Closing on: Sep 30, 2021

Group leader: Geneviève TASTEVIN and Pierre-Jean NACHER
Tel: 01 44 32 20 25 and 01 44 32 34 28

Web page: http://www.lkb.upmc.fr/polarisedhelium/polarised-helium-and-quantum-fluids/
Laboratory location: ENS – 24, rue Lhomond – Paris 5e

Scientific project:

Field-dependent polarisation processes in helium plasmas

Nuclear spin-polarised ³He gas currently produced by laser optical pumping has extensive scientific or practical applications. These include pre-clinical lung MRI, spin filters for neutron beams, targets for high energy physics, nuclear magnetometry, etc. [1]. The growing needs of end-users constantly drive new efforts to boost up nuclear polarisation, gas density, or production rate, and to extend the range of operating conditions.

Metastability exchange optical pumping, MEOP, yields very high ³He nuclear spin polarisation (up to 90%) in “standard” conditions: room temperature, low gas pressure, low magnetic field. The high efficiency of MEOP (typically, 1 polarised nucleus per absorbed photon) relies on two processes that involve a minority of atoms promoted in the metastable 2³S level by a weak rf discharge: laser-driven OP cycles on the closed 2³S-2³P optical transition at 1083 nm and polarisation transfer by ME collisions between 2³S and ground state atoms (a quick binary exchange of electronic excitation with no change of nuclear spin orientations).

We have also shown that MEOP remains very efficient in “non-standard” conditions involving high-pressure gas and high magnetic field (up to 4.7 T), a remarkable result when strong hyperfine decoupling prevails. Alternatively, the recently discovered PAMP scheme (Polarization of Atoms in a Magnetized Plasma) for hyperpolarizing 3He without laser OP has raised questions regarding the reasons for its unexpected high efficiency and opened prospects for new applications [2]. 

PhD project: In the frame of a new research project focused on high-field hyperpolarisation of noble gases, experimental work will combine optical and NMR studies [3]. Optical polarimetry on 2³S and 2³P He atoms will be performed in radiofrequency gas discharges at various magnetic field strengths, in samples of pure ³He gas and of ³He-4He gas mixtures. This will allow time monitoring of ³He nuclear polarisation build-up and decay in the atomic ground state and detailed characterisation of all relevant processes in excited states, both with applied OP or in OP-free PAMP situations. Development of suitable models for the involved processes, including alignment-to-orientation conversion in He at moderate magnetic field, is expected to provide key clues to understanding (and hopefully bypassing) current limitations of both MEOP and PAMP.

Start date: Fall 2021 – As soon as possible.

[1] Rev. Mod. Phys. (2017) 89:045004 –  https://arxiv.org/abs/1612.04178

[2] Phys. Rev. A (2018) 98:063405 –   https://arxiv.org/abs/1806.07624

[3] HELPING project home page   

Read more: http://www.lkb.upmc.fr/polarisedhelium/polarised-helium-and-quantum-fluids/meop/

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