Experiments with Supersonic Beams as a Source of Cold Atoms

Magnetic Field (T)
1.9
1.7
1.5
1.3
1.1
0.9
0.7
0.5
0.3
0.1
Initial Pulse
.5 Hz Steady State
3 3.5 4 4.5 5 5.5 6 6.5 7
x 10 −4
−0.1
Time (s)
Figure 5.6: The time profile of the magnetic field in the hydrogen coilgun coils as
measured by Faraday rotation. The blue curve shows the field measured for the
initial current pulse in the coil, while the red curve shows the field after the coil and
resistors in the switching circuit have warmed up by being pulsed at .5Hz. The noise
in the measurements that appears when field is near zero is due to the resolution of
the digital oscilloscope used to record the signal on the photodiode.
capacitors to drive each coil. These are needed for a few reasons. First, the lack of
Permendur around the coil means that the field extends much farther from each coil
than in the previous coilgun iterations. This means that in order for the atom to feel
the full effect of the field, the coil must be turned on earlier. This is especially true
for the slow atoms at the end of the coilgun. Hence, the pulse length is increased
to 200 μs. Additionally, the increased current in the coil means that the capacitors
discharge faster. A longer pulse of greater current requires larger capacitance to keep
the current nearly constant through the end of the pulse. Since the coils no longer
share IGBTs, the pulses can overlap at the beginning of the coilgun without causing
any problems.
The field in the coils and the temporal switching profile of the magnetic field
is measured using the Faraday effect. The field turns on exponentially with a time
constant of 30 μs, and turns off linearly in 10 μs. Thetimeprofileofthemagnetic
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