[Dan Evans]

That can't be required just to burn metal; it must be either for penetrating a media or for the radius of the beam (as one poster pointed out).

Yes, and a lot of other things too. Maybe that figure was was the power input to the laser and the laser was very inefficient. Do you have a link to that source? Was it referring to an xray laser? If so, they weren't very collimated. The beam could have expanded a thousand miles wide. And also what does this have to do with atmospheric absorption? SDI was to shoot down missiles during the boost phase after they left the atmosphere.

[ableChair]

SDI was to shoot down missiles during the boost phase after they left the atmosphere.

No, see my previous posts. These numbers were for a land based laser that fired at mirrors in orbit. It's a two-way attenuation by the atmosphere.