Experiments with Supersonic Beams as a Source of Cold Atoms

of the IGBTs used for the coilgun coil switching circuit, has to do with the “on” state
behavior of these devices. In their “on” state, IGBTs behave like diodes with a
relatively fixed voltage drop as a function of current. They are also susceptible to
thermal runaway, as the forward voltage drop decreases with increasing temperature.
This makes it difficult to place multiple IGBTs in parallel. However, the forward
voltage drop will even then lead to significant losses. While MOSFETs do not have
blocking voltages as high as an IGBT, they behave like a resistive load in their “on’
state, and their positive temperature co-efficient stops thermal runaway, enabling
simple parallelling of multiple devices. Parallelling MOSFETs reduces the “on” state
losses since the current is distributed across what is effectively many resistors in
parallel, allowing the effective resistance to be reduced. For the low voltage but high
current switching needed in the magnetic trapping coils, using MOSFETs enables
lower losses in the switches.
The final element of the circuit is a high voltage capacitor and an IGBT
(Powerex CM600HA-24H) in parallel with the power supply. This part of the circuit
is used to turn the coils on rapidly by briefly increasing the voltage across the circuit.
This voltage boost is particularly necessary when the front coil is turned on to create
an anti-Helmholtz trap. The front coil must switch on quickly to capture the atoms
once they have been stopped.
As the switching of the front trap coil is particularly important in the trapping
sequence, the temporal switching profile is shown in figure 5.12. The profile is mea-
sured by Faraday rotation with the crystal located in the middle of the front trapping
coil. This shows that the front coil switches off and on linearly in 150 μs. The IGBT
provides a voltage boost to shorten the switch on time (it is over 600 μs without the
IGBT in the circuit).
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