Progress towards a precision measurement of the n=2 triplet P fine structure of atomic helium*

We report progress on the measurement of the $J$=1 to $J$=2 2$^3$P fine-structure interval of atomic helium. The measurement uses a liquid-nitrogen-cooled DC discharge source of metastable helium and the atomic beam is laser cooled in the transverse directions. The atoms are excited to 2$^3$P by a 1083-nm diode laser, and the fine-structure transition is driven by microwaves using the frequency-offset separated oscillatory fields technique [Phys. Rev. A 92, 052504 (2015)]. The transition is detected by further laser excitation to a Rydberg state, followed by Stark ionization.