Design and Commissioning of an Octupole Magneto-optical Trap for Sub-Doppler Cooling of ³⁹K

Potassium has proven to be a difficult atom to Bose-condense, with initial condensates coming about due to sympathetic cooling from other species, e.g. ⁸⁷Rb. The difficulty with achieving a high phase-space trap is largely due to the hyperfine atomic structure of potassium, with splitting between the ²P_3/2 F'=3 and F'=2 levels only 20 MHz (3.3Γ) in ³⁹K and about 17 MHz (2.8Γ) in ⁴¹K, preventing the use of dark-spot magneto-optical traps (MOTs). Low density and high photon rescattering make loading a potassium optical dipole trap challenging. Sub-Doppler cooling techniques have been developed for potassium, but dipole trap loading remains inefficient due to the low initial density.

To this end, we have developed and built a trapping field using an octupole magnetic field, which combines elements of both near-zero fields at the trap center for sub-Doppler cooling, and restoring forces at the boundary to generate cold atoms for long enough times to load an optical dipole trap at high density. In this talk, we discuss and design and construction of our octupole symmetry trap from two sets of quadrupole symmetry anti-Helmholtz coils. We also elucidate techniques used to align and test the coils and show results on the lifetime and temperature of atoms trapped in the MOT. This work allows for more consistent and rapid loading of an optical dipole trap for atoms with poorly resolved hyperfine structure such as potassium.