As far as I understand, for thrusters pressure limit is 10-4 - 10-3 Torr. For 50Pa accelerating cells should too small. I think it is engineering problem.

has it to do with space charge limitations? or mean-free-path?

Space Thrusters? Small because of too much gas?

Mean-free-path is too small.

I think the accelerating cells mentioned by Valentin are the extracting electrodes of the ion source. However, the 50 Pa mentioned in the question is about the metal plasma usually inside a discharge chamber, it may not be the pressure where the exracting electrodes work. For example, the extracting hole or slit is very small so that there should be very large pressure gradient existing. But a sacrifice shall be very low extracting beam current.

MEVVA+ECR is then the best choice..  What is the critical density of electrons at 35 GHz (conductivity of plasma strong enough to repulse wave fields) ? does MEVVA reach this limit? would 2,4GHz not be enough?

good question