kiwamu.izumi@LIGO.ORG - posted 07:31, Thursday 20 February 2014 - last comment - 11:01, Thursday 20 February 2014(10199)
Done with the morning red lock
I am done with the morning red lock and handed the interferometer over to Keita and Jax. Here are some notes for the green and blue teams:
- I decreased the FSS threshoold (see alog 10195) to 0.8 from 0.9 because it was dropping the lock frequently.
- I decreased the MC2_M3_LOCK_LIMIT from 6.4e6 to 4e5 which is the nominal value.
- I touched up the alignment of PR2. So be aware of it when one aligns the red light into the arm cavity.
- PRMI is left aligned. Feel free to misalign it.
- ETMX is now aligned without the oplev damping loops running (see alog 10197).
PRMI locks:
Today I was able to lock the PRMI with the sidebands resonant in the PRC. There were three key points:(1) the alignment was not great, (2) the notches in FM6 of MICH (see alog 10127) was too aggressive for the initial acquisition and (3) a 30 Hz low pass was not engaged in MICH's FM9 which was usually set up by the guardian.
My first guess for the MICH and PRCL gains were 40 and -0.4 respectively (see alog 10168) because these are the nominal values we have been using in the past week. However, it turned out that the alignment of PRMI was not good enough so that optical gain was smaller by a factor of between 2 and 3 for both MICH and PRCL. So I empirically ended up with gain settings of 80 and -1.4 for MICH and PRCL respectively to acquire a lock for a long period. Then tweaking PRM and PR2 gave me a high build up which was approximately 30000 counts in POPAIR_B_RF18 and this is about the same amount we saw on 11th of February. The attached is a trend of the power build up and alignment sliders. The misalignment was mainly in pitch.
At the end, the gain was at 40 and -0.6 in MICH and PRCL respectively. I didn't get a chance to measure the UGF.
Next steps:
Our short term goal is to do the "one arm + PRMI 3f" test and therefore the stability study of the 3f locking is the most critical at this moment. However I (re-)found that the daily alignment is time-consuming and is something we must automate. So I would like to get the dither system running at first before entering a serious 3f study.
Even though the PRMI didn't spontaneously drop the lock at the end of the morning commissioning, fluctuation in the intracavity power was large. The power could drop to the half of its maximum and was oscillating mainly at 0.9 Hz. Looking at the PR3 gigE camera (VID-CAM09), I found that the oscillation of the cavity power synchronized with scattered light off of the PR3 cage which looked oscillating mainly in pitch. So I tried to identify which optic was moving by using the data from this morning.
According to a coherency test (see the attachment), ITMY is the most suspicious at this point.
ITMY was oscillating at 0.4-ish Hz and shows a moderately high coherence with the POP_B_RF18. It is possible that this 0.4-ish Hz motion of ITMY then produced a fluctuation in POP_RF18 at the twice higher frequency due to the quadratic response of the cavity power. This issue is not a killer at this point, but the study will continue.