Experiments to Control Atom Number and Phase-Space Density in ...
Experiments to Control Atom Number and Phase-Space Density in ...
Experiments to Control Atom Number and Phase-Space Density in ...
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large safety marg<strong>in</strong> on the current is implemented by us<strong>in</strong>g 10 MOSFETs <strong>in</strong> parallel.<br />
Figure 7.22: MOSFET H-bridge setup.<br />
The electrical circuit for the switch<strong>in</strong>g of the current direction through one of the<br />
Feshbach coils is shown <strong>in</strong> figure 7.23. This design allows for switch<strong>in</strong>g from Helmholtz-<br />
<strong>to</strong> anti-Helmholtz-configuration <strong>in</strong> only a few ms. The current direction is controlled<br />
by a TTL <strong>in</strong>put <strong>to</strong> the H-bridge driv<strong>in</strong>g circuit. The H-Bridge driver (HIP4081) opens<br />
either the MOSFET groups A <strong>and</strong> D (each MOSFET symbol figure 7.23 represents a<br />
group of 10 MOSFETs run <strong>in</strong> parallel), or B <strong>and</strong> C simultaneously, switch<strong>in</strong>g the current<br />
direction through Feshbach coil 2. To reduce switch<strong>in</strong>g <strong>and</strong> turn-off times the overall<br />
<strong>in</strong>ductance of the setup is m<strong>in</strong>imized. Therefore all current carry<strong>in</strong>g wires run <strong>in</strong> pairs<br />
with m<strong>in</strong>imal distance between them.<br />
Because of the ability <strong>to</strong> switch the Feshbach coils between Helmholtz- <strong>and</strong> anti-<br />
Helmholtz configuration it is possible <strong>to</strong> use the Feshbach coils <strong>to</strong> create the quadrupole<br />
field for the MOT. In contrast <strong>to</strong> the MOT coils, the Feshbach coils can be run con-<br />
t<strong>in</strong>ously at the current required <strong>to</strong> generate the MOT magnetic field gradients (20 A).<br />
The load<strong>in</strong>g rate <strong>and</strong> the a<strong>to</strong>m number <strong>in</strong> both situations is comparable. The speed<br />
at which the system can be aligned however, is greatly <strong>in</strong>creased when the quadrupole<br />
fields can be run cont<strong>in</strong>ously. The experiment therefore now typically uses the Feshbach<br />
coils <strong>to</strong> create the MOT magnetic field gradients.<br />
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