Stefan, Dan, Gabriele, Evan

Summary

Tonight we worked on making the ASC more robust, and the signals more decoupled.

Details

We are able to close the same loops as last night (LHO#17124), with the same issues:

1. In order to engage cETM or PR2, these optics must be manually adjusted beforehand to bring the error signals close to zero.
2. The PR2 and PRM loops seem to make the sideband buildups less stable.
3. No matter what loops we close or what optics we touch, we cannot seem to improve the recycling gain (as measured by the arm buildups). Over the last few days, we seem to get about 26 W/W. However, we can seemingly improve the visibility (mainly by adjusting cETM) as seen in LSC-REFL_A_LF. So it seems that we can make more power enter the interferometer, but we cannot get it to circulate in the arms.

The goal for tonight was mainly to try fixing (1) and (2). For (2), Stefan worked on decoupling the POPA → PRM loop by applying a pitch step to PRC1, and then recording the resulting steps in the control signals of INP1, PRC1, PRC2, SRC1, and SRC2. These step sizes (normalized by the step in PRC1) are then put into the output matrix. This gave a noticeable decoupling of the loops.

For (1), we noticed that for every 0.1 µrad step in ETMX (pitch or yaw), PR2 required a step of 0.1 µrad in the same direction. Therefore, we are now experimenting with a new loop: REFLA9I + REFLB9I → ETMX + ETMY + 2×PR2. The factor of 2 was established empirically using the same method as above. The full cETM drive vector is −5.2 for PRM, 2.0 for PR2, 1 for ETMX, 1 for ETMY, −9.5 for SRM, 1.8 for SR2, and −0.15 for IM4, but we have not yet employed it successfully. It causes a very sudden unlock of the interferometer, whose cause we haven't pinned down yet.

Some notes about housekeeping:

1. We rephased REFLA9I and REFLB9I by driving an 8 Hz line into PR2 pitch and putting all of that signal into the I phase.
2. The turning off of the DRMI WFS and the top-stage suspension offloading is now handled in PREP_TR_CARM instead of CARM_ON_TR.