Sheila, Corey, Cheryl

We have been struggling with oscillations around 4.2 Hz which show up in our DARM, CHARD and CSOFT signals for the last several days.  Motivated by that, I checked on the L2A decoupling for ETMX.  It seems that the L2A is actually increasing the coupling to both pitch and yaw above 2.5 Hz, but the DARM loop is unstable at 8 Hz without these L2A filters on.  We haven't fixed anything yet, but we probably do need to spend some time to make length to angle decoupling measurements this week.  We haven't see a 4.2 oscillation all day, that might be because Daniel changed the whitening on ASC REFL A 45.  

Background:

This is a similar frequency to the problems that were mitigated by increasing the relative gain between the PUM and ESD stages in the DARM loop in late Febuary 47164.   In the last week the spot position was moved on ETMX (P2L gain went from 4 to 5) to get an improvement in the power recycling gain, 47609.  There have also been several changes to the pum coil driver states, we've reduced the cut offs in CHARD P 47750 to make that loop more stable, and there have been changes to the offloading gains in the DARM actuator 47784   See the two attached PDF to see what happens to the DARM loop with the PUM gain reduction people tried on Friday, there is nearly a crossover instability at ~9 Hz, although it doesn't seem that the loop was actually unstable.

L2A checks:

After we lost lock because of some ADS errors, I took a bit of time to check the ETMX length to angle decoupling for L1 +L2.  The first attachment shows the response of the optical levers to L2 length drive with the decoupling filters on and off.  The decoupling does it's job at 0.5 Hz, but above 2 Hz both the pitch and yaw decoupling are actually increasing the cross coupling.  (The measurements were made with the same excitation level, with the decoupling off there is no coherence above ~2.5 Hz, but with the decoupling on the coherence is high). I also tried this measurement with the L2Y decoupling off and the L2P on, the results were what one would expect.  Hang redid the fitting of these decoupling filters for the new suspension, 42692 based on measurements Jeff K posted in 42662.  The coherence of several of those measurements petters out above ~2.5 Hz, so it's not surprising that the decoupling isn't good. I attempted to make some measurements that would have coherence to higher frequencies, but didn't get good results.  We might need to take the time to do swept sine measurements. 

For L1 the situation is different, the L2P decoupling doesn't have much of an impact at all.  L2Y reduces a peak at 1.37 Hz.  (second attached screenshot) There is an alog that describes the L1 L2P filter here: 43150 and in comments. The coherence of these measurements isn't good above about 2.5 Hz. For L2Y we have not used a fit to the data that Jeff collected, we've just been using a DC gain since January: 46236

Impact of L2A on locking:

We tried locking several times without L2 L2A engaged, and had a DARM instability at 8 Hz ring up within a minute or 2 of transitioning DARM control back to EX with the low noise ESD each time.  I reverted the changes to the LOCK gains, but we still see the 8Hz instability unless the L2 L2A decoupling is back on. 

Configurations we tried:

• no L1 L2Y, no L2 L2P or L2Y, L2 lock gain 15 L1 lock gain 1.16, coil drivers state 1  8 Hz lockloss
• no L1 L2Y, no L2 L2P or L2Y, L2 lock gain 23 L1 lock gain 1.06, coil drivers state 1 8 Hz lockloss
• no L1 L2Y, no L2 L2P or L2Y, L2 lock gain 23 L1 lock gain 1.06, coil drivers state 3 8 Hz lockloss
• no L1 L2Y, L2 L2P and L2Y back on, L2 lock gain 23 L1 lock gain 1.06, coil drivers state 1  This was stable for ~ 30 minutes before something rang up at 3.8 Hz.  I was trying to measure the PUM/ESD crossover, with an exctation at 20Hz when we lost lock, which I think is unrelated to the lockloss.  
• L1 L2Y and L2P on, L2 L2P and L2Y back on, L2 lock gain 23 L1 lock gain 1.06, coil drivers state 1.  Again stable for ~30 minutes before unlocking, no sign of any oscillation in LSC or ASC.  Unlike what Craig observed here, 47784, there  was plenty of range on ETMX, L2 is using about 1/10 of the range for the microseism.  

Other notes from today:

• I have added 10dB of gain to ADS YAW4+YAW5, this should help to make things a little bit less slow while relocking
• I have tried a couple more times to engage the green WFS to TMS loop on the X arm in locking arms green, this seems fine and helps increase the build ups. 
• I've added SQZ_MANAGER to the list of nodes that ISC_LOCK will release for inital alignment.  
• Daniel changed the whitening state of REFLA45 WFS, because of the error seen here: 47666  We haven't seen 4.2 Hz at all.