Experiments with Supersonic Beams as a Source of Cold Atoms

and a titanium locking ring, with an exposed crystal diameter of 9 mm. The back of
the threaded plate has a tapped hole, which screws onto a third piece which fits into
the rotor via the smaller crystal mounting threads. This system has the additional
advantage of simplifying the mounting and removing of crystals from the rotor, re-
ducing the time the crystals and the vacuum chamber are exposed to air during a
crystal exchange.
3.3.2.2 Vibration and Balancing
The spinning parts of the experiment are all constructed with dimensional
tolerances of .001 inches, which ensures that the rotor apparatus should be nearly
balanced to begin with. To verify the initial balancing, the rotor was mounted with
the motor axis horizontal, instead of vertical as it is in the chamber, and statically
balanced. This provides the rough balancing needed to initially spin the rotor, since
even a small static imbalance would lead to potentially large vibrations when spinning.
To balance the spinning system, the vibration of the rotor must be measured.
This is accomplished in two ways. The first method uses a laser beam to measure
the amplitude of the pendulum modes of the shaft. The laser is directed though the
chamber, with half of the beam blocked by the lower part of spindle. As the shaft
vibrates, the spindle moves in the laser beam, blocking more or less of the beam,
which is detected by monitoring the intensity of the light leaving the chamber via a
photo diode. Vibrations are detected to an accuracy of 5μm using this setup. A knife
edge mounted to a micrometer is used to calibrate the readings. Having assembled
the system with the greatest accuracy possible, using feeler gauges to ensure that
parts are properly aligned, a runout of 70 μm is observed (over the course of one slow
rotation the spindle moves over a range of 70 μm).
The second method of monitoring the vibrations does not directly measure
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