Attaching a starship to a great big mass of frozen deuterium seems a good idea, in light of Dr. Friedwardt Winterberg’s updated e-print from June…
Advanced Deuterium Fusion Rocket Propulsion For Manned Deep Space Missions
…of course the trick is igniting the reaction and getting a high fusion-burnup fraction out of the fuel-target. Not so easily done, but Winterberg’s work makes the prospects look good with sufficient effort. The estimated exhaust velocity is fairly high, a bit over ~4% of c, [according to Brian Wang it can go as high as 6.3%] which means a 120,000 ton starship attached to 12,000,000 tons of deuterium can do a delta-vee of ~0.2 c. With an efficient magnetic sail that means the journey speed approaches ~0.2 c, albeit with the mass-penalty of the sail. Perhaps a plasma-magnet can be formed at such speeds, with a quite different decceleration profile to the mag-sail, since the artificial magnetosphere balloons to match the plasma ram-pressure. Essentially the size goes up as the relative speed goes down, thus allowing a more-or-less constant braking force. A decceleration of 0.1 m/s2 will bring the vehicle to a halt in ~19 years over about 1.9 light-years from 0.2 c.
In case you didn’t realise it’s a LONG way between the stars. Fusion-ship trip-times are decadal at best.
PS Even a Vex of 6.3% c, the maximum burn-out velocity is just 0.28 c. While that’s an incredible 84,000 km/s it does still mean that the nearest star, Proxima Centauri, is at least a 15 year trip.