Design and characterization of collimated effusive gas beam sources: Effect of source dimensions and backing pressure on total flow and beam profile

The total flux and the flow profile of gas-phase molecular beams generated by a number of assemblies involving different combinations of apertures and capillary arrays were measured for a wide range of backing pressures covering the molecular and viscous flow regimes. Specifically, δ=10 and 50 μm diameter, $\text{L=2\hspace{0.05em}}\mathrm{mm}$ thick glass capillary arrays were combined with single apertures of φ=0.17, 1.0, and 11.4 mm diameters and $\text{L=0.1,}$ 0.5, and 2 mm thickness in order to design high-flux beam dosers with a high degree of collimation. The variations in the total flux and the spatial profile of the beams were tested as a function of the backing pressure, which was varied between ${10}^{\text{−4}}$ and ${10}^2 \mathrm{Torr},$ by a sampling movable skimmer. The data obtained under the molecular flow (low backing pressure) regime corroborate some conclusions from previous reports. In particular, it is shown here that the conductance of the dosers (the ratio of the total flux of the beam to the backing pressure) depends only on their geometry in that regime. The beam profile, on the other hand, deteriorates with increasing backing pressure because of the increase in gas–wall and gas–gas collisions in the intermediate “opaque” flow regime where the mean free path of the gas, λ, is larger than the diameter of the capillaries, δ, but smaller than the length of the tubes, L. As λ approaches δ, a transition “slip” regime is reached, and a drop in the conductance of the doser is observed. Finally, by pressures where λ<0.05δ, a viscous laminar flow is established where the conductance of the doser increases with pressure and its directionality improves as well. The implications of our results to the design of molecular beams for specific applications are briefly discussed.