Multiple-Collision Free-Electron Laser Compton Backscattering for a High-Yield Gamma-Ray Source

We observed multiple-collision free-electron laser (FEL)-Compton backscattering in which a multi-bunch electron beam makes head-on collisions with multi-pulse FELs in an optical cavity, using an infrared FEL system in the storage ring NIJI-IV. It was demonstrated that the measured spectrum of the multiple-collision FEL-Compton backscattering gamma rays was the summation of the spectra of the gamma rays generated at each collision point. Moreover, it was demonstrated that the spatial distribution of the multiple-collision FEL-Compton backscattering gamma rays was the summation of those of the gamma rays generated at each collision point. Our experimental results proved quantitatively that the multiple collisions in the FEL-Compton backscattering process are effective in increasing the yield of the gamma rays. By applying the multiple-collision FEL-Compton backscattering to high-repetition FEL devices such as energy recovery linac FELs, an unprecedented high-yield gamma-ray source with quasi-monochromaticity and wavelength tunability will be realized.