The Polarized 3He Target for the JLab SBS GEn Experiment, E12-09-016

The Super Bigbite Spectrometer (SBS) program at JLab will measure the elastic nucleon form factors at high momentum transfer, including a measurement of the neutron electric form factor GEn using the double-polarization technique with both a polarized electron beam and a polarized ³He target. In order to obtain adequate statistics, the polarized ³He target will operate with a polarization-weighted luminosity approximately a factor of three above any previous polarized ³He target used in an electron-scattering experiment. The target itself consists of a double-chambered glass cell, filled to approximately 7 atm of ³He at room temperature, with a long cylindrical "target chamber" 60 cm in length. In order to maintain high polarization with electron beam currents up to 60uA, the target cells contain roughly 6 STP liters of ³He, twice the quantity of ³He used in previous targets at JLab. This cell is polarized using alkali-hybrid spin-exchange optical pumping using up to 200W of near-infrared light from high-power diode-laser arrays. The target system used in the experiment includes multiple Helmholtz coils which create a magnetic “holding field” that determines the polarization direction. The system also includes another set of coils, the RF coils, used for NMR, and components for performing EPR spectroscopy that are used for absolute polarization measurements. To shield against stray magnetic fields associated with the SBS system’s magnets, a large soft-iron enclosure surrounds the target system. In addition to a description of the SBS GEn target system, data from bench tests of individual ³He target cells will be presented.