Actually I would expect a number of different reactor designs to become prevalent in the future for fusion. Each planet or section of space would likely have different designs based on the most abundant fuel for a fusion reaction. On the Earth sea water. The moon HE-3. Europa water again other outer moons maybe ram scoops collecting massive amounts of Hydrogen from neighboring gas giants. Some other locations... depends on what they have.
Edit: And don't forget that even with a .5c engine it would likely take longer than 50 years. First you have to accelerate to .5c and at some point you will have to decelerate unless you want to hit some large solid body or even not so solid body at 1/2 the speed of light.
Those two considerations add a nice solid chunk of time to any transit to another star.
[ December 02, 2003, 22:29: Message edited by: Cyrien ]