Abstract : Metastability exchange optical pumping of helium-3 usually operates at low pressures. This is not well suited for the production of a highly polarised and dense gas of helium-3 required in particular for lung MRI applications. We show that an improvement results from performing the optical pumping at higher pressures in the presence of a sufficiently high magnetic field.
We first point out the importance of correlations between atomic velocity and nuclear orientation for excited atoms in the plasma which are coupled to the 1083 nm optical pumping light. We have studied the effect of these correlations on optical pumping efficiency through systematic experimental measurements in conjunction with a detailed model.
We then focus on non standard conditions for optical pumping in high magnetic field and high pressure. Here we performed a detailed spectroscopic study of Zeeman effects on the 1083 nm transition of helium. It gave a precise
identification of positions and intensities of all transitions, and in turn allowed us to we develop a new optical measurement method of nuclear polarisation in arbitrary field. We have demonstrated that the presence of
intense 1083 nm light significantly increases the population of helium metastable molecules in the plasma, in particular at high pressure.