we are evaluating some design options for an ultracold-atom vacuum chamber.
In particular, I would like to estimate the background pressure due to outgassing and permeation from the material. However, outgassing and permeation depend a lot on the different molecule types, e.g., N, H, Ar, as well as the pumping speeds.
I started evaluating if Moflow would be an option to simulate the background pressure but I failed to find a configuration for the different molecule types.
It appears to me that Molflow assumes a single molecule, is this correct?
Hello Bodo, and thanks for inquiring about Molflow+.
Yes, Molflow+ simulates only one has specie at a time, if you need to simulate a gas mixture you need to run single specie simulations and add up the results, eventually with proper weights.
The reason behind it is historical and also due to the fact that in molecular flow molecules are independent of each other.
Hope this helps and do not hesitate to come back here should you need any further info, clarifications or need for support, we do it all the time with new users.
Simulating Helium and Hydrogen separately and writing a small post-processing script to combine the simulation results is fine for us.
However, another ambiguity arose today concerning the pumping speed. We have two differential pumping chambers with different ion pumps attached.
How do we model such a setup? Do we need to estimate some effective pumping speed accounting for the differences between the pumps and the effective conductance with respect to the source?
Is there some information available about such a procedure?
Hello again:
Yes, you need to determine separately the effective pumping speed of your differential pumping, for each gas specie. Typically it is just a different equivalent sticking coefficient for the surfaces mimicking the entrance of the pumps, but may be different depending on the level of simulation of your pumps. For instance once I needed to model a large 500 l/s ion-pump with an internal neg pump module(cartridge) and therefore I needed to find the equivalent sticking coefficient for the openings where the pumping elements (starcell) where installed and also for the external surface (geometrical cylindrical envelope) of the NEG cartridges. I did that by trial and error, finding what sticking coeff. gave the measured pumping speed at the pumps mouth.