One project over the last several weeks has been winding coils for the atomic fountains. There are two different requirements, one is the so-called “C-field” coils and the second is the MOT (magneto-optic trap) coil pair.
The “C-field” is the bias field in an atomic clock that essentially tells the atoms which way is up, i.e. it defines the quantization axis. It also shifts the frequency of the transition, so in a frequency standard you need to know what the field is. In a clock (there is a subtle difference) you care about the stability, i.e. you don’t want it to change, so it’s enough to feed this with a precision current source to give a bias field of a milligauss or so. Two layers, up and back, so the pitch on each layer should tend to cancel and leave you with a vertical field, and about 600 turns per layer. There are also extra shim windings at each end to better simulate an infinite solenoid — a real solenoid’s field drops off at the ends, so we boost it back up a little. The drift region, where the atoms oscillate between the two hyperfine states (the “tick” of an atomic clock), sees a very stable field.
Pretty easy, but time-consuming (as it were); the basic winding took more than four hours. What you see is the jig I used, which has a stepper motor and a home-built feed system that wets the wire with alcohol to activate the bonding material. Square wire is used so it doesn’t have any gaps.
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