Experiments with Supersonic Beams as a Source of Cold Atoms

eceding crystal. Each curve is the average of 50 signals, which artificially broadens
the peak. The time-of-flight profiles of the individual shots are slightly narrower, but
there is some slight variation in their arrival time due to slight differences in the rotor
velocity and a small jitter in the valve triggering signal sent from the encoder.
At the higher crystal velocities, a shoulder appears in the signal before the
arrival of the slow beams in figure 3.16, and in all cases the detected signal is higher
after the beam passes through the detector than before. This is due to a rise in the
background helium in the chamber, which results from the atoms in the beam that
do not hit the rotor and are inelastically scattered from the chamber walls, or from
atoms that inelastically scatter from the rotor itself. Since the chamber and rotor are
at room temperature, these atoms are faster than the atoms in the beam, especially
when the beam has been slowed by reflecting from a fast receding crystal, which is
why the rise in background helium pressure in the chamber can be observed even
before the slowed beam arrives for the fastest crystal velocities.
Another element of time-of-flight profiles shown in figure 3.16 is the decreasing
detected amplitude of the slowed beam for increasing crystal velocity. One cause of
this is the temporal extent of the incoming beam, which arrives at the position of
the rotor over a period of around 115 μs. During this time, the angle of the rotor
changes in proportion to the angular velocity, and the reflected beam is spread in
plane of rotation. This fanning effect is mitigated by a large rotor, which allows for
lower angular velocities for the same corresponding crystal velocity, but at the highest
crystal velocities, the approximation of linear motion by a rotating object can still
become important. The fanning effect was quantified in the simulations performed in
[33]. The other cause for the decrease in the signal height is the decay of the crystals
over time in the vacuum chamber. This is discussed in more detail in section 3.4.3.
A summary of the slowing results is shown in figure 3.17. The slowed beam
50