Sheila Kiwamu Patrick
We looked at a couple of the locklosses that people had from NOISE_TUNINGS last night. We found that the problem is when the ISS second loop becomes DC coupled it saturated. The attached screen shot shows some of the symptoms, where MC2 trans power is ramping up right before the lockloss, the IMC guardian changes state to turn off the ISS, the AOM driver mon and ISS second loop outputs have dramatic changes in the seconds before the lockloss.
I followed the instructions by Jeff K and D Sigg in alog 31942 to reset the ISS diffracted power. (H1:PSL-ISS_REFSIGNAL and H1:PSL-ISS_SECONDLOOP_AC_COUPLING_OFFSET)
I am not sure if turning on the ISS 3rd loop (done to help avoid CSOFT Pitch problems over the weeked) contributed to this being a problem last night.
In this afternoon, after a lockloss at NOISE_TUNINGS, we updated the offset (H1:PSL-ISS_SECONDLOOP_AC_COUPLING_OFFSET) again following Kissel's instruction. We set it to -0.12 which had been +0.05. However, this tuning wasn't good enough -- it still caused a power variation as the DC coupling was engaged by 10% or so. In the end, we set it to -0.095 as a result of a few iterative trials.
Note that when the offset was at +0.05, this caused a reduction in the power sent to the interferometer as the DC coupling was engaged. When it was -0.12, it increased the interferometer power instead.
2nd loop AC coupling offset H1:PSL-ISS_SECONDLOOP_AC_COUPLING_OFFSET is fine as of now.
No need to adjust. The fact that the diffraction stays at about 4% when the 2nd loop is ON and AC coupling is ON means that the offset is good. (That offset should not have any impact on the kick to the ISS when you turn off AC coupling.)
The problem seems to be the timing you turn on the third loop.
When the third loop is on while the 2nd loop is AC coupled, AC component of the AC coupling output becomes huge. We're talking about 10-something% RMS in the equivalent RIN (see the first attachment, top left, beginning of the plot).
In that plot, when AC coupling is turned off, instead of settling on the average of 6000, it settled on 6400. Suddenly the DC increased by a factor of 1.07, and this is seen by the second loop PDs (Ch15).
The output is held at the exact time when the AC coupling is turned off (except that it still slowly changes due to power scaling). Since the AC component is so huge, each time AC coupling is turned off, there's a big chance that the output is held at some number that is totally off from the average.
Smooth DC coupling transition requires that the AC coupling output settles on the value close enough to the average. This is only possible when the AC component is small enough to start with. Enable 3rd loop after ISS is DC coupled. If that's impossible we need a fancier solution.
If you look at the 2nd attachment, which looks at the same channels for 7 days, you can see that the AC coupling output never got huge until the 3rd loop was put in place in the guardian. It seems like we got 2 failures out of 3 trials.
Guardian changed, no 3rd loop for now.
With a telephone support from Sheila, ISC_LOCK.py line 3184 was commented out so that the third loop is NOT turned on during INCREASE_POWER.
We aren't turning on the 3rd loop in the guardian. If you think that CSOFT instability comes back, for now follow the instruction in alog 37046 and manually turn on the 3rd loop. You'll risk kicking yourself out, but if the noise is terrible you might choose to gamble.
I forgot earlier on the phone, we commented out the line in the guardian which requests the third loop, but this isn't enough to make sure it doesn't come on. We also have to turn off the input. The reason is that in Noise Tunnig the request for IMC_LOCK is set to ISS_DC_COUPLED, which will require it to pass through ISS_TR_CLOSED, which turns on the third loop unless the input is switched off.