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@Hobbes' answer mentions that Deep Space 1 used special refractive solar concentrators attached to photovoltaic cells.

According to SCARLET:

The innovative array uses 720 lenses to focus sunlight onto 3600 solar cells, each converting the light into electricity to power the ion propulsion system and the rest of the spacecraft. The array produces about 2500 watts when at the same distance from the Sun as Earth. That may not sound like a lot (it's only enough to run 3 microwave ovens), but it's much more than most spacecraft exploring the solar system.

The NASA Spinoff article Concentrators Enhance Solar Power Systems says:

“Everyone remembers the ion engine on Deep Space 1, but they tend to forget that the SCARLET array powered it,” says Piszczor. “Not only did both technologies work as designed, but the synergy between the two, solar power and propulsion together, is really the important aspect of this technology demonstration mission. It was the first successful use of solar electric propulsion for primary propulsion.”

I can see some disadvantages; these bulk refractive lenses may add moe mass than simply covering the same area with photovoltaic cells, and they are probably more directional; after a certain amount of misalignment power would probably drop more rapidly than for a flat panel without focusing optics.

Why then did this mission use this technology instead of an equal area panel without 720 lenses?

"bonus points" for other examples where similar refractive concentrators were selected and used in space missions.

Concentrator-related:

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    $\begingroup$ "The concentrating lens of Entech’s design had a curved outer surface with Fresnel patterns, or grooves, on the inside." They used a nearly 200 years old technology to reduce the mass of the lenses developed by French physicist Augustin-Jean Fresnel for lighthouses. $\endgroup$
    – Uwe
    Feb 10, 2019 at 11:17
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    $\begingroup$ These cells have a higher efficiency and where also used on some GEO satellite busses. Check upload.wikimedia.org/wikipedia/commons/3/35/… - all the in-lab efficiency records of the past years where reached with concentrator-based setups (the symbols with a dot in the middle) $\endgroup$
    – asdfex
    Feb 10, 2019 at 14:13
  • $\begingroup$ @asdfex I see, so per square meter, more power is available by using a substantially smaller area of PV with concentration than for a full square meter of PV. I think you could probably write that up as an answer. Considering that this was a solar-electric demonstration, they probably wanted as absolutely much power as possible. btw I just added some related links at the end. $\endgroup$
    – uhoh
    Feb 10, 2019 at 15:52

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DS1 was a technology demonstrator mission. The goal of the solar concentrators was to lower the cost of the solar array.

SCARLET uses cylindrical silicone Fresnel lenses to concentrate sunlight onto GaInP2/GaAs/Ge cells arranged in strips. Including the optical efficiency of the lenses, a total effective magnification greater than 7 is achieved. With relatively small panel area actually covered by solar cells, the total cost of cells is lowered, and thicker cover glass becomes practical, thus reducing the susceptibility to radiation. The dual junction cells display significant quantum efficiencies from 400 nm to 850 nm, and achieved an average efficiency in flight of about 22.5%.

DS1 was a very early user of triple-junction cells (launched in 1998, while the first mention I can find of commercial triple-junction cells was 2000), so those cells would have been more expensive than usual.

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  • $\begingroup$ More information about the line-focus Fresnel lens concentrators here plus some nice images to include into the answer. $\endgroup$
    – Uwe
    Feb 10, 2019 at 18:54
  • $\begingroup$ Found the images, but the main focus of that paper is about a successor to the SCARLET array, using thin-film Fresnel concentrators. I get the impression the SCARLET array used a solid wedge of plastic rather than a thin Fresnel lens. $\endgroup$
    – Hobbes
    Feb 10, 2019 at 19:00
  • $\begingroup$ Despite the block quote, I have a hunch that the use of higher intensity concentrated light was necessary for reasons of device efficiency as well, as pointed out here. $\endgroup$
    – uhoh
    Feb 11, 2019 at 23:42

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