Experiments with Supersonic Beams as a Source of Cold Atoms

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