Rarefied Gas Flow through Long Rectangular Channel with Very Small Gap-to-Width Ratio: Experiments and DSMC Calculations

Abstract

A non-destructive, fast 2D profile monitor for an ion beam accelerator can be achieved by creating a high-density gas-sheet target in vacuum. It is known that gas flow through a long rectangular channel with a very small gap-to-width ratio is effectual in forming a gas sheet. To obtain a gas sheet with the desired performance, the spatial distribution of the emitted molecules in the candidate channel should be examined in more detail. As part of the gas-sheet development, the gas load through the channel should also be investigated to maintain the accelerator tubes in ultra-high vacuum. For efficient investigation, we explored the possibility of using direct simulation Monte Carlo (DSMC) method instead of experiments. We measured the gas flow (N₂ and Ar) through the channel with gap a = 0.1 mm, width b = 50 mm, and length L = 100 mm and derived the vacuum conductance C for the Knudsen number (Kn) in the range of 0.03–10⁴. Then, for comparing with the Kn dependence of the measured conductance, the DSMC method was used to calculate the transmission probability (the Clausing factor Ck) of the molecules passing through a channel with infinite width. The changes in Ck and C are similar. As Kn decreases from ∼10³, Ck and C begin to decrease rapidly and reach a minimum when Kn is between 1 and 0.5. Then they increase as Kn decreases further to <0.5. Furthermore, we successfully simulated the spatial distribution of the molecules ejected from the channel when molecular collisions occur with increasing inlet pressure.