the form of a magnetized block and a nonmagnetized steel helix. ___________________________________________ #nezahb@ariel.ac.il Proceedings of FEL2015, Daejeon, Korea MOP009 FEL Theory ISBN 978-3-95450-134-2 43 C op yr ig ht © 20 15 C C -B Y3. 0 an d by th e re sp ec tiv e au th or s