Last night, I left the XARM locked with PR2 ADS. According to the IMMT TRANS QPD signal and PR2 drift, the IMC seems to be the evil of these days IFO drift. And now, for some reason, the IMC got calm down. Attached figure shows the IMMT TRANS QPD signal (top two) and PR2 oplev (bottom).

The first attached figure shows the degrading XARM TRANS with IMMT TRANS QPD and IMC oplev signals, and the second is same plot when I dare to misalign the MC mirrors.

The IMMT TRANS QPD moves 0.1 cnts or more in the first figure, but degradation in IMC TRANS (top mid) is not shown. However, obvious digradation of TRANS power is shown in the second figure, although IMMT TRANS QPD moved less than 0.05 cnts. Also, the IMC_REFL image has changed obviously.

This indicates that the injection beam moves, although IMC itself does not move. What's going on?

We noticed that, because we aligned the PRM reflected beam to the beam dump, the heat conducted to the optical table warms up the optical componenent on the table (STM1, STM2, IFI, IMMT2). That's why IMMT1 TRANS and IMMT2 oplevs moved. The attached plot is the IMC TRANS power v.s. IMMT1 TRANS and IMMT2 oplev.

In the plot,
t =-800 IMC TRANS power was incleased to 5.6W
t = -300 PRM is aligned (beam not hitting on the dump)

[Kenta, Masayuki]

We put beam dump to dump the misaligned PRM reflection light on the REFL table. If you request MISALIGN_FOR_PRFPMI to PRM guardian, the PRM will be misaligned with smaller angle where the PRM reflection light will be dumped with the beam dump on the REFL table.

Actually, the main beam is slightly clipped with that dump. We should modify it at some point.