While spin exchange optical pumping is attractive, higher 3He polarisation is presently available with MEOP when optical polarisation is applied to selected excited energy levels in metastable 3He. This process takes place in a discharge at low pressure and a polarisation preserving compression by a factor of a few 1000 has to follow. The compression phase requires some technical effort, and state-of-the-art compressors are still prototypes which are quite complex, but permit the filling of detachable NSF cells in less than one hour (see photo).

Very recently, higher polarisations (> 80%) have been reached in a sealed-off optical pumping cell with the new commercial Yb-fibre lasers delivering a band-width which matches the Doppler broadening in the plasma better. However, when increasing the power of the light, the 3He polarisation goes through a maximum and then decreases. Further investigation of the laser stability and spectrum, and the power dependency of some optical elements is clearly necessary (participants P1 and P6).

A loss of polarisation still results from the flow through type of operation of MEOP filling stations, where the polarisation achieved in the optical pumping volume is significantly lower than in sealed-off cells. Partners P1 and P4 have considerable experience with piston compressors and will take over this part in a very close collaboration when designing a continuously circulating pumping station.

When dealing with very high polarisation rates, the magnetic field misalignment due to the earth-field contribution may lead to the saturation of the optical pumping process. As of now, this effect has not been taken into account but it may be useful determining its amplitude. Partner P5 will investigate this issue when building its compressor, and partner P3 plans a separate experiment to study this dependence in a sealed-off cell in an apparatus capable of arbitrary orientation relative to the earth’s field.

Because a number of instruments would very much benefit from using very large cells which cannot be dismounted easily (e.g. Time-Of-Flight spectrometers), the remote type of operation is not always feasible. Therefore, one has to conceive a compressor with the ability to provide a continuous flow of highly polarised gas. While the performance of the advanced piston compressor developed by P1 is already impressive, several other ideas have been proposed and alternative compressors will be studied (e.g. peristaltic or diaphragm pumps). This part will involve the participants P1, P4, P5, O1, O2 and O3.

Here are the main objectives:

• Investigation of the Yb-fibre laser stability/spectrum, the power dependency of the optical elements and the flow through type of operation with the aim to increase the 3He polarisation,
  
• Development work on absolute 3He polarisation measurement and monitoring at the compressor output,
  
• Establish a continuously circulating MEOP pumping station with provision for testing several novel compressors,
  
• Development and evaluation of novel compressors (peristaltic, etc.),
  
• Investigation of the dependence of the polarisation on the relative orientation of the earth’s field compared to the main field and optical axis,
  
• Development of a compact continuous gas flow MEOP system,
  
• Neutron beam testing of continuous gas flow systems.