Backward Compton scattering of picosecond CO₂ laser pulses using relativistic electron beam for the bright X-ray generation

Laser-Compton scattering can generate X-rays or y-rays with high brightness by applying highpeak- power laser pulses to relativistic electron bunches. We have proposed laser-Compton scattering to generate polarized y -rays which can induce pair-creation to produce polarized positrons for JLC (Japan Linear Collider) project[ 11. As a photon source of the Compton scattering, a picosecond CO₂ laser system is going to be applied for our experiment because of the higher scattering cross section than near-infiared solid-state lasers, laser power scalability, the laser efficiency and the possibility of high-repetition rated operation. Not only for the JLC positrons source, Compton scattering with Ca-laser has a possibility to be a compact high intensity X-ray source as an alternative of synchrotron radiation source[2]. We have been developing a laser-Compton scattering chamber for a preliminary experiment, in which COz laser pulses and electron bunches propagate along the same axis in opposite direction and collide at the focal point. Fig. 1 is the sketch of the Compton chamber to be applied in the beamline of the Accelerator-Test-Facility in Brookhaven National Laboratory. Picosecond CO₂ laser beams are introduced &om the side window and focused by an off-axis parabolic Cu mirror of 50 mm diameter with the focal length of 15 cm. Cu mirror has 5 mm diameter hole along the electron beam axis to have electrons pass through. An axicon telescope system made of ZnSe is placed on the CO₂ beam axis just before the Compton chamber in order to modify spatial profiles as shown in Fig. 2-(a) to accommodate to the mirror with holes. Fig. 2-(b) represents a numerically calculated CO₂ beam profile at the focal point. The estimated beam waist size is about 100 pm, which is almost same as the electron beam size at the colliding point. After the focal point, the diverging laser beams are paralleled by another Cu mirror and extracted fiom the chamber.