Over the last few weeks we've done some work on ARM ASC loops, mostly for the hard modes.  

We've been in the HARD soft basis for a few weeks (19752 19775).  Changing to HARD/SOFT was didn't seem to have negative consequences for us, we don't think that it changed our A2L decoupling much. Our method tuning the coefficents before the change was not very precise, some of the coefficents are now different by up to 50%, but they were probably just not correct a few weeks ago.

In the past two weeks we have been retuning the loop shapes for the hard arm loops. There were three motivations:

• We had locklosses both durring the CARM offset reduction (REFL trans 19243) that were due to a DHARD instability (20055), and from full lock that seemed to be from arm signals contaminating other ASC signals, in particular the SRC alignment (19855).
• We had 10% or more fluctuations in optical gain with frequencies around 0.05-0.1 Hz, which we monitored with a BLRMS around a calibration line.  Although there was no conclusive evidence, watching ASC error signals it seems that the fluctions in CHARD (Yaw especially) on the same timescale could be the cause of the optical gain fluctuations.
• We have been in the hard soft basis, but we are running optical levers on the ITMs and not the ETMs.  We have had some instabilties when we turn them off.  We don't understand why the damping helps us stay stable. 
• The DHARD yaw loop had several marginally stable ugfs, and
• CHARD loops had very low ugfs, below 0.1 Hz.

DHARD retuning was fairly sucsesfull, we added boosts to both DHARD loops that seem to help our stability at high power.  Neither of these loops has a cut-off, which we need to add.  We have locked a few times without op damping, but turned them back on due to instabilities. 

We then moved on to CHARD, where we have rather bad SNR in the sensor (the sum of the REFL 9 I signals).  We had very low gain loops here (both less than 0.1 Hz) because we had previously seen that increasing the gain added noise in DARM.  We tried rephasing the refl 9 wfs by driving a line into the mode cleaner, and by driving CHARD in pitch.  (20364 and at a higher TCS power: 20368) This rephasing did seem to cause any stability problems, and could be fine tuned. We then increased the bandwidth for the CAHRD loops.  This introduced large non-stationary noise into DARM, which might have been made worse by a non-optimal alingment.  Last night we reduced the amount of non-stationary noise by reverting to our old, low gain loops (with cut offs and leads still added).  There was some remaining coherence with CHARD (non-stationary, up to 90 Hz).  Today we think that we fixed some SRC alignment issues (20519) which we think is the reason why the coherence with CHARD has been reduced.  However, we still have coherence with CHARD from time to time. 

Nominally, our next steps would be to try to find a better error signal to use for CHARD, by driving excitation in CHARD, im4, and one of the PRs to check if our input matrix can be improved. We could also think about running DC loops to TMS to create an AC coupled signal to blend with refl for CHARD. To improve the stationarity of DARM we could be more agresive with cut off filters or decrease the bandwidth more. UPDATE: Evan and I looked at the sensing matrix for refl tonight and it seems like the phasing of 45 is way off.

Open loop gain measurements:

DHARD PIT 20144  Since this measurement Evan has added a resonant gain at 0.46 Hz tonight to squash the instability we sometimes see.  

DHARD YAW 20084 no changes to this loop that I know of since the measurement. 

CHARD PIT 20497 This gain has since been reduced by 6dB, and neither of the boots are used.  

CHARD YAW 20499 Since this measurement 20 dB of gain was removed, the MS boost was removed (a gentle boost with a pair of complex zeros at 0.5 Hz that adds 25 dB at DC), a 2:3Hz lead filter was added and a 30Hz elliptic low pass was added.  

Spectra of the error and control signals are attached.  So are the control filters, CSOFT and DSOFT are the same, so only CSOFT are plotted. Obviously, the soft loops could be improved, but we haven't gotten to that yet, and aren't sure if we will.