Experiments with Supersonic Beams as a Source of Cold Atoms

10 mm
2.0 T
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0
Figure 5.2: The spatial profile of the magnetic field produced in the hydrogen coilgun
coils at 825 A. The fields are calculated numerically using finite element analysis.
5.2.1.1 Slowing Coil Geometry
Larger coils have a bigger physical aperture, allowing more particles to enter
the coilgun and increasing the number of particles slowed. Larger coils also create a
smaller field gradient. This means that more of the atoms in the beam experience
approximately equal fields, again increasing the phase space volume which can be
slowed and thereby improving the slowed flux. The reduced field gradient is especially
important at lower velocities. Small differences in the energy removed result in a larger
spread in velocities for lower speeds, due the quadratic relationship between energy
and velocity. In addition, the same velocity spread in a beam will have a much larger
effect at lower velocity. For example, a Δv of 5 m/s on a beam moving at 500 m/s
results in the beam spreading by less that .1 mm between coil, but a Δv of 5 m/s on
a beam moving at 50 m/s results in a spread of more than 2 mm over the length of a
single coil. Larger coils and lower field gradients are therefore particularly beneficial,
in terms of final slowed flux, for low final beam velocities.
The coils consist of 24 windings (4 layers of 6 turns) of 1mm Kapton insulated
copper magnet wire. The inner diameter of the coil is 11.5mm and the outer diameter
120