Magnetic trapping and evaporative cooling of metastable triplet helium

A cloverleaf magnetic trap is loaded from a magneto-optical trap containing 2 × 10 9 helium atoms in the metastable 2 3 S 1 state. With optical molasses and a spin polarization pulse up to 1.5 × 10 9 atoms are magnetically trapped. The vacuum limited trap lifetime is ∼12 s. Compression in 0.5 s yields a density increase to 8 × 10 9 cm −3 . With rf-forced evaporation using an exponentially shaped frequency sweep with a 5 s time constant the phase space density is increased from 1 × 10 −7 to 4 × 10 −3 . The central density increases to 3 × 10 11 cm −3 , while an increase in elastic collision rate from 40 to 220 s −1 indicates runaway evaporation.