A linear particle accelerator permits to accelerate ions to high speed and there is no need to use big, heavy magnets to divert the beam. The space vacuum remove the need of pipes, so it could be delivered compact and then open to reach the right length. An RTG or a bunch of it could power the accelerator for years, maybe the exhausted plutonium could be used as ion source. Is it possible?
A gridded ion thruster is a linear particle accelerator. So yes.
If you're looking for higher exhaust velocity, which is higher Isp, sure, you can make it whatever you like. However you end up with proportionally lower thrust for the same amount of power. You very quickly end up having to take forever to produce a useful amount of $\Delta V$. The sweet spot is where you can consume your propellant in units of years, as opposed to decades or longer.
I suppose if we had much longer lifespans and more reliable equipment, we would be tolerant of something like a century-long mission. Then again, humans have spent centuries building cathedrals, so maybe our lack of patience is a cultural thing.