In a helium plasma, a variety of excited states can be populated by electronic impact. The rf discharge used for optical pumping, OP, promotes of a small fraction of the atoms (a few ppm) to the metastable 2³S state. A very high nuclear polarisation can be obtained in ³He gas by MEOP (hyperfine OP of 2³S-2³P transition at 1083 nm and metastability exchange during binary collisions between metastable and ground state He atoms), with major applications in several fields: gas probe for lung MRI, polarising spin filters for neutron beams, polarised targets for high energy physics, investigation of new fundamental spin-dependent interactions, study of polarised Fermi liquids, high resolution NMR magnetometry, etc.
Our latest systematic investigations on MEOP have focused on the fundamental limits of this technique in standard conditions (room temperature, p ≈ 1 mbar, B ≈ 1 mT) as well as on the potential benefits of operation in non-standard conditions (10 < p < 400 mbar, 0.1 < B < 4.7 T).
In pure ³He gas, we have systematically observed that the maximum nuclear polarisation achieved in steady-state is lower than expected and, indeed, limited by a strong enhancement of the angular momentum loss rate at high pump light intensity. Read more in a conference proceeding [M. Batz et al, J. Phys. : Conf. Series 294 (2010) 012002 – 21 pp. – “Fundamentals of metastability exchange optical pumping in helium” and find adescription of more recent findings in a co-authored review paper [T. R. Gentile et al, Rev. Mod. Phys. 89, 045004 (2017) – 59 pp. – Optically polarized ³He]
A primary challenge is to elucidate the physical process(es) causing very strong pumping-induced additional polarisation losses. Dedicated low-field investigations were undertaken to study two potential sources of relaxation, both enhanced by 2³S-2³P excitation: 1/ The presence of metastable helium dimers, He2*, in the rf discharge (B. Glowacz, PhD thesis 2011), 2/ The re-absorption of 1083 nm fluorescence light by the metastable atoms, i.e., radiation trapping (2011 internships). None could not account for the observations (Batz, PhD thesis 2011).
Current work on MEOP focuses on collisional excitation transfer in He gas discharges (A. Dia, PhD thesis 2021) and its potential impact on MEOP efficiency. Accurate characterization of the collisional redistribution of atoms has been achieved by laser absorption spectroscopy. Qualitative evidence of 1S-2³P excitation transfer as well as an estimate for the excitation transfer cross-section have been obtained. Further measurements of collisional rates are planned, in isotopic mixtures as well as in pure ³He gas. A reliable measurement of the excitation transfer cross section and comparison with ab initio computations [Vrinceanu et al, 2010] are needed. Impact on low-field MEOP dynamics must be established.
The objective is a better understanding and modeling of MEOP. This would lay the ground for the development of improved tools and contribute to a rapid development of new low- and high-field applications of hyperpolarised ³He.
New challenges are provided by prospects for use of ³He magnetometry for high resolution mass spectrometry in ion traps. This will require in-situ polarisation and sensitive detection at low gas pressure, operation of MEOP at both high field (7 T or more) and low temperature (down to a few K). Extension of high field MEOP down to cryogenic temperatures will be tested in the super-wide bore magnet of the 7-T NMR spectrometer/imager which has been purchased thanks to the WideNMR collaborative project (2016-2018) and implanted at CEA Saclay.
A new, laser-free, mechanism for hyperpolarisation of ³He nuclear spin has been discovered. This opens broad new lines of investigation of atomic processes up in rf He gas discharges at high magnetic field. A scenario has been tentatively proposed to explain discharge-induced hyperpolarisation of ³He. [A. Maul et al, Phys. Rev. A (2018) 98, 063405 – 12 pp., Nuclear hyperpolarization of ³He by magnetized plasmas]
Current work on PAMP
– In-depth investigation of PAMP at high magnetic field is included in the collaborative HELPING project (2021-2025, LKB and LDSRM/IRAMIS/CEA Saclay, ANR funding).
– Investigation of PAMP at moderately low magnetic field (0.1T) is performed at LKB.
– NMR measurements of PAMP-induced nuclear orientation are performed in Kazan (MRS Lab) on ³He gas samples in 3.66 T [Makarchenko et al, Instrum Exp Tech 64:911–916 (2021), Phys. Rev. A 106, 023101 (2022)].
Short internship projects
Motivated and talented students are always welcome. Application letter should be addressed to the team leader, together with a detailed CV and relevant supporting material. Project duration: 2 – 3 months.
M2 internship projects (atomic physics and optics)
The student will carry out experimental work in ³He gas discharges at moderate magnetic field strength (0.1 T) in connection with new research projects that will shortly be launched at much higher field.
- Polarisation of Atoms in Magnetised Plasmas (PAMP):
Work will aim at demonstration and investigation of discharge-induced nuclear polarisation at 0.1T. Spectroscopic measurements with probe lasers will be performed in pure ³He gas and in isotopic gas mixtures. Polarisation buildup and decay will be monitored as a function of relevant experimental parameters, including the magnetic field strength.
- Metastability Exchange Optical Pumping (I): MEOP of isotopic mixtures
Work will aim at quantitative assessment of the efficiency of laser-induced polarisation of ³He in isotopic mixtures at 0.1 T. Metastability exchange optical pumping will be investigated in He gas discharges at various atom number densities and ³He concentrations. Results will be compared to those obtained in pioneering studies of ³He-4He MEOP carried out at millitesla field strengths.
The student will use a dedicated measurement setup as well as tools for rf excitation and optical polarimetry that have been developed in prior internship work.