Polarized Electron Source

A longitudinally polarized electron beam may be created by irradiating a GaAs semiconductor with a circularly polarized laser beam. The energy of the laser's photons must be slightly greater than the band gap energy in the GaAs for photoemission to occur. Angular momentum conservation is responsible for the electron polarization since the P(3/2) valence band electrons are promoted to the S(1/2) conduction band by the circularly polarized photons in the ratio 3:1. Therefore, the maximum polarization possible in a conventional GaAs semiconductor is P = (3-1)/(3+1) = 50%. Experimentally, the electron polarization is always below this limit due to excitation of the conduction band electrons.

More recently, strained GaAs semiconductors have been shown to produce polarizations well over 50%. The strained lattice removes the degeneracy of the m(j) = 3/2 and m(j) = 1/2 levels of the P(3/2) valence band states and theoretically allows for 100% electron polarization.

The figure below shows the electron polarization for different types of semiconductors as a function of laser wavelength. In 1992 the SLC ran with a bulk GaAs cathode and a laser wavelength of 715 nm. This produced an electron beam with ~28% polarization. In 1993 a new strained GaAs cathode was used. For the first portion of the 1993 run the laser wavelength was 850nm which produced a polarization of ~55-60%. For the remaining 80% of the 1993 run the laser wavelength was 865 nm and produced an electron beam polarization of~62-67%. These results are shown in the figure below. The photo-emitted electrons are accumulated into a bunch by a 178 MHz RF field and then accelerated through a high-gradient electromagnetic filed to an energy of 50 MeV. The figure below shows the layout of the polarized electron gun and laser. The laser strikes the cathode two times per 120 Hz machine cycle. One bunch of electrons will eventually collide with a positron bunch at the interaction point. The other electron bunch is used to create positrons. Typically, between 60 and 100 billion electrons are liberated from the cathode per pulse.

After reaching an energy of 50 MeV the electrons enter the first section of the linear accelerator (LINAC) where they, along with one positron bunch from the positron target, are accelerated to an energy of 1.19 GeV. The electrons and positrons are then diverted from the LINAC into two different damping rings.