Sorry to go all science police on you, but true controlled nuclear fusion hasn't even been achieved yet... Right now we only have nuclear
fission, which is quite a bit weaker than fusion is supposed to be. The only fusion devices we've ever created are hydrogen bombs, and after 60 years of design improvements developed from previous experiments, scientists are
still in the process of producing enough energy to ignite the reaction.
Basically, you've got to have a huge atomic fireball just to jumpstart the process. Fusion isn't a likely candidate for powering ships anytime in the near future.
I'd go with smaller fusion reactors, and as Xahnel said, multiple. But I can't imagine how a few small reactors couldn't power a space station. A single spaceship should only take one or two.
Also, lasers aren't a good weapon against asteroids... Most lasers can't even cut wood, and those that can cut through metal (like asteroids) at a decent speed take ridiculously insane amounts of power to operate... I'd go with missiles, or avoiding asteroids altogether - and this is easily done. Contrary to what movies would have you believe (Star Wars comes to mind), asteroid belts are incredibly thin. Actually, if one was sighted on a trip through the Solar System's belt, it would bring all the passengers flocking to the windows, taking pictures. Space is huge, and asteroids are few and far between. Remember, you're stretching the mass of a single planet all the way around the sun in a huge torus. Here's a pic for clarification:
Imagine Earth's mass in that pic split up and stretched all the way around the sun. Then look at it again and realize that the pic
isn't even close to scale. In reality, the Earth
ought to be a little speck in that picture, smaller than they have the moon drawn. Really, you'd be lucky if you
ever saw an asteroid by accident.
science and astronomy is kind of my thing.
sorry.