@annodomini2: "...colossal, mile-wide Dark Sky air/spaceports..."
Well, they're talking about platforms that are a "mile wide." Not sure this relates to the gas envelope, or the platform itself, but if airship history is a guide, the human-habitable section of an airship tends to be quite a bit smaller than the envelope.
So, for sake of argument, let's conclude that the gas envelope is "a mile wide," and is shaped like a thick disc, or an extremely oblate spheroid.
This means that if the sun were shining perpendicularly on the upper surface of said photovoltaic envelope, there would be (at minimum) about 0.785... square miles of incident area:
-- -- A = pi (r^2) = pi * (0.5 mi ^ 2) = 0.785398163 square miles
-- -- -- -- (It's a "mile wide," which means 0,5 mile radius.)
(Probably more, since the "top" of the envelope "disc/spheroid" would probably have a noticeable curved "bulge" in the middle, increasing surface area, but to keep things simple, we'll presume it's perfectly flat.)
0.785... square miles translates into 2.034... * 10^6 square meters:
-- -- 0.785398163 square miles * (2,589,988.11 square metres / square mile) = 2,034,171.9 square metres
Let's subtract, say, 7% of that area to factor for zones that can't be used for solar collection (seams between cloth sections, cabling, envelope expansion control bladders, etc.) and we're left with about 1.892... * 10^6 square meters of photovoltaic area:
-- -- 2,034,171.9 square metres - 7% = 1,891,779.87 square metres
If each square metre produces just one watt of power, our photovoltaic balloon-disc generates almost 1.9 **megawatts** of electricity at its relative "high noon" point, well in excess of the 200 kilowatts needed to drive a VASIMR engine.
So, the problem isn't available power; rather, it's the weight of everything else that goes into getting that power to the ion drive, like cabling, control systems, and other-and-sundry components.
Making a cloth photovoltaic envelope that generates the necessary amount of electricity isn't the issue. It's getting that power to the engine in a manner that still allows enough lifting capacity for useful cargo...
