Grigori M. Kazakevitch

Bunching properties of a classical microtron-injector for a far infrared free electron laser

Abstract Longitudinal bunching properties of a classical microtron have been investigated by the numerical simulation of the longitudinal motion of accelerated electrons. The simulations were performed for the 12-turn microtron that has been used as an injector for the KAERI far infrared free electron laser. Based on the bunching properties of the electron beam, the temporal distribution of the coherent undulator radiation power during a macro pulse from the free electron laser was calculated. In the calculations, we took into account the dispersion properties of the accelerating cavity and deviations of the bunch repetition rate that were measured by the heterodyne method in real operating conditions of the microtron. The calculation results are compared with the experimental data.

Magnetron driven classical microtron as an injector for a wide band tunable compact far infrared free electron laser

A compact 12 turn classical microtron driven by a 2.8 GHz magnetron has been improved for use as an injector of a compact wide-band far infrared (FIR) free electron laser (FEL). The microtron provides an accelerated beam current of up to 70 mA by total energy of up to 7.2 MeV in a 5.5 /spl mu/s duration macro pulse. The frequency of the magnetron was stabilized in the range of (3-4)/spl times/10/sup -5/ by the frequency pulling of the magnetron with the reflected wave from the accelerating cavity. The measured energy spread and emittances in vertical and horizontal components of the electron beam were less than 0.4%, 1.5 mm/spl middot/mrad and 3.5 mm/spl middot/mrad, respectively. A measured temporal deviation of the bunch repetition rate of less than 120 kHz could be obtained by optimization of the microtron operating conditions for the FEL. With a total energy of electrons and macro pulse current of 7 MeV and 45 mA respectively stable FIR lasing could be obtained in the wavelength range from 97 to 150 /spl mu/m by changing the undulator K-parameter. The accelerating cavity moving system was upgraded to increase the variable range of the electron beam energy from 4.3 to 7 MeV, which provides wide tuning range of the FIR FEL wavelength up to 300 /spl mu/m.

A high-precision electromagnetic undulator for a compact FIR FEL

A high-precision electromagnetic undulator has been developed for a compact FIR free-electron laser (FEL) driven by a magnetron-based microtron. Permanent magnets are placed between iron poles to compensate for the high-flux magnetic field generated by the main coils. The period of the undulator is 25 mm and the number of periods is 80. We can change the K-parameter from 1 to 1.6 (Bw ¼ 4:526:8 kG at a fixed gap of 5.6 mm) in order to cover the FEL wavelength range from 100 to 300mm at an electron energy of 6.5–4 MeV. The undulator has a modular structure of iron poles and a novel correction scheme of field distribution to improve significantly the accuracy of the field distribution. The r.m.s. deviation of the field distribution was improved from 0.2% to 0.05% with the correction method. The undulator shows considerable advantages in performance, compactness, cost effectiveness, and simplicity in manufacturing and construction. r 2002 Elsevier Science B.V. All rights reserved. PACS: 07.55.Db; 41.60.Cr

Start-up of lasing in compact far-infrared free-electron laser driven by 8-MeV microtron

We have developed a compact Far Infrared Free Electron Laser (FIR FEL) on the base of 8 MEV microtron, which can provide 6 microsecond(s) up to 70 mA macropulse beam current. Beam line consists of bending and steering magnets. Optical Transition Radiation screen and 3 doublets of quadruples to provide matching of microtron output electron beam parameters with optimized parameters for FEL operation. A 2 m length, 25 mm period and 5.7 mm gap undulator has extremely low field error of 0.05% for the magnet field peak amplitude in the range 5.5 - 6.5 kG. Inside the undulator the electron beam passes through 2 mm X 20 mm aperture 2779 mm length planar waveguide. On both sides of the waveguide are installed two cylindrical 3 m curvature mirrors. They form confocal type free space mode in horizontal plane and waveguide mode in vertical plane of the optical resonator. A hole in output mirror provides coupling ratio about 1 - 2%. During experiments we have observed the coherent effect in spontaneous emission with power enhancement approximately 1000 times. It was observed too the long wave radiation, which can be explained by vertical betatron oscillation of electrons in undulator. The experiments with FIR resonator and Ge-Ga liquid He cooled detector shown coherent generation and tunability of it power with tuning of the length of the resonator. The enhancement of outcoupled signal in 50 times was detected with changing of resonator length. This phenomenon can be explained as startup of the lasing process in FIR resonator.

Sideband instability in the compact THz free-electron laser at KAERI

A frequency offset of the Sideband instability has been observed in a compact waveguide-mode terahertz (THz) free-electron laser (FEL). The spectra of the FEL pulses were measured by using a Fabry-Perot spectrometer having a resolution of 10−4 of the central wavelength at 2 − 3 THz range. The shift of the sideband was measured to be 0.5 − 1.2 μm, depending on the FEL wavelengths from 110 to 165 μm. An increase in the sideband shift for a longer wavelength could be explained by a change in the wave’s group velocity in a plane-parallel waveguide. Mode competition between the sidebands and the primary wave was observed by changing the cavity length of the FEL. We could decrease the number of modes and reduce the linewidth of the spectra by controlling the cavity detuning. We discuss the dependence of the complexity of the sideband instability on the FEL wavelengths and its gain characteristics.

Application of a Compact FEL on THz Imaging

We have developed a compact terahertz free electron laser operating in the 100-1200 μm range. In this paper we will show and discuss the main results of THz imaging by using the KAERI compact FEL.