Rip it apart with magnetic fields
If the first plate was followed by a negatively-charged ring electrode around the perimeter the plasma would be pulled apart before it struck the second metal plate, then the plasma would be cooler and the plates wouldn't have to be placed so far apart. This set up might even deflect some of the plasma outwards, perpendicular to its original trajectory, so that it missed the second shield altogether. Another positively-charged electrode down the center might be even more effective in pushing the ionized plasma away from the central axis of the ship, before striking the second shield-plate. Any plasma that hits the second plate would be highly dissipated over a huge area.