Upgrade of a compact FIR FEL driven by a magnetron-based microtron for the wavelength range of 100–300 μm

A compact far infrared (FIR) free-electron laser (FEL) driven by a magnetron-based microtron has been upgraded to extend the wavelength range from 100–160 μm to reach 300 μm. The energy of the electron beam from the microtron can be varied from 6.5 to 4.3 MeV by changing the position of an RF cavity inside the microtron. We have extracted a macropulse current of the electron beam of more than 50 mA for 5.4 MeV, and the signals in the wavelength range of 160–280 μm have been observed with the electron beam. The radiation is transported to an experimental stage through a 10-m-length vacuum pipe with a collimating lens and gold-coated mirrors. The measured spatial distribution of the transported radiation shows a small diverging angle less than 1 mrad and spot size (FWHM) less than 20 mm. The pulse width and macropulse power of the lasing signals measured at the experimental stage are approximately 3 μs and 10 W, respectively. The main features of the FEL have been investigated in this report. The possibility of the FEL covering the wavelength range up to 1 mm is discussed and investigated by the measured signal of the coherent radiation. The application experiments of the FEL to the FIR imaging, molecular spectroscopy, and solid-state physics are under preparation.