For use in a vacuum.
The gun strips hydrogen atoms and accelerates the electrons one way on a ring magnet and the protons in the other direction (not in the same path but using as much of the same hardware as possible). It then discharges both beams in succession (electrons, then protons).
This idea comes from reading George O. Smith's The Complete Venus Equilateral (one of my favorite SF books).
In the story he pointed out several problems with using an electron beam as a weapon.
- As you fire the beam you acquire a high positive charge (the electrons have to come from somewhere).
- The negative charge that the target acquires will start to deflect the beams of subsequent shots.
- Beam dispersal.
The twin beam seems to have a few advantages:
- The charge of both the firing ship and the target remains neutral.
- Any negative charge picked up by the faster moving electron beam will, in a small way, add to the accuracy of the heavier and slower proton beam. The overall neutral effect on the target will not significantly affect succeeding shots.
- Beam Dispersal. Well, no real help here that I can see except that the spread out electron beam can paint the target and help the proton beam focus a tiny bit more of the near misses onto the target.