CHARACTERISTICS OF INVERSE COMPTON X-RAYS GENERATED INSIDE THE NIJI-IV FREE ELECTRON LASER OSCILLATORS*

Inverse Compton X-rays were obtained during lasing of the NIJI-IV free electron laser (FEL) in the infrared (IR) range. The position of the Compton collisions between electron bunches and laser pulses inside the FEL oscillator clearly influenced the energy spectrum of the Compton X-rays. Collisions outside the undulator magnetic field led to a sharp and clear Compton peak, while collisions inside the undulator magnetic field made it obscure. The maximum X-ray energies were 0.7 – 2.1 MeV for the FEL wavelengths of 2.6 – 0.88 μm with a fixed e-beam energy of 310 MeV. Relative energy spread was observed to be 11 % for 1.2- MeV Compton X-rays with a lead collimator of 10 mm in bore and 100 mm in thickness, when the collision point was outside the undulator magnetic field. X-ray yield was of the order of 10 6 photons/s when the NIJI-IV was operated in a threebunch mode. Detailed characteristics of the inverse Compton X-rays are discussed in various Comptoncollision conditions.