Wide aperture periodic lens system for multiple Compton backscattering

Polarized γ-ray generation by Compton backscattering in a periodic focusing system of electron and laser beams is discussed for the production of polarized positron beam in a linear collider. Circularly polarized CO2 laser beams are focused by an optical lens series and collided with a 5.8 GeV electron beam to generate circularly polarized γ-rays with 60 MeV at a maximum. In the present work, the basic concept of periodic lens system discussed previously is reconsidered to reduce the laser power required for a γ-ray yield of 7×1015 γ/s and the peak laser power density at lenses as much as possible for technical practice. The electron beam is focused by a series of permanent quadrupole magnets with a FODO structure. The power is reduced to six sources with 5.6 kW each, and the peak power density is reduced to 1.4 GW/cm2. These values can be reduced further by using a longer laser pulse length and a damping ring for the electron beam.