[Sudarshan, Gabriele]

Following up on yesterday's activity, today we properly dumped the beams on IOT2R. We tried different configurations: beam dump directly in place of the first mirror of the periscope, razor beam dumps, black glass plate, black galss beam dump. Finally, the best configuration is the one that lets the beams go through the periscope (recentering all three mirrors) and dump the REFL beam far away with a  black glass angled beam dump, and the IMC trans beam with a razor blade dump. See the attached pictures for more details.

With this configuration, we measured the out-of-loop intensity noise with MC2_TRANS, as before with the PRM aligned and the BS misaligned to avoid flashes. After many trials, we could geth the 18 Hz peak down by a factor 10, as shown in the 5th figure. Green is PRM aligned, ISS second loop off, before our modification. Brown is PRM aligned, ISS second loop on, before our modification. Red same as the last one, but aftr the modification.

The IMC transmission beam is not critical, and even leaving it undamped doesn't change much. Instead, the dumping of the REFL beam is critical. By simply tilting the beam dump of a very small angle (order of one degree), we could change significantly the amount of intensity noise. We tried many times to tweak the angle. Once we could make the 18 Hz completely disappear, but then just the action of clamping down the beam dump wornsed the situation. We think it's not really worth to spend more time on this, since a small change of the PRM alignment will in any case move the beam and probably spoil the noise reduction. The last figure shows a comparison of the intensity noise with different beam dump positions. Blue is before the modification, the other traces correspond to sligthly different beam dump positions. We also tried to rotate the beam dump 90 degrees, to have the black glass plates oriented "horizontally', but this didn't help either.

In conclusion, now the situation is much better than before, but the REFL beam transmitted by IM4 and collected on IOT2R is very critical. We should consider a better solution for the long term. For future reference, the last picture shows which beam we are talking about: the purple beam is the REFL beam transmitted through IM4 and sent otto IOT2R where it is reflected/scattered back to IM4, then reflected off the IM4 HR face and finally going to the ISS aray (green beam)