CAPABILITIES OF TERAHERTZ SUPER-RADIANCE FROM ELECTRON BUNCHES MOVING IN MICRO-UNDULATORS N. Balal † , V.L. Bratman 1 , Yu. Lurie, and A. Friedman Ariel University, Ariel, Israel 1 also at Institute of Applied Physics, Russian Academy of Sciences, Nizhny Novgorod, Russia Abstract An available frequency range of powerful coherent radi- ation from sub-picosecond and picosecond bunches with high charge and comparatively moderate particle energy of 3 - 6 MeV that are formed in laser-driven linacs signifi- cantly extends if one uses a micro-undulator. Such an un- dulator with a helical symmetry and a high transverse field can be implemented by redistributing a strong uniform magnetic field by a helical ferromagnetic or copper inser- tion. According to simulations and experiments with pro- totypes, a steel helix with a period of (8-10) mm and an inner diameter of (2-2.5) mm inserted in the 3T-field of a solenoid can provide a helical undulator field with the same periodicity and an amplitude of about of 0.6 T. Using a more complex hybrid system with a permanently magnet- ized helical structure can increase this value up to 1.1 T. The necessary helices can be manufactured on the ma- chine, assembled from steel wires, formed from powder placed into a hollow helical shell or 3D - printed. Simula- tions based on the WB3D code demonstrate that using such undulators with the length of (30-40) cm enable single- mode super-radiance from bunches with charge of 1 nC and duration of 2 ps moving in an over-sized waveguide in fre- quency range of 3-5 THz. The calculated efficiency of such process many times exceeds efficiency that can be obtained with short bunches of the same initial density. INTRODUCTION Advanced laser-driven photo-injectors make possible formation of very dense picosecond and sub-picosecond electron bunches with charge of the order of 1 nC and larger at moderate relativistic energy [1–4]. Such bunches can be attractive for simple production of powerful THz electromagnetic pulses using various mechanisms of the so-called coherent spontaneous radiation and super-radi- ance [5-16]. In particular, it is planned to use a coherent spontaneous Doppler-upshifted undulator radiation of bunches whose longitudinal size or period of preliminary density modulation is smaller than the wavelength of radi- ation in first experiments at the Israeli THz source [13]. The modulation can also arise self-consistently in extended bunches during their interaction with the radiated electro- magnetic pulses in process of super-radiance (see, e.g., [10- 12] and literature cited therein). At the fixed electron energy, the radiation frequency of the Israeli source can be obviously increased by decreas- ing of the undulator period. Simple and efficient ways for creation of helical undulators with small periods and strong amplitudes of a transverse magnetic field were inde- pendently proposed in [15-18] and [19-22]. They present modifications of old ideas and based on redistribution of a uniform magnetic field by ferromagnetic [23-25] or con- ducting bodies placed inside a solenoid. When using a small-period undulator for implementa- tion of coherent spontaneous radiation it is necessary to provide a very short initial bunch duration. Because of very strong mutual Coulomb repulsion of the particles in the dense bunch this can be only fulfilled at a very limited length of bunch propagation. Other opportunities are opened when one uses radiation of pre-modulated bunches or super-radiance of extended bunches; in the latter case a self-modulation of density and particle bunching in the field of the radiated wave occur [10-12]. HELICAL MICRO-UNDULATORS BASED ON REDISTRIBUTION OF UNIFORM MAGNETIC FIELD The undulator field with a small period and a large trans- verse amplitude can be created by redistributing the strong uniform field on a periodic ferromagnetic insertion [15-18, 20-22]. This method was successfully demonstrated many years ago in planar systems with periodic planar ferromag- netic insertions [23-25]. In papers [20-22] and [15-18], it has been proposed to use a helical insertion for creation of a helical undulator field. Experiments with helixes includ- ing ones with small periods (Fig. 1a) have demonstrated a satisfactory coincidence with calculations [17]. A steel he- lixes with the period of 8-10 mm and inner diameter of 2- 2.5 mm placed into a strong uniform field of 3 T can pro- vide a helical undulator field with the amplitude of 0.6 T (Fig. 1). The larger amplitude can be obtained using a hy- brid system (Fig. 2). [15] consisting of a steel helix (“bolt”) placed inside a permanently magnetized helical block. To avoid changing the direction and value of the magnetic field of the permanent magnet we performed the calcula- tions for the latter system with a low solenoid field of 1.2 T; the rest parameters (Fig. 1a) are as follows: a=4 mm, R 1 =0.75 mm, R 2 =4 mm, and d=8 mm. According to simu- lations this system can provide the helical undulator field with the amplitude of 1.1 T at the axis. ___________________________________________ † nezahb@ariel.ac.il 39th Free Electron Laser Conf. FEL2019, Hamburg, Germany JACoW Publishing ISBN: 978-3-95450-210-3 doi:10.18429/JACoW-FEL2019-WEP086 Photon Beamline Instrumentation and Undulators WEP086 517 Content from this work may be used under the terms of the CC BY 3.0 licence (© 2019). Any distribution of this work must maintain attribution to the author(s), title of the work, publisher, and DOI