Experiments with Supersonic Beams as a Source of Cold Atoms

TTL pulse to the data acquisition card when the rotor passes though the triggering
position, which is 30 ◦ before the static reflection position. The card then waits an
empirically determined time before pulsing the Even-Lavie nozzle to create the beam.
This ensures that the beam will arrive at the nozzle at the correct time for the beam
to be reflected to the slowed beam detector. The control program is capable of
running the experiment at one shot per rotor rotation, but the vacuum pressure in
the nozzle chamber rises too much when this is done at high rotor velocity. As such,
the control system can be told to skip rotations, limiting the nozzle to a certain
maximum repetition rate.
3.4 Data and Results
Because the maximal rotation rate of the rotor is limited by vibrations to
speeds well below the initial design parameters, as explained in section 3.3.2.2, it is
necessary to reduce the speed of the helium beam below that obtained by cooling the
nozzle to 77 K. At this temperature, a pure helium beam has a velocity of around
900 m/s. Even at the maximum rotation rate achieved of 60 Hz the rotor tip velocity
is 190 m/s, which is not enough to significantly slow the beam. However, by mixing
the helium with neon in a 50:50 mixture, the beam velocity is reduced to around
500−550m/s, depending on the backing pressure of the nozzle. Lowering the velocity
of the incoming helium beam also has the advantage of improving the Debye-Waller
factor, which should lead to a larger fraction of the helium being specularly reflected.
The data described here is taken with the old spindle, limiting the rotor to 42 Hz and
a tip velocity of 133 m/s.
For the experiments and data described below, the specific gas mixture used
is a 48:52 mixture of helium and neon, with a nozzle reservoir pressure of 42 psi. This
mixture produces a helium beam with a velocity of 511 ± 9m/s.
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