lisa.barsotti@LIGO.ORG - posted 00:08, Tuesday 28 June 2016 - last comment - 09:12, Tuesday 28 June 2016(27990)
40W towards low noise, more PI damping tests
Jenne, Carl, Sheila, TJ, Stefan, Lisa We have a quite reliable locking sequence to 40W at this point (recycling gain ~28.5, same SOFT offset strategy as over the week-end with one set of offsets engaged before power up, one at 40W; SRM alignment done by end), so tonight we started going back to low noise while doing PI testing. Here is the list of things we successfully did; we still need to modify the ISC LOCK code to be compatible with some of those (note that we are still on POPX, so the POP beam diverter is open):
- Coil driver switched to low noise
- ASC cut-offs: JLP35 for DHARD (P/Y) and CHARD (P)
- Modified CHARD-Y 35 Hz cut-off (the new one is called "modcut"), because the current one had gain peaking between 20 and 30 Hz that was preventing us from switching ESD Y to low noise; CHARD-Y gain also lowered by 40% (from -0.7 to -0.4)
- ESD Y switched to low noise
- ISS3 loop off, ISS 2 ON (instructions from Kiwamu; Jenny had to reduce the actuation delay from 0.2 to 0.1 to engage the second loop)
- OMC whitening on
- MICH feed-forward: the -15.9 nominal gain seemed ok
- Nominal SRCL tuning: LP 80, gain reduced by a factor of 2, feed-forward engaged with nominal gain -1
- REFL and AS beam diverters closed (POP open as we are still on POPX)
- ESD X in state 2 (tested bias off -- no impact on the noise)
- ITMY ESD state switched to 2
- Jenne realized that the OMC ASC output had railed --we switched to QPDs and stayed on QPDs for the rest of the lock, but awe were already transitioned back to high noise at that point..
Plot 1: Noise spectrum for tonight. Below 60Hz is due to ASC, and can still be addressed.
Plot 2: Auxiliary loops noise. Note the increased coupling of the mechanical resonances just below 60Hz. I suspect that we still are not quite centered in the recycling cavity.
Plot 3: Aux loop coherences
After this lock broke Carl and I turned on the ITM ring heaters to 0.5 Watts each, and left the end stations at 1.5 Watts each. Carl thinks that this will help with PI, and this will also set us up to try some common TCS tuning tomorow.
Some of the things that Lisa mentioned are now in the guardian.
Attached is a screenshot of when (in PSL power terms) the OMC ASC rails.
For the first almost 3 hours of this lock we were toggling the gain of CSOFT P by 20 dB every 10 seconds because of a logic problem in the guardian. Should be fixed now.
Filters for PI damping have been broadened to 10Hz as the phase change over 0.5Hz for some of the filters being used was greater than 200 degrees.
These 10Hz wide filters may be problematic for the 15541.9 and 15542.6Hz modes. I have tested damping these modes at low amplitudes with the 10Hz wide filters and to damp, however as we reached 3 hours into this lock the 15541.9 and 15542.6 were slowly pushing their way up, the filters I have put in have about 60 degrees phase shift for 1Hz change in frequency.
I tested iwave on this pair of modes, it tracks the largest mode very well, however once damped to a level lower than the next highest mode it runs off and tracks that mode. This meant two iwave blocks running one on each etm pi model generally pushed their test mass with the largest amplitude mode. I was using tau of 10.
We have a SUS_PI guardian now. The gaurdian has 4 states managed by ISC_LOCK. IFO_DOWN, OMC_TRACKING, PI_DAMPING and ETMX_PI_DAMPING. The tracking state just turns on tracking for long term testing. The damping states turn on the bandpass damping chains with settings that have been tested today and very low gains for the settings in Terra's PI wiki. I will update the wiki tomorrow with some new settings.
The 0.5W increase in the ITM ring heaters should push the optical mode to lower frequencies, as we see PI in the 15540Hz group of modes close to the end of the RoC thermal transient (1.5-2hour in a 2-3hour transient) I am hoping this will be enough to make these instabilites a little less agressive. I have been doing some testing of the 15kHz mode in anticipation that this TCS change will make these modes ring up more at the beginning of lock.
Before leaving I stepped the ETMY Ring heater by 0.5W total to test the idea that we can push the 15542Hz modes appart with a little heating.
The ITM ring heaters that Sheila activated seemed to have introduced a substrate lensing of about 8 uD on each optic according to the TCS simulator. The ITMX HWS saw the consistent amount of change in the substrate lensing (~ 18 uD due to the round trip lensing which gives an extra factor of two). After the ring heaters were activated, the power recycling on average was higher than the previous 41 W stretch by 1%; this could be because the last lock stretch with the ring heaters was with a slightly lower PSL power of 39 W. I attach trend of the relevant channels.