Aireal,
Very interesting post. Anything that can help with the shielding problem is good because I think it is a major hurdle. I wonder if you have looked at the Helicon Doube Layer Thruster ion engine design that has been under development for the last couple of years at the ANU.
(http://prl.anu.edu.au/020research/13...0research/HDLT).

The advantage of this design is that the electrons don't need to be funnelled back into the ion beam, they are accelerated backwards down the thrust tube. This might present a handy way to separate out the electrons for use in your shieldng mechanism.

Anyway just thought I'd bring it to your attention, don't think it has had much press outside of Australia. Good luck with your design.