Experiments with Supersonic Beams as a Source of Cold Atoms
Experiments with Supersonic Beams as a Source of Cold Atoms
Experiments with Supersonic Beams as a Source of Cold Atoms
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from tubular aluminum welded together to support the 600 kg chamber m<strong>as</strong>s. The<br />
Even-Lavie nozzle is shown in green, <strong>as</strong> is the skimmer and skimmer mount. The<br />
SRS residual g<strong>as</strong> analyzers used for beam detection are shown in blue. The rotor and<br />
spindle are depicted in yellow, while the rotary feedthrough and motor which drive<br />
the rotor are shown in orange.<br />
3.3.2 The Rotor<br />
There are several design requirements for the rotor itself, which informed its<br />
final construction. As can be seen from equation 3.2, to significantly slow a supersonic<br />
beam, the crystal should be capable <strong>of</strong> reaching velocities up to half <strong>of</strong> the initial beam<br />
speed. A supersonic beam <strong>of</strong> pure helium from a nozzle cooled <strong>with</strong> liquid nitrogen to<br />
77 K will have a velocity <strong>of</strong> 900 m/s and so the rotor w<strong>as</strong> originally designed to reach<br />
tip velocities <strong>of</strong> up to 450m/s. Another design constraint comes from the use <strong>of</strong> rotary<br />
motion to approximate linear motion <strong>of</strong> the crystal. Clearly, over small distances, a<br />
larger rotor will more closely approximate linear motion. This is important because<br />
the helium beam h<strong>as</strong> a finite spatial and temporal extent when it arrives at the crystal<br />
(≈ 100 μs). The angle <strong>of</strong> reflection changes during the arrival <strong>of</strong> the beam, limiting<br />
the fraction <strong>of</strong> the reflected beam which travels in the desired direction. This fanning<br />
effect leads to a loss <strong>of</strong> flux, and should ideally be minimized by making the rotor <strong>as</strong><br />
large <strong>as</strong> practical. Finally, an effective and simple method <strong>of</strong> mounting the crystals<br />
to the tip <strong>of</strong> the rotor, which does not overly compromise the rotor’s strength, needs<br />
to be found.<br />
3.3.2.1 Initial Design<br />
The centripetal force needed to hold a m<strong>as</strong>s in circular motion, F = mv2<br />
r<br />
mω 2 r,whereF is the force, m is the m<strong>as</strong>s <strong>of</strong> the body, r is the radius <strong>of</strong> the orbit, v<br />
34<br />
=