Generation of Intense Free-Electron Laser Radiation in the Terahertz Regime
Y. Pinhasi, Y. Lurie, G.A. Pinhasi and A. Friedman
Ariel University, Ariel, Israel
Received: May 21, 2015
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Free-electron lasers are high power radiation sources that utilize a distributed interaction between an accelerated electron beam and the electromagnetic field. In these devices, the electron beam serves as the amplification medium generating electromagnetic radiation, while propagating in a periodic magnetic structure called "wiggler" or "undulator". When electrons pass in the wiggler, they oscillate and act as a moving dipole emitting a wave packet of undulator synchrotron radiation. Incoherent summation of the wave packets results in a spontaneous emission. When the electrons are bunched into a short pulse, they all emit their wave packets in the same phase. The radiation wave packets combine coherently, resulting in super-radiance (where the energy radiated is proportional to the square of the electric charge). Such short bunches can be generated by an RF linear accelerator, driven by a photocathode injector. The radiation wavelength is determined by the velocity of the electrons and the spatial period of the undulator. The super-radiance mechanism enables the generation of intense radiation in frequency bands, whereas conventional sources fail to produce a powerful coherent radiation. In this article, we describe the design and analysis of ultrashort pulse free-electron laser operating at the sub-millimeter and terahertz regimes. The free-electron laser is based on a magnetostatic planar wiggler, in which super-radiant emission is emanated by accelerated electron bunches. A three-dimensional, space-frequency theory is developed in order to study radiation excitation in the wiggler. The total electromagnetic field (radiation and space-charge waves) is presented in the frequency domain as an expansion in terms of transverse eigenmodes of the (cold) cavity, in which the field is excited and propagates. The mutual interaction between the electron beam and the electromagnetic field is fully described by coupled equations, expressing the evolution of mode amplitudes and electron beam dynamics. The approach is applied in a numerical particle code WB3D, simulating wide-band interaction of a free-electron laser operating in the linear and non-linear regimes. The model is utilized to study spontaneous and super-radiant emissions radiated by an electron bunch at the sub-millimeter regime, taking into account three-dimensional space-charge effects emerging in such ultrashort bunches.

DOI: 10.12693/APhysPolA.128.315
PACS numbers: 41.60.Cr, 52.59.Rz