The vacuum is not perfect at LEO. The free molecular particles are a mixed bag (some are ionized and some are neutral). The neutral particles would stream into that electrostatic trap and react with the anti-particles resulting in lots of gamma radiation.

Also I don't like the idea of harvesting significant amounts of anti-matter anywhere near the Earth.

This brings to mind an amusing science fiction scenario. What would you do with a wrecked UFO, if you found one with the crew dead and the motor still running? Naturally one would want to reverse engineer it but chances are it would be fueled with anti-matter. No way you'd want to take it apart anywhere near the Earth. The way I'd write the story would be to build a machine shop / science lab on the far side of the Moon. Then transport the UFO to this lab and have volunteers slowly take the thing apart. There would be video cameras watching their every move and transmitting their progress back to the Earth. Whenever the volunteers pulled off a part that was obviously safe, they'd stop work and the new part would be promptly shipped back to the Earth for further study. I'd end the story with the volunteers getting down to the bare motor. There would be a heated discussion, maybe one of the volunteers would chicken-out, then there would be a bright flash forming a corona around the Moon as seen from the Earth. Lots of fun ways you could play with that story.

Gary