Interstellar material will go right through it, leaving a hole no bigger than the material. This should have a negligible impact over it’s lifetime. Space junk would be a bigger problem. It would need to be well outside of the envelope of space junk. One possibility would be for it to not orbit the earth, but rather trail the earth as the earth revolves around the sun. If placed correctly this should prevent it from being pulled into the earth and will indefinitely maintain it’s relative position.

Japan is proposing a similar system, where the sunlight is converted into a laser beam before it is sent to earth: http://www.treehugger.com/files/2007/09/orbiting_space.php
I’m thinking the system I proposed would have to do something like this as well especially if we’re talking about the distances I’m proposing here.

Another thing to consider: using these beams to propel spacecraft. This has already be proposed with the “solar sails” attached to the spacecraft, but rather than put a bunch of huge sails on the spaceship … the spaceship would only have one small sail that would collect the concentrated rays as sent by the huge solar array I’m proposing.

I also think the challenge of overheating optics can be addressed by using only reflective optics (mirrors) with nearly perfect reflectivity - which is entirely possible. Given an overheated condition a bimetal contraption could move them out of the focus point until sufficient heat has radiated into space afterward the bimetal system would naturally bring the optics back into play.