Experiments to Control Atom Number and Phase-Space Density in ...

4.2.1 Upper MOT Coils
A pair of anti-Helmholtz coils creates the quadrupole field required to create a
MOT in the upper chamber. The radius of these coils is 4 cm, spaced 6 cm apart.
The coils consist of 91 windings of AWG 20 wire, creating a magnetic field gradient of
15 G/cm at a current of 2.5 A. To dissipate the generated heat, the coils are mounted to
water-cooled copper blocks. The current in each coil is controlled individually, so that
the location of the magnetic field minimum can be adjusted in space.
4.2.2 Magnetic Trap Coils
A second pair of circular coils are mounted around the science chamber cell.
At small currents they create the weak magnetic field gradient required for producing
a MOT in the lower chamber, at large currents these coils are able to generate the
magnetic field gradients required to produce a quadrupole magnetic trap. Up to 30 A
are typically used for creating the magnetic trap.
These coils are made from 176 windings of AWG 14 wire in three layers (layer A:
53 windings, layer B: 53 windings, layer C: 70 windings). They have an inner diameter of
34 mm, an outer diameter of 69 mm and a width of 42 mm. The separation between the
centers of the coils is 75 mm. At large currents up to 261 W resistive heat is produced in
these coils. In order to effectively cool the coils, they are surrounded by a PVC enclosure,
through which a continous flow of water is directed. Figure 4.4 shows a schematic of the
magnetic trap coils.
Near the center of the magnetic trap, these coils produce a magnetic field gradient
of Bz = 9.7 G/(cm A) in the axial direction and Br = 4.8 G/(cm A) in the radial
direction. They are wired in series and powered by three power supplies (Lambda
Gen80-19) that are wired in parallel. A home-built PID control circuit regulates the
current flow using seven power op-amps (OPA549).
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