Nuclear hyperpolarization of He-3 by magnetized plasmas

We describe a method, referred to as PAMP (polarization of atoms in a magnetized plasma), that allows hyperpolarization of He-3 nuclear spins at high magnetic field solely by excitation of a rf gas discharge. A magnetized plasma is obtained when the mean free path of the free electrons is much larger than their gyration radius in the rf gas discharge. Investigations of PAMP are carried out in the 1-15-mbar pressure range with rf excitation around 100 MHz. Quantitative NMR measurements at 4.7 T and room temperature show that, for different cell sizes and gas densities, He-3 nuclear polarizations in the 1 to 9% range are achieved (i.e., larger than the Boltzmann equilibrium spin polarization of the free electrons). A description involving alignment-to-orientation conversion in the excited 2(3)P state is proposed. The PAMP method appears as a very attractive alternative to established laser polarization techniques (spin exchange or metastability exchange optical pumping). Application to He-3 nuclear magnetometry with a relative precision of 10(-12) is demonstrated.