Maybe this has been hashed out already, but how is the optimum (+) potential for the magrid arrived at? If it's too low, electrons won't recirculate. If it's too high, ions would be repelled from the center (and electrons sucked out of the plasma). Too bad it can't be high enough to keep He+ from impacting the grid.
Is this the kind of thing that you start out with a reasonable guesstimate, then tweak it for best results? Or is there a more analytical approach?