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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 work on MEOP has recently focused on the fundamental limits of this technique in standard conditions (room temperature, p ≈ 1 mbar, B ≈ 1 mT), as well as on its 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 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 current primary challenge is to explain the measured additional loss rate of polarisation and its linear increase with the absorbed 1083 nm laser power. Dedicated investigations have been undertaken. at low magnetic field, so far.
- Studies conducted before a 3-year shutdown (for renovation work, in the hosting Physics building) have lead us to exclude significant contributions from two potential sources of relaxation, both enhanced by 2³S-2³P excitation: the presence of metastable helium dimers, He2*, in the RF discharge (B. Glowacz, PhD thesis 2011), the re-absorption of 1083 nm fluorescence light by the metastable atoms, i.e., radiation trapping (2011 internships).
- Work has restarted in 2016, in the renovated optics lab. On-going studies by A. Dia (PhD work, 2017-2020) focuses on of collisional excitation transfer in He gas discharges and its potential impact on MEOP efficiency at low field.
Our current strategy is to find ways to optimally probe the helium discharge and to quantitatively monitor the time evolution of populations in all electronic states and hyperfine Zeeman sublevels directly or indirectly involved in ³He MEOP.
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 [A. Maul et al, Phys. Rev. A (2018) 98, 063405 – 12 pp., Nuclear hyperpolarization of ³He by magnetized plasmas]. This opens broad new lines of investigation of atomic processes up in rf He gas discharges at high magnetic field.
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.
Topics for January – July 2020:
M1 internship: Laser spectroscopy of He in rf gas discharges (II): nuclear polarisation buildup and decay at 0.1T
M1 internship: Optimisation of operating conditions for high-density or high-field MEOP.
M2 internship projects (2019 – 2020, 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.
- Metastability Exchange Optical Pumping (II): Laser spectroscopy of the first excited triplet energy levels of helium
Recruitment for this position is now closed.
Recruitment of one PhD student is planned: “High-field hyperpolarisation in helium plasmas”
PhD project : “High-field hyperpolarisation in helium plasmas”
A one-page description of the context and the scientific objectives can be found here . Useful links are included.
The project is focused on pioneering investigations of optical and non-optical techniques that allow huge enhancements of ³He nuclear spin polarisation in high magnetic field.
It provides a unique opportunity for getting experience not only with advanced atomic physics and laser spectroscopy, but also with rf gas discharges, cryogenics, or NMR, within ongoing cross-disciplinary collaborative work.
Financial support may be obtained through ANR funding (if granted) or application for PhD contract (EDPIF).