You should be able to see quite clearly the discoloration of the copper between the two yellow arrows (click any picture to enlarge). The two yellow arrows are located near the ends of the concentrator reflector and show where the heat originated.
As I wrote in about the stagnation test, the collector does not normally operate with no liquid flow to cool the absorber yet the system must be designed so that it is capable of withstanding this condition without damage.alloys vary between manufacturers. I do not have technical information on the silver solder that I used so I will need to find some for the next build that has a known, defined melting temperature.
Still, the effect of the stagnation heat are quite stunning. Clearly this is something that should not happen regularly.
The fibreglas bung has deteriorated somewhat from the neat roll I originally inserted in the end of the evacuated tube. This is because I had partially removed the absorber from the evacuated tube after the test and left it partially out for a couple of days of wind and rain which battered the fiberglass somewhat. Normally the bung would be fully inside the end of the evacuated tube.
The absorber overall is about five feet long and my impression of it sitting on the bench is of several electric kettle heaters unwound and straightened out. I am estimating that the absorber was subjected to about 800 watts of power over several hours in an enclosed space (the evacuated tube). The effect on the copper tube and mesh is dramatic.