Investigation of Removal of $^{3}$He from Liquid $^{4}$He Solution for the Neutron Electric Dipole Moment Measurement

The measurement cycle for the proposed experiment to measure the neutron electric dipole moment at the SNS includes the injection and removal of polarized $^{3}$He, which is used as a comagnetometer in the same 15 liters of superfluid $^{4}$He which trap the ultracold neutrons. A critical part of the process is the removal of $^{3}$He atoms at the end of data collection, reducing the $^{3}$He concentration from 10$^{-10}$ to 10$^{-12}$ in a period of 100-200 seconds. It is proposed to accomplish the task via diffusion of the $^{3}$He from the target cell to an evaporator which preferentially removes $^{3}$He vapor. The efficiency of the process is strongly sensitive to the temperature dependent diffusion rate and vapor pressure of $^{3}$He as well as the superfluid film flow in $^{4}$He. We describe the design of this process and initial results from a prototype evaporator implemented at NC State University.