Unfortunately, they aren't powerfull enough unless you can get them to produce fusion (Bussard's idea), since then they would just be extremly energy hungry and inefficient ion drives.

Also unfortunately, we can't yet keep fusion stable even in closed fields, let alone open ones.  Tho there is evidence that on a large (very large) scale we could create something like that.  It would be much easier (Dyson this time I think) to take an already existing and self sustaining fusion reactor, (a star) and use it for the drive.  Which was one use he suggested for a ringworld.  Line the ringworld with supercondutors (probably both radially and crosswise wrapped) and use it to generate the containment field that turns the star (possibly our sun) into a stellar fusion drive, as well as generating the attraction and catchment field (I forget the proper name) to scoop up the interstellar medium to refuel.  Take your hole system with you when you run from whatever is chasing you.  (marauding alien race, all the stars at the galactic core going nova, that sort of major earth-moving thing lol)

Also get some interesting side effects, a super laser that would put the deathstar to shame (swallow it whole), control of how fast your star burns (unnecessarilly lengthen its life), modify the primary frequency of its output (keep it yellow longer than expected unless you happen to like it pink), use it as a very high power tranmitter (good for burning out any nearby radio antennas doing the alien equivalent of seti).  Of course your magnet setup and the computers and models that run them have to be very good to keep from accidently frying yourself.  But humans never go trying out nifty unknown technologies or pushing little red buttons that say not to, do we?