Philippe, Robert
Summary: We found no evidence of an overall reduction in coupling at the septum, associated with replacement of the main window. This could be because the window was not angled to retro-reflect light from the beam, so additional angling made no improvement. Another possibility is that scattering coupling was not dominated by the window. We found a candidate for the latter explanation: a beam spot on the HAM5 side of the septum. The septum moves more than the ground and is a better reflector than other parts of the vacuum enclosure, so we may want to consider shrouding it.
Figure 1 shows a comparison of coupling before and after the break. Some frequencies look better by factors of 2 or 3, others look worse by similar factors. We don’t know how much of this is due to differences associated with the scaffolding. The average of “before” and “after” coupling factors is about the same. On the whole, I think we can say that any improvement was less than a factor of 2.
We had hoped for more of an improvement, so we checked for other problems by redoing the other measurements that had led us to the septum before the break (none led specifically to the septum window). Figure 2 shows that both the arrival time and the DARM amplitude for impulse injections are still most consistent with coupling near to the septum accelerometer. And, the frequency structure in the signal from the septum accelerometer is still the best match to the frequency structure in DARM.
In addition, we tried the new beating-shakers technique, and preliminary results are also consistent with coupling at the septum.
We photographed the spot on the new septum window. Figure 3 shows the new and old spots and scattering centers on the window. The brightness at 3 degrees was roughly similar to what I had photographed before the window replacement, as determined by camera settings used to capture it.
One possible explanation for un-improved scattering coupling is that we weren’t getting a specular reflection off of the window, so angling it did not help, and the improvement in window quality or cleanliness did not change the scattering by much.
Another possibility is that there is/was another beam on the septum that dominates the scattering from the septum. So, we looked again for other beams on the septum. We found one on the HAM5 side of the septum near the central port (Figure 4) that I had not noticed before. I’m not sure whether or not this beam spot was present before the break because I could only see it from the very edge of one view port and from the illuminator port, which I had not used previously. I’m not sure of the origin of the beam spot on the septum. According to camera settings it is quite bright, requiring about the same camera settings as the beam spot from the window, but at a much larger angle than 3 degrees.
Unfortunately, the presence of the spot on the septum makes it difficult to conclude whether or not we must remove the window in order to make further reductions in coupling. I think that one thing we have learned is that we should consider a scheme to shroud the septum further since it moves more than the ground, is highly reflective, and is normal to the beam path.
