Clicking through NASA.spinoff.gov I came to the page for spinnoff 2001 Glenn Research Center and found this:
Glenn Research Center's sapphire refractive secondary concentrator will be used with primary collector-concentrators to focus solar energy. The solar energy can be used in power conversion systems, thermal propulsion systems, and solar furnaces.
In Archive.org I found more images. Starting at this https://archive.org/details/GRC-C-2000-454 you can view many more at the bottom.
I then found these:
- NASA Technical Memo 2000-208401 A High-Efficiency Refractive Secondary Solar Concentrator for High Temperature Solar Thermal Applications
- NASA Technical Memo 2009-215802 Failure Analysis of Sapphire Refractive Secondary Concentrators
Near the bottom of page 3 of TM-2000-208401 in the section labeled Prototype Hardware Fabrication & Assembly, it says:
The secondary concentrator DTIRC has an 8.9 cm. inlet diameter, a 1.9 cm. exit diameter, and is 12 cm. long. The flux extractor is 15 cm. long and has 3 equilateral facets.
I think it means that the total length is 12 + 15 = 27cm.
I think the idea is that they are non-imaging and use what I have heard called non-Liouvillian optics; in other words they don't obey Liouville's Theorem and conserve phase space or étendue.
But I still don't understand what these are "for" or how much they actually concentrate.
Question: So I'd like to understand how the "Glenn Research Center's sapphire refractive secondary concentrator" works, and in what situation it is used with respect to the application (a solar rocket engine?), and with respect to the primary concentrator.
Here's the abstract for NASA TM-2000-208401:
A refractive secondary solar concentrator is a non-imaging optical device that accepts focused solar energy from a primary concentrator and redirects that light, by means of refraction and total internal reflection (TIR) into a cavity where the solar energy is used for power and/or propulsion applications. This concept offers a variety of advantages compared to typical reflective secondary concentrators (or the use of no secondary at all): higher optical efficiency, minimal secondary cooling requirements, a smaller cavity aperture, a reduction of outgassing from the cavity and flux tailoring of the solar energy within the heat receiver. During the past 2 years, NASA Lewis has been aggressively developing this concept in support of the NASA Marshall Shooting Star Flight Experiment. This paper provides a brief overview of the advantages and technical challenges associated with the development of a refractive secondary concentrator and the fabrication of a working unit in support of the flight demonstration program.