Single–pass Laser Polarization of Ultrarelativistic Positrons

The new method for producing of the polarized relativistic positrons is suggested. A beam of unpolarized positrons accelerated up to a few GeV can be polarized during a head-on collision with an intense circularly polarized lazer wave. After a multiple Compton backscattering process the initial positrons may lose a substantial part of its energy and, as consequence, may acquire the significant longitudinal polarization. The simple formulas for the final positron energy and polarization degree depended on the laser flash parameters have been obtained. The comparison of efficiences for the suggested technique and known ones is carried out. Some advantages of the new method were shown. The experiments with polarized electron-positron beams in future linear colliders will furnish a means for studying a number of intriguing physical problems [1]. While the problem of generation and acceleration of longitudinally polarized electron beams seems to be solved [2], the approach for production of polarized positron beams with the required parameters has not been finally defined yet. In [3-7] methods were proposed for the generation of longitudinally polarized positrons during ee– pair production by circularly polarized photons with the energy ω ∼ 10 MeV, which are, in their turn, generated by either passing electrons with the energy ∼ 10 GeV through a helical undulator [3], or through Compton backscattering of circularly polarized laser photons on a beam of electrons with the energy ∼5 GeV [4,5], or through bremsstrahlung of longitudinally polarized ∼50 MeV electrons [6,7]. To achieve the needed intensity of a positron source (Ne+,pol ∼ 10 10 particles/bunch) it is suggested to use an undulator of the length L > 100 m [8], or to increase the laser power [9], or to use a high-current accelerator of polarized electrons [10]. The present paper considers an alternative way to approach this problem. A beam of unpolarized positrons from a conventional source being cooled in a damping ring and preliminary accelerated to an energy E0 can be polarized during a head-oncollision with a high-intensity circularly polarized laser wave. It is well known that during Compton backscattering of circularly polarized laser photons on unpolarized positrons (electrons) with the energy E0 ∼100 GeV the scattered photon takes up to 90% of the initial positron energy while the recoil positron aquires ∼ 100% longitudinal polarization [11,12]. At E0 ≤10 GeV, however, the positron loses too little of its energy during single Compton backscattering (a few percent), and the longitudinal polarization of the recoil positron is, therefore, of the same order of magnitude. Current advances of laser physics make it possible to obtain parameters of laser flash such