Experiments with Supersonic Beams as a Source of Cold Atoms

A comparison of the time-of-flight profiles for a direct and statically reflected
beam is shown in figure 3.15. The direct beam is shown in black and the reflected
beam is shown in red. Both curves are averages of 50 beam pulses. The temperatures
of these beams are calculated from Gaussian fits to be 249 ± 36 mK for the direct
beam and 254 ± 41 mK for the reflected beam. These temperatures are effectively
the same, indicating that the beam is specularly reflecting from the crystal and is
not being heated by inelastic processes. The reflected beam arrives earlier than the
direct beam in the because the distance traveled by the reflected beam is shorter than
that traveled by the direct beam. This also explains the slightly narrower time-of-
flight profile of the reflected beam. Because different detectors, set up in different
configurations, are used to detect the direct and reflected beams, the amplitude of
the signals is not comparable.
3.4.2 Beam Slowing
To spin up the rotor and reflect the beam from the receding crystal, the beam
must arrive at the correct time so that the rotor is at the correct position and angle
to reflected the beam to the detector. A pulse is sent from the motor controller when
the rotor reaches a position 30 ◦ from the optimized static reflection position. After
a delay, the nozzle is pulsed, allowing the beam and rotor to interact at the optimal
position. To find the correct delay for a particular rotor speed, the delay times which
cause the rotor to just miss the beam on either side are found, and the optimal delay
is taken to be the average of these delays. At the highest rotor speeds, these delays
are optimized to the microsecond.
Time-of-flight profiles of the reflected beam for different crystal velocities are
shown in figure 3.16. The arrival time of the reflected beam is delayed with increasing
mirror velocity, which shows that the beam is being slowed by reflection from the
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