Hi John. Having founded the L5 Society, I was deep into moon mining technology. The mass driver turned out to be much more difficult than originally thought, though now it might be relatively easy.
The problem is velocity scatter from the ejection end of the mass driver. I don’t remember the exact numbers, but they would not be hard to find in old papers by Tom Heppenheimer. The source of the problem was the switching speed and jitter of the SCRs that controlled the mass driver coils. Over the last 40 years, this has become perhaps 1000 times better with things like GaN fast power transistors.
In the days when Heppenheimer was working on the problem, the only solution was “achromatic” orbits where (by analogy to lenses that bring different wavelengths to the same focus) mass driver launches from a particular point on the moon with a little variation in velocity wind up (with some scatter) at the same place, the “catcher” out near L2.
I spent close to ten years looking into power satellites built from the earth simply because the complications of a moon mine, mass driver, and all the processing needed to fabricate SPS parts were beyond me. (And an estimate of the cost was in the trillion-dollar range.) Power satellites make economic sense if you can get the cost to lift parts to GEO under $200/kg. At high flight rates, Skylon and StarShip should be able to get the cost to LEO down to $100/kg. If electric propulsion with Ve of 20 km/s is feasible, the cost of the reaction mass to get to GEO is another $100/kg.
I gave up because the space junk will not let power satellites be constructed in LEO and if you build them above the junk, the radiation is lethal in hours. That means teleoperation. The cost and time to develop robots or teleoperation were out of my skill range.