Metastable optical pumping of 3He for neutron spin filter cells at ISIS

We report on developments in building a compact neutron spin filter based on metastable optical pumping. The apparatus incorporates a high-power Yb-fibre laser, and polarisations of >70% are now routinely achieved in sealed cells prepared at ISIS. A circulating system has also been constructed and initial results with polarised gas are reported. Central to the project is a compact polarisation-preserving compressor and progress on the development of two devices is reported.