[ufmace]

Curiously, the other people who responded all think it'll be downhole. Anyways, this does make the device heat practical to handle, but now you have to figure out how you're going to get the vaporized rock out and shoot your microwave laser thing through 20km of rock vapors. And we're also presuming you can keep 20km of borehole exposed to atmospheric pressure with only this supposedly glass wall thing preventing formation fluids from flooding the borehole. If that cracks anywhere and floods it with high pressure hydrocarbons, maybe you could seal it up before all of your surface hardware gets blown up, but what do you do next? Oil wells have procedures for how to circulate heavier fluid back in to get the well back under control, but I have no idea how you fix a failed glass wall thing.