POLARISED HELIUM : OPENINGS FOR INTERNSHIPPOLARISED 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.
Internship project 1:
RF discharges in helium gas: optimisation for optical pumping
The metastable 2³S state of He, which lies about 20 eV above the ground state, is populated by electronic collisions. To avoid quenching by residual impurities, high purity gas is used and generally excited by a weak, electrodeless, rf discharge (< 5 W, < 3 MHz). The low abundance (a few ppm) of 2³S atoms is spatially non uniform in the MEOP cells, which usually consist in cylindrical glass containers.
- One objective is to increase the 2³S atom number density, without substantial increase of the nuclear spin relaxation rate.
- Another objective is to achieve a good matching between the pump laser intensity and 2³S atom density transverse profiles. The latter is actually influenced by the rf discharge intensity, as well as by other parameters (e.g., the cell dimensions) and the operating conditions (gas pressure, magnetic field).
Internship project 2:
Collisional redistribution in the excited 2³P state
New tools are being developed to better probe the helium discharge and, in particular, to quantitatively monitor the time evolution of populations in selected atomic (sub)levels during OP. Recently, a widely tunable laser diode has been implemented and tested for absorption-based investigations of collisional mixing between 2³P levels, using the 2³P-3³S atomic transition at 706 nm (2016 internship).
Internship projects (Cont’d)
Internship project 3:
Metastable He dimers: spectroscopy at 465 nm
A tunable blue laser has been developed for absorption-based monitoring of the abundance of He2* dimers in the RF discharge, with and without atomic 2³S-2³P excitation. For absolute number density measurements, detailed knowledge of the rotational structure and of the absorption line shape is needed for the molecular transition probed at 465 nm, a³Σu+(0)-e³Πg(0).
A first series of measurements have been performed in ³He, ⁴He, and isotopic mixtures. The recorded (Doppler- and pressure-broadened) spectra provide a clear evidence of underlying (hyper)fine structures for a majority of rotational components.
The objective is to resolve and to quantitatively analyse the absorption spectrum of ³He2* or of its isotopologues, ⁴He2* and (³He-⁴He)*.
Experimentally, the linear Zeeman effect may be used to separate out and identify the various rotational line components.
The student will use the available pieces of equipment to build a dedicated set up, perform the measurements, and analyse the compiled data.
Internship project 4:
Towards optical pumping of helium-3 at low temperature
Thanks to the WideNMR project, our group will have privileged access to a very wide bore NMR spectrometer. A dedicated cryostat will be built (at LKB) to perform high-field MEOP experiments down to cryogenic temperatures and to develop new optical tools, if needed, in Saclay.
The work may include: implementation of experimental devices for RF discharge and beam control inside a cryostat; absorption measurements and OP at low and high field; bibliographic search for specific technical aspects.
The light sources are commercial solid-state lasers (fibre laser / amplifier, tunable laser diodes). A cryostat as well as full NMR equipment are also available for operational tests at low magnetic field.
The internship will provide an opportunity for hands-on experience with optical techniques for laser OP and in-situ plasma diagnosis in pure ³He gas.