Upgrade of the microtron-FEL injector for deep variation of the FIR radiation wavelength

To make the compact FIR FEL as a convenient source for different applications including investigation in surface science, solid-state physics, etc., the microtron-injector has been upgraded to provide variation of the energy by the extraction of the beam from 9-th ÷ 12-th orbits. This permits to vary FIR FEL wavelength from 100 μm to more than 300 μm. The microtron RF system including the RF cavity, RF-line elements and the magnetron has been strongly mounted on the movable frame of the stage. A flexible bellows having ∼170 mm stroke provides vacuum sealing at the microtron with the movable RF cavity. The efficiency of the beam extraction, has been measured to optimise the RF cavity position and position of the extracting channel for each maximum orbit number at a constant magnetron power. Basing on the dispersion relation, the FIR FEL first harmonic wavelength is calculated as a function of the undulator field strength.