A focussing funnel for metastable helium

We have constructed a magneto-optical funnel for He atoms and studied its p roperties using a laser cooled, highly mono-energetic atomic beam. A simple model of its action allows us to quantitatively understand the observed sp ot size and "focal length". We show that for a fast beam, the velocity damp ing coefficient plays an important role in determining the focal length of the device. The observed spot size is limited mainly by transverse heating processes which impose a transverse velocity spread. The device also permit s easy scanning of the focussed spot.