physics question

I have a discharge tube with an argon plasma at approximately ^-5 torr. The argon is partially ionized by electrons emitted from a tungsten filament and accelerated across a 500 volt potential. First I need to calculate the plasma frequency, which I have a pretty good idea how to do, and it’s in the Feynman lectures somewhere also. Next I need to determine wether or not it will be collisionless for electron oscillations on the order of 1.5 inches- the diameter of the discharge tube. I’m not sure if it matters, but the direction of the oscillations is perpendicular to the gradient of the potential. My first problem here is I can’t manage to find the total cross section for electron collision in argon. Also, even if I did it would be dependent on the kinetic energy of the electron and i don’t really know how to come up with that.
Also I need a way to compute the current that would flow through the plasma, which should be a function of the potential and the density.