Experiments with Supersonic Beams as a Source of Cold Atoms

3.3.3 Detection
An SRS [61] residual gas analyzer (RGA) is used to detect the beams. The
experiment uses two RGAs, one to detect the slowed helium beam (RGA 200), and
one which detects the beam when the rotor is out of the way (RGA 100). Both
RGAs work by ionizing the helium via electron bombardment in an ionization region.
The ions are then focused into a quadrupole mass spectrometer which provides mass
filtering, and detected by a continuous dynode electron multiplier. The output of
the electron multiplier is fed directly into a current pre-amplifier, and from there the
signal is read by the data acquisition card in the computer.
Because the beams are detected by ionizing the helium, the detection efficiency
for helium is on the order of 10 −5 [62]. The ionization efficiency should scale with the
time the atoms spend in the ionization volume, a slow beam should have a greater
detection efficiency than a fast one.
3.3.4 Control Systems
The experiment is controlled by a data acquisition card made by National
Instruments (NI PCI-6120), run by a custom control program written in LabVIEW.
The motor driving the rotor is a 380 watt Moog motor fitted with an 8000 line
encoder, and is servo controlled by a Galil controller. This permits the rotational
frequency of the rotor to be controlled to a few thousandths of a Hz. The servo
controller is also capable of controlling the static position of the rotor to within 0.25 ◦ ,
and communicates with the computer control system through a serial connection.
The data acquisition card also controls the timing of the experiment. The
nozzle must pulse at the correct phase of the rotor’s rotation for the beam to reflect at
the correct position. Figure 3.14 illustrates the method used to achieve the required
phase locking between the rotor and the nozzle. The motor controller delivers a
44