CAS 2017 Vacuum simulations Individual work - exercises

Gauge 1 Gauge 2 Coated sample

Gauge 1 Gauge 2 1000 mm ~250 mm 63 mm Coated sample ~150 mm ~150 mm

Setting up the geometry in Molflow 1000 mm 63 mm Coated sample Create the base tube with the Test / Pipe command Use the (distorted) Scale command to get dimensions right

Setting up the geometry in Molflow Coated sample ~150 mm ~150 mm Use the Extrude command to create two more sections

~250 mm Coated sample Use the Rotate facet and Move facet commands with the copy option to create the perpendicular tubes Then use the build intersection command to create the union of the two pipes (select intersecting facets, and select all vertices that you’d like to keep) The build intersection command works correctly if one pipe is slightly thicker than the other (use Scale facet)

Formula for pressure on facet N: “PN” Gauge 1 Gauge 2 Coated sample Use the end caps of the tubes to represent the gauges (use formulas), pumps (use sticking) and gas injection Q S = 300 l/s

Gauge 1 Gauge 2 Coated sample Recommended: create a transparent facet that runs along the centerplane, and add profile and texture to it

Exercise 1 We start inject gas at a constant rate. After some time the pressures stabilize. We observe the following pressures on the gauges: Gauge1: 1E-6 mbar Gauge2: 2.5E-9 mbar Approximately what’s the sticking factor of the sample? What’s the gas injection rate? 1E-6 mbar 2.5E-9 mbar Sticking = ? Coated sample Q=?

Exercise 2 NEG coating is an effective pump, but it can’t pump gas forever: eventually it saturates. Its pumping ability drops quickly when the pumped gas forms one monolayer on the surface – corresponding to about 1E15 pumped molecules / cm2 In our setup, how much N2 is that? How much time does it take until our setup saturates?

Exercise 3 In the first exercise you have found the sticking factor for a given pressure ratio, observed on the two gauges. What was the transmission ratio of the gas (the probability that an injected molecule makes it through the tube) for that pressure ratio? By changing the simulation parameters, you could help an engineer to solve exercise 1 in the future. Make a “transmission ratio as a function of the sample’s sticking factor” plot: X axis: different sticking factor values (around 5-7 different values) Y axis: gas transmission ratio