A "heat pipe"-style recirculating lithium oven for entrainment into supersonic helium beams

This thesis presents the work on a new oven design optimized for the entrainment of lithium into a pulsed supersonic beam of helium. The proposed oven does not pollute the vacuum with lithium atoms that are not entrained, but instead is designed to capture and recirculate all atoms that are not carried away by the supersonic beam. The transportation of lithium inside the oven is reminiscent of a heat pipe, coining the nickname of the oven: Lithium is evaporated in a hot reservoir, recondensed at a cold surface, and recirculated to the hot reservoir using capillary action.

The supersonic helium beam is generated by an Even-Lavie valve yielding a cold and collimated flow of gas. The supersonic beam can be further collimated by using a skimmer. Two different heat-pipe oven designs are investigated in this thesis, one for entrainment downstream of the skimmer and on for entrainment upstream of the skimmer.

Downstream of the skimmer, the lithium that is carried in the helium beam behind the oven is much hotter and slower than the carrier gas. This is due to the low density of the helium beam past the skimmer, which does not support enough collisions with the lithium atoms to reach equilibrium.

Upstream of the skimmer, the helium density is higher and the beam behaves in part like continuous flow and the interaction with the oven heats the supersonic beam considerably. The lithium content in the beam that was measured past the skimmer is faster and hotter than the helium flow. It is also not properly entrained, but pushed along by the helium beam.

From these experiments it is obvious, that entrainment depends crucially of the local properties of the supersonic beam at the entrainment site. To be more flexible in the choice of this parameter, a new design for a heat-pipe oven is proposed, which allows for linear translation along the propagation direction of the beam, upstream of the skimmer.