I've often read that beam-powered propulsion is the only basic interstellar propulsion concept without physics problems. To me, that doesn't seem far from the truth. However, for a long time I've felt that the biggest obstacle to overcome is the beam divergence problem. Most of the concepts I've read about use a single laser, or a laser array stationed at a single location. It seems to me that - unless your spacecraft can withstand enormous G-forces - you're going to need an acceleration path that's billions of kilometers long if you hope to reach 0.1 c or greater. Could you ever have that with a single laser? I've imagined that you would need many powerful space-based lasers along the acceleration path, but that would obviously introduce a lot of complication. How small of a laser "spot size" could you realistically hope to achieve over a distance of a billion kilometers? Is there a theoretical best that you could shoot for?