Solar Wind Concentrator Assembled at LANL (08-00)
Installation of the targets into the Genesis concentrator sample collection device was completed August 21, 2000 at the Los Alamos National Laboratory. It was accomplished by two LANL mechanical technicians and checked by a payload quality assurance support person from JPL.
Workers in the LANL assembly area wore full body suits and cleanroom boots, rather than the smocks that are typically used during spacecraft component construction. This step was taken to meet stringent cleanliness guidelines for Genesis collector materials. A particle counter was used to monitor room cleanliness throughout the two-day procedure.
The concentrator is shaped like an open-topped cylinder. It is positioned under the sample collection arrays in the canister. Once the arrays are swung out, solar wind will be able to flow directly into the concentrator.
Solar wind consists mainly of hydrogen and helium, the two lightest elements. The Genesis scientists are interested in collecting samples of all the other elements in solar wind, from lithium to uranium. Their abundances in the solar wind are very low. Unfortunately, one of the most interesting elements, oxygen, tends to have high backgrounds in almost all materials. It is thus desirable to increase the implanted solar wind over this oxygen background. The Genesis concentrator does this by first applying an electric field through a grid or screen that repels the ionized hydrogen. The incoming solar wind particles are reflected near the back of the parabolic concentrator surface onto a target. The back surface is not smooth, because that would reflect sunlight onto the target, making it way too hot. Instead, the mirror is milled into tiny micro-steps so that the sunlight is reflected straight back out the way it came in. The microsteps (below) do not prevent the mirror from directing ions toward the target because the ions do not actually bounce off the mirror. Rather, they feel the repulsive force of the mirror and are turned around some distance above the surface. At this distance the mirror still has a force field similar to a completely smooth surface!
The target onto which the solar wind ions will be concentrated is made of four individual quadrants. The two that appear greenish in the photo are silicon carbide made by Cree Research, Inc. The upper dark quadrant is C-13 enriched diamond made by Raytheon. The other dark quadrant is semiconductor-grade silicon with an amorphous diamond coating made by Sandia National Laboratory.
The diamond target is inspected after assembly. Technicians are surprised by the color of the C13 diamond segments. Having worked with Raytheon's isotopically normal diamonds before, they expected the Raytheon-produced quadrant to be transparent. They suspect the dark appearance is caused by very minor impurities of boron, which will not cause problems for the solar wind sample collection procedure.
After the target is completed, the electrostatic grid that will cover the concentrator is inspected. This looks like a very fine-mesh screen held inside a circular framework. The grid is attached to the frame by spot welds, a shim ring, and screws. Technicians are looking for obvious unevenness in the tension of the grid material, which is also mapped microscopically.
After assembly the concentrator endured operational tests in a vacuum chamber to simulate conditions at L1. After passing all its tests, the Genesis concentrator was shipped from LANL to JSC on August 23, 2000 for installation into the science canister.
For additional technical information read "Solar Wind Concentrator" by McComas, et al. |
