Wall collisions, angular flux, and pumping requirements in molecular flow through tubes and microchannel arrays

Abstract

Microchannel arrays can be used to produce intense molecular beams with free molecular flow. Arrays can attain much higher intensities than a single orifice small enough to be in free molecular flow. Pumping requirements are greatly reduced compared to supersonic jets producing similar beam intensities. Theoretical descriptions of the angular flux and wall collisions in single tubes and microchannel arrays are developed. These results are extended to the situation of a finite mean free path and the consideration of wall collisions which occur before transmission through a tube. Despite the large average number of wall collisions experienced by molecules passing through a long tube, molecules along the axis of the beam experience zero or few wall collisions. The near absence of wall collisions on the beam axis allows reactive species to be sampled with microchannel arrays. Tubes with a length to radius ratio of up to ∼40 are useful for reactive species. Most wall collisions for molecules transmitted along the beam axis through microchannel arrays are collisions in which a molecule enters the array, collides with the wall, goes back to the source region, and then is transmitted through the array. The nearly effusive beam allows the selection of species based on their thermal velocities. A long single tube used as a skimmer will provide a greater pressure drop than an orifice.