Experiments with Supersonic Beams as a Source of Cold Atoms

The gas throughput of the nozzle can be estimated by
Φ= n0w
4
(2.26)
where Φ is the throughput in molecules/m2 /s, n0 is the number density of the
molecules in the reservoir, and w is the speed of the flow. Consider neon at 77 K
with a reservoir pressure of 5 atm as an example. At this pressure and temperature,
the ideal gas law (equation 2.1) gives a number density of 5 · 10 26 molecules/m 3 .The
velocity of such a flow is approximately 400 m/s giving an estimated throughput of
5 · 10 28 molecules/m 2 /s. Using a nozzle opening time of 10 μs and accounting for the
200 μm diameter of the valve gives a flux of 1.5 · 10 16 molecules/shot. Operating the
valve under optimal conditions results in a peak brightness of 4 · 10 23 molecules/sr/s
[33], which is greater than other sources [34–36] by at least an order of magnitude.
A CAD representation of the valve is shown in figure 2.2. The nozzle hangs
fromapooltypecryostatthatallowsthenozzletobecooled.The200μm diameter
trumpet shaped nozzle is depicted on the left side of figure 2.2 (a), with the gas
entering from the stainless steel tube on the right. The gas flows through stainless
steel pressure tube that is sealed at both ends by DuPont Kapton washers. The
actual valve mechanism is depicted in figure 2.2 (b). The plunger forms a seal with
the Kapton washer on the valve exit side of the high pressure tube and is held in
place by a spring which sits inside the pressure tube. The plunger is moved by
≈150 μm via an electromagnetic coil that produces fields of around 2.5 T, allowing
gas to adiabatically expand into the vacuum. The plunger is guided by two alumina
pieces that keep it centered in the pressure tube. The plunger is returned to the
sealing position by the spring when the coil is turned off. The entire plunger cycle
takes 15 μs [37].
The Even-Lavie valve is a good match for the experiments described in the
remainder of this dissertation. The methods of slowing and control described here
18