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

Abstract

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 µs duration macro pulse. The frequency of the magnetron was stabilized in the range of (3-4)×10-5 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⋅mrad and 3.5 mm⋅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 µm 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 µm. Microtron magnetic-vacuum system.A 12-turn microtron-FIR FEL injector developed on the basis of its prototype [1] has an unusual 750 mm diameter and 267 mm high chamber-less magnetic-vacuum system in which the vacuum volume is composed of magnet poles and the back magnet yoke. Magnet coils are sealed with extended surface of magnet poles to back yoke by turning from the same metal block. Thus only one indium wire seals the magnetic-vacuum system of the microtron as whole. Different sizes of microtron magnetic system were calculated by varying the thickness, location, shape of shims, coils geometry and insets cross section to get the maximal diameter of vacuum space with a magnetic field inhomogeneity approximately 0.1%. Measured under microtron operating conditions (the magnetic field was in the range of 0.9-1.1 kG) the value of the magnetic field inhomogeneity for 12 turns was less than 0.2%.The beam-extracting channel was made from low carbon steel. A compensation system having low carbon steel pivots positioned above and below the extracting channel was used to avoid magnetic field distortion near the entrance of the channel. The extraction efficiency measured under this condition was approximately 90%. RF -system of microtron.The microtron RF-system was designed to provide the intra-pulse frequency stability for FIR FEL operation. A 2.8 GHz, 2.5 MW pulse power MI-456A magnetron fabricated in Russia was chosen as the RF generator. The wave-guide line of the microtron (Fig. 1 (a)), including a ferrite valve with back losses approximately 15 dB by 2 MW pulse power, a directional coupler for measurements of the forward and reflected waves and vacuum waveguide window, was optimized for size to serve at the same time for stabilization of the magnetron frequency by the frequency pulling.The microtron RF-system in...