Magnetic trapping and evaporative cooling of metastable triplet helium

A cloverleaf magnetic trap is loaded from a magneto-optical trap containing 2 × 109 helium atoms in the metastable 23S 1 state. With optical molasses and a spin polarization pulse up to 1.5 × 109 atoms are magnetically trapped. The vacuum limited trap lifetime is ~12 s. Compression in 0.5 s yields a density increase to 8 × 109 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 × 1011 cm−3, while an increase in elastic collision rate from 40 to 220 s−1 indicates runaway evaporation.

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