Experiments with Supersonic Beams as a Source of Cold Atoms

Figure 4.12: An oscilloscope trace measuring the voltage across the .25 Ω resistor in
series with the slowing coil. The probes are 1:100, since the voltages would be too
high for the oscilloscope to measure with 1:1 probes. The green trace is the voltage
on the capacitor side of the resistor, while the yellow trace is the IGBT side. The
purple trace shows the voltage drop across the resistor. Figure Courtesy Christian
Parthey.
the IGBT of 550 V, due to the back EMF in the coil trying to maintain the current
in the coil even though the connection to ground no longer exists. This voltage spike
would be even higher, but a 2 μF snubber capacitor across the IGBT absorbs much
of the current, as well as limiting voltage and current oscillations. Additionally, the
voltage on the collector is reduced by a freewheel diode and 2 Ω resistor in parallel
with the coil, permitting the current to briefly circulate while being damped by the
resistor. These measures are necessary because the blocking voltage of the IGBT and
thyristors is limited to 600 V.
While the measured current drops quickly, Lenz’s law indicates that eddy
currents may be formed in the permendur surrounding the coil, which could affect
the switching of the magnetic field. For this reason, the field is measured directly
using a pickup coil. A single turn coil is inserted into the bore of the slowing coil, and
the current in the coil is monitored. Integrating this current gives a temporal profile
of the field produced by the coil, as shown in figure 4.13. As seen here, though the
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