A 1.15GW solar farm here on earth would produce 8GW on mercury, it takes 8.8 kj to launch a kg of mass from mercury to mercury sun l1, such a plant could launch 915 tonnes a second from a railgun or 28.85 Giga tonnes per annum (firing in 15 minute batches or 825,000 tonne masses). The first initial fraction of a percent of mass will be iron tubing, this will be assembled into a 1 km long closed hollow core 10-20 meters across, the other 99% of the cylinder mass will be Mg (noncombustible high strength alloy 98% Mg 1% al 1%Ca) this will be placed over the core and the outer layer tacked to keep the chunks together, a thin concave mirror (1-3km diameter long and high and mm thick) will then focus light onto the alloy core structure, this will fuse all the pieces together and form a small proto cylinder. the hollow core will be removed then the space it occupied will be filled with pure Sulphur. the process will be repeated and this time at the point the alloy is at its melting point the S will already be gas and the whole structure will inflate like a balloon, the mirror will be removed, S vented and now you have a 19km x 3.8km 9m thick walled Mg cylinder.