Experiments with Supersonic Beams as a Source of Cold Atoms

magnetic flux density (T)
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
(a)
-6 -4 -2 0 2 4 6
4.3
4.2
4.1
4.0
3.9
3.8
3.7
3.6
3.5
(b)
-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5
axial coordinate (mm) transverse coordinate (mm)
Figure 4.10: Finite element calculations of the magnitude of the field in the proof-ofprinciple
experiment coils are shown. The field calculated along the axis of the coil is
shown in (a), while the transverse field magnitude at the center of the coil is shown
in (b).
that the atoms are still 3−4cm from the coil when the current is turned on, and they
are still well outside of the extent of the field when the current reaches its maximum.
Thus the atoms will see a steady state field whentheyenterthecoil,andthetime
variation in the field experienced by the atoms is only due to their motion and the
deliberate switch-off of the field.
The current is produced by allowing a capacitor to discharge across the coil,
with two solid state switches controlling the current. The switches used are an
integrated gate bipolar transistor (IGBT), and a thyristor. The IGBT (Powerex
CM400DY-12NF) is a fast switch, with well-defined switching characteristics, while
the thyristor (Littelfuse S6040R) is slow, with relatively poorly defined switching pro-
files. Using both switches allows the fast switching but much more expensive IGBT
to be used to switch multiple coil channels, while the thyristor is used to isolate the
channels from each other.
A schematic of the switching circuit is shown in figure 4.11. The parts within
the dashed box exist for each coil channel, while those outside the box are shared
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