Spatial distribution of the pulsed neutral helium beam

A supersonic He beam source, which consists of a commercial pulse generator and a pulsed nozzle with a diameter of 0.8 mm, is fabricated. The helium beam is ejected from a pulsed nozzle, and then collimated through a skimmer. We observe that the helium density is inversely proportional to the square of the distance. At the longitudinal position of Y = 12 cm away from the nozzle, the atom densities of the helium beams of 8.4 x 1014 cm-3 (26 mTorr) and 6.1 x 1014 cm-3 (19 mTorr) are obtained for the nozzle backing pressure of 101 kPa and the gaps between the nozzle and the skimmer of D = 1 and 3 mm. Experimental data indicate that the radial width of the helium beam is around 64 mm at a position 19 cm away from the nozzle for D = 1 mm. In contrast, for D = 3 mm, the radial width of the helium atom beam is decreased to 44 mm. From the time shift of each peak, the speeds of the pulsed beams of 1.1 km s-1 at Y = 14 cm and 508 m s-1 at Y = 21 cm are also obtained. The background of signal due to gas diffusion at the position far from the nozzle is always larger than that at the position close to the nozzle. This suggests that the speed of the He beam is affected by He atom collision and gas diffusion. A high-density short-pulsed supersonic metastable helium beam was obtained. After discharge, a metastable density of 1.33 x 1011 cm-3 was obtained. Its fraction (N2S1/NHe) was up to 1.65 x 10-5