With current technology NASA get 3100s of specific impulse for ion drive, 2.3kw, 92mN. For a larger ship, not just small satellite, it should be doable to increase the voltage and get 10x specific impulse at 10x power with the same thrust. A nuclear reactor without much protection - on a long stick with small screen protecting only direction of the payload - can be something like 5-10kw/kg. That allows to have 90% of the mass to be propellant, and thus reach the speed, in about 30 years of acceleration, of more than 600km/s. With 2 stages, in 60 years with some optimization/etc. we can get 1500km/s, or 800 years to Alpha Centauri.