In article .com,
wrote:

See:

http://blog.wired.com/defense/2007/0...tron_spac.html

I'm not sure whether it could ever work for Earth-to-Orbit launch... but
it might make a dandy launcher from the lunar surface (where there is no
atmosphere to slam into), or an engine for large spacecraft that can
make use of a wide variety of materials for reaction mass.

Best,
- Joe