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 pumpingOverview of team's activitiesAperçu des activités de l'équipe
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.
A 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. They 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).
After a 3-year shutdown for renovation work in the Physics building, a new lab is available for further experimental work on MEOP.
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.
Today, prospects for use of ³He magnetometry for high resolution mass spectrometry in ion traps provide new challenges: efficient 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).
A new collaborative project (WideNMR, 2016-2019) offers the opportunity to extend the tests of high field MEOP down to cryogenic temperatures.
Short internship projects
Motivated and talented students are always welcome.
January – July 2019: Interns are invited to join the on-going PhD work on low field MEOP of ³He (2017-2020), which currently aims at investigation of the collisional processes involving the excited 2³P state in He gas discharges.
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.