craig.cahillane@LIGO.ORG - posted 01:36, Wednesday 30 October 2019 - last comment - 02:54, Wednesday 30 October 2019(52792)
Checked CARM for locking, no luck so far
We are having problems powering up. The problem happens while the rotation stage is rotating to increase power incident on the IMC. To compensate our analog CARM and IMC loops for the increase in optical gain, we decrease the analog gain using the H1:IMC-REFL_SERVO_FASTGAIN slider. This slider goes in steps of 1 dB, so it's not exactly a smooth ride, but usually it's fine. Now it is not. Dips in power everywhere after the IMC (MC2 trans, MC4 trans, PRG) are associated with some, not all, FASTGAIN steps. It's not clear what changed. This wasn't a problem Saturday or Monday. I checked the IMC and CARM OLGs just before we power up (attached), and both looked fine. We tried decreasing the CARM gain on H1:IMC-REFL_SERVO_IN2GAIN by 3 dB (from -22 dB to -25 dB) just before running MAXIMUM_POWER, to test if the three UGFs in the CARM loop was the problem. It was not, the decrease in CARM gain did not help with the power glitches. (btw CARM has three UGFs because at ~20 kHz the 9 MHz sidebands become resonant in CARM, increasing the CARM plant gain. This is why the loop wins phase there.) Anyway, the "ISS troubles" from earlier were the symptom that kills us. Jenne moved the second-loop turn on to after powerup, so the ISS no longer immediately ends the lock. Unfortunately, other lock-enders include the ADS system, whose output is not limited during powerup and is susceptible to large glitches (the impulse causes the ADS lowpasses to accumulate to push the IFO off alignment).
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We lost lock, probably due to a brief loss of the PRC2 error signal. I took a MCL crossover, things are increased overall compared to the old reference, which was probably taken at full lock, but nothing too dangerous with 30 degrees of phase margin. On relock, we tried to increase the IMC gain by 2 dB. On the second click, IMC REFL camera immediately became far brighter, so I retreated the gain slider. We basically triggered a power glitch by raising the IMC gain, seen in MC2 trans, etc. After my heroic save of the lock I almost killed, we decided to lower the IMC gain by 2 dB. This solved our powerup power glitches problem. Likely, the IMC UGF was getting too close the phase = 180 deg point at 100 kHz while we were adjusting power. We are continuing to try to lock. We lost at LASER_NOISE_SUPPRESSION, the next state that tries to adjust these gains. None of this is in the guardian yet. (If anyone knows how to rotate pdfs made by diaggui that would be great. I've tried like three different methods unsuccessfully.)
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Adjusted CARM_TO_ANALOG (ISC_LOCK line 2322) such thatezca['IMC-REFL_SERVO_IN1GAIN'] += 1 # from -1 to 0 dB, CRC 20191030rather thanezca['IMC-REFL_SERVO_IN1GAIN'] += 3to push the IMC gain down. This state works. Put the +2 dB of IMC-REFL_SERVO_IN1GAIN gain back into LASER_NOISE_SUPPRESSION. Hopefully this will avoid the CARM gain increases here colliding with the IMC, so LASER_NOISE_SUPPRESSION will no longer cause locklosses. EDIT: it worked