Maybe, but they say that it's focused in the center of the chamber because that's where you want ignition to start. In that case, you might not have that high flux at the entry aperture (you might not want this for other material reasons, too). In any case, I don't think you want to start combustion at the edge because that's the least efficient place to do it.
Maybe the beam is wide enough before it is focused so that the soot wouldn't block enough of it to be a problem before the soot is blown out of the chamber.
No. But I only intended to address the point that if the beam is focused in the center then it would take more than a speck of soot to block the wider, lower flux, defocused beam where it enters the cylinder.
