Experiments with Supersonic Beams as a Source of Cold Atoms

etween the gate and cathode, and the current from the anode to the cathode have
both stopped. An additional 20 μs is added to this time to ensure that the thyristor
is truly closed. For 80 μs coil pulses, an IGBT can be fired once every 140 μs. With
8 IGBTs, and a coil spacing of 1.41 cm, as described in section 4.4.4, this permits
beams of up to 800 m/s to be slowed. Faster beams would require a larger number of
IGBTs. The IGBT itself is controlled by a gate driver and DC/DC converter package
(Powerex VLA-500), and mounted on a dedicated board (Powerex BG2A). When the
IGBT is closed and current is allowed to flow, the gate driver lifts the voltage of the
gate to 15 V compared to the emitter of the IGBT.
The timing of the coil pulses is a critical part of the magnetic slowing exper-
iment. Each thyristor requires a 5 V signal to open, as does each IGBT. Thus the
timing of the pulses going to the 18 thyristors, 8 IGBTs, the Even-Lavie nozzle, the
discharge driver, and the data acquisition system must all be carefully controlled.
This is done using a field programmable gate array (FPGA). The FPGA board (Xil-
inx DS-BD-3SxLC-PQ208 REV 2) has 140 digital outputs, and an external clock at
10 MHz providing 100 ns timing resolution. The outputs of the FPGA are buffered
and distributed using a custom board, and all outputs from this board are passed
through opto-couplers to isolate the digital electronics from the coil drive circuitry.
When the two TTL pulses arrive at the IGBT and the thyristor, each open and
allow the capacitor to discharge. The measured peak current across the .25 Ω resistor
in series with the coil is 400A and it reaches this value after 35μs. The current remains
constant for the remaining 45 μs of the pulse. An oscilloscope trace of the voltage
across the .25 Ω resistor is shown in figure 4.12. When the IGBT opens the circuit,
the current in the IGBT stops very quickly (a few hundred nanoseconds), however
the current in the coil does not shut off immediately, but instead drops linearly to 0A
over 7 μs. This continued current produces a large voltage spike on the collector of
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