Cumulative Ionization in Optically Pumped Helium Discharges: A Source of Polarized Electrons

Polarization analysis of electrons extracted from a weak helium discharge in which the $2{}_{}{}^3{}_{}{}^{}S_1^{}{}_{}{}^{}$ metastable atoms are spin oriented by optical pumping indicates that the reaction $\mathrm{He}\left(2\mathrm{}{}_{}{}^3{}_{}{}^{}S_1^{}{}_{}{}^{}\right)+\mathrm{He}\left(2\mathrm{}{}_{}{}^3{}_{}{}^{}S_1^{}{}_{}{}^{}\right)\to \mathrm{He}\left(1\mathrm{}{}_{}{}^1{}_{}{}^{}S_0^{}{}_{}{}^{}\right)+{\mathrm{He}}^{+}+e^{-}$ is the predominant source of ionization in the discharge. The reaction conserves spin angular momentum and therefore produces polarized electrons. The process has been exploited to produce continuous beams of electrons with polarization as high as 10%. For a source-gas pressure of 0.035 torr, a continuous 4-μA beam with 8% spin polarization has been realized. Electrons extracted from the late afterglow of a pulsed discharge have 17% polarization at an optimum pressure of 0.1 torr.