Undulator-based production of polarized positrons

Abstract

a b s t r a c tFull exploitation of the physics potential of a future International Linear Collider will require the use of polarized electron and positron beams. Experiment E166 at the Stanford Linear Accelerator Center (SLAC) has demonstrated a scheme in which an electron beam passes through a helical undulator to generate photons (whose first-harmonic spectrum extended to 7.9 MeV) with circular polarization, which are then converted in a thin target to generate longitudinally polarized positrons and electrons. The experiment was carried out with a 1-m-long, 400-period, pulsed helical undulator in the Final Focus Test Beam (FFTB) operated at 46.6 GeV. Measurements of the positron polarization have been performed at five positron energies from 4.5 to 7.5 MeV. In addition, the electron polarization has been determined at 6.7 MeV, and the effect of operating the undulator with a ferrofluid was also investigated. To compare the measurements with expectations, detailed simulations were made with an upgraded version of GEANT4 that includes the dominant polarization-dependent interactions of electrons, positrons, and photons with matter. The measurements agree with calculations, corresponding to 80% polarization for positrons near 6 MeV and 90% for electrons near 7 MeV.

Multiphoton effects in coherent radiation spectra

“…The only question is whether ω 0 belongs to the gamma-range, for which the calorimetric method of spectrum measurement is pertinent. In the SLAC experiment E-166 [44], ω 0 reached 7.5 MeV, though only under moderate photon multiplicity w 1c = 0.35. For the TESLA design (E e = 250 GeV, L = 135 m, N = 10 4 , K = 1), ω 0 must rise to 25 MeV, and correspond to w 1c ∼ 10 2 [5].…”

“…Therein, the δ-term is suppressed, since W 0 → 0, and for the present case it will be neglected. Once (44) is inserted to Eq. (20), 13 all ω-and E-dependencies factor out after a simple rescaling of the integration variables, giving…”

Brilliant positron sources for CLIC and other collider projects

“…Henceforth, a rotating target with high velocity (100 m/s, tangential) is considered [39]. A proof of principle, at a reduced power, was successfully operated at SLAC (E-166) [40]. Another approach has been recently studied [41].…”

Survey of top quark polarization at a polarized lineare+e−collider

“…When discussing our predictions we shall attempt to explore the whole four-and three-dimensional parameter space for the rate and polarization, respectively. We mention that the beam polarizations envisaged at the ILC are h À ¼ AE90% for electrons and h þ ¼ AE80% for positrons [11].…”