LKB - Polarised Helium, Quantum Fluids and Solids

POLARISED HELIUM : OPENINGS FOR INTERNSHIP AND / OR PhD

Optical measurements and optical pumping in helium plasmas

Metastability exchange optical pumping in exotic conditions – Study of ³He nuclear polarisation loss mechanisms during optical pumping

Overview of team's activitiesAperçu des activités de l'équipe

Context

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 (2016-2019)  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.

Topics for January – July 2019 were:

M1 internship: He laser spectroscopy   – 1 student has been recruited. Position is closed.

M1 internship: infrared laser beam shaping 1 student has been recruited. Position is closed.

M2 internship project (2018 – 2019, atomic physics and optics)

The student will carry out experimental work in ³He gas discharges, at low (mT) or moderate (0.1 T) field strength.

  • Implementation of annular beam shaping with aspherical surfaces. The objective is to boost up MEOP efficency in somewhat extreme operating conditions, as met in our studies of low field MRI (high gas pressure) or high field optical pumping (magnetised plasma). Optical and NMR measurements will be combined for validation.
  • Investigation of alignment-to-orientation conversion in helium at moderate field strength. Spectroscopic measurements with probe lasers will be performed in pure ³He gas as well as in isotopic gas mixtures.

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).