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Hi I have a question about time dependent vacuum simulations. I have a small chamber inside an outer vacuum chamber, with only a few small openings. I want to pump out the inner chamber, and wanted to simulate the time it takes to get to a specific preasure starting at normal preasure. So that i can estimate if i can get to the preasure in reasonable time or if i need more/bigger holes in it. Is this possible with Molflow? I found this article: https://molflow.web.cern.ch/node/214 but i think that is not what i am looking for, i can't find a way to set a start preasure or am i missing something? Thanks in advance Tobi
Submitted by Marton Ady 6 years ago

Dear Tobi,

One of the core principles of Molflow is that in can only generate, reflect or absorb molecules on surfaces (and not in the volume). This is the reason that you can't set a starting pressure: that would require recognizing "valid" space between the facets and uniformly fill it with initial particles.

In theory, you could create such an initial condition by injecting some particles for a very short burst (controlled by parameters, as described in the time-dependent mode), closing injection and waiting until gas reaches equilibrium, then open the holes leading to the outer chamber and plot the pressure decrease.

I would advise against using this approach though: in time dependent mode all hits are simulated, so it is suitable up to processes taking a few seconds (few million collisions) but not longer.

Luckily for you, your problem can be solved easily if we go back to vacuum theory :)

If you have an inital pressure p(0) in a volume V and the pumping speed is S (everything in SI units), then the pressure evolution will be:

p(t) = p(0)*exp(-t * S/V)

The S/V is the time constant of the system: basically the faster we pump (S), the faster the pressure decrease will be, on the other hand the larger the volume is (V), the slower.

You know already p(0), the target pressure, the volume V, so to get t you need the pumping speed S.

Basic UHV vacuum theory says p=Q/S where Q is the outgassing in SI units

It means the more you pump, the lower pressure you'll get, and the more you inject, the higher.

Do the following: forget Molflow's time-dependent mode, do a steady state simulation:

  • Set uniform (per area) outgassing to your inner chamber walls. The absolute value doesn't matter as long as it's uniform.
  • Set pumping in the outer chamber with the real pumping speed of the simulated system
  • Run the simulation and see what pressure Molflow calculates in the inner chamber. For this you can create a large transparent facet in the middle.

Then you'll get the effective pumping speed of your outer pumps in the inner chamber. You know Q (the outgassing sum, you can check it for example in the Global Settings window), you just calculated P, therefore you'll get S.

Plug that S in the first equation and you'll get the pressure solution of your system.

Submitted by e4a758e7bc0b5c69a3f5 6 years ago
Hi Thank you for your detailed answer, this realy helps. Thats what i thougt. I will try doing the calculation you discribed, i already did a steady state simulation, so it should be easy to calculate.