Modelling of a Seeded Supersonic Jet Cold Atom Source

We present a Monte-Carlo simulation of the capture of lithium atoms by a supersonic helium jet. We have calculated fully quantum, differential, lithium-helium scattering cross-sections for use in this simulation. The seeding conditions are similar to those of a new cold atom source we are developing. This source contains a thermal atomic lithium beam directed into the helium jet at a distance of about 1 cm from the nozzle. The nozzle is cryogenically cooled to approximately 5 K to reduce the forward jet velocity. Lithium atoms thermalize with the helium jet, and are then extracted from the jet with a magnetic lens. This experimental approach allows us to reach milliKelvin temperatures in the moving frame, which would not be possible with a pure lithium expansion. A further simulation is then carried out, tracing the atoms through the focusing magnet. Simulation results show reasonable agreement with the experimentally observed focused lithium flux of up to 2 × 10¹² s^-1  and spot size of 3 to 5 mm. We will also discuss the use of these simulations for further optimization of our cold atom source.