Pulsed helium droplet beams

Highlights: • Cryogenic pulsed helium droplet beams. • Large helium droplets up to 10¹¹ atoms and 2 μm in diameter. • A factor of ∼1000× higher droplet flux than in continuous beams. • Formation of droplets via the fragmentation of the helium fluid. We report a systematic study of pulsed cryogenic expansion at temperatures ranging from 20 K down to 4.5 K, which enables us to span a large range of helium droplet sizes from ∼10⁵ to ∼10¹¹ He atoms, as obtained via titrations of the droplet beam with helium gas at room temperature. The measured peak flux is a factor of one thousand larger than in a continuous beam. Comparison with continuous nozzle results show similar droplet sizes at low T < 6 K, indicating in both cases the droplets are created via the fragmentation of the fluid.