Following up on the DARM bicoherence observation and the non-stationarity of low frequency noise: the noise in DARM between 20 and 40 Hz is correlated with the amplitude of the 2.6 Hz peak
The attached plot is an histogram of the DARM RMS in the 20-37 Hz region (computed by summing bins in a whitenend spectrogram) and the DARM RMS around the 2.6 Hz peak (computed with a band-pass filter between 2.4 and 2.7 Hz).
There is a clear correlation between the DARM noise in the 20-40 Hz region and the RMS in the 2.6 Hz region.
An exploration of where the 2.6 Hz peak is visible and coherent with DARM
The ETMX M0 / R0 / TMSX are interesting signals, maybe something isn't working properly in the R0 tracking loop?
The usual ASC suspects, especially CHARD_Y.
Several DC centering loops.
Some OMC signals.
No smoking gun
I have some evidence that could indicate that this peak is related to some instability in the HARD loops.
First, we noticed this 2.6 Hz peak is very prevalent in the OMC and OM3 suspensions. Evan was tracking the presence of the peak in OM3 yaw and noted it appeared sometime around April 9-14. This was a tricky time because many things happened: we powered up, we adjusted the compensation plates and OMC to reduce scatter, etc. Evan also noticed it doubled in size on June 22, which corresponds to when we went down in power. I used the OMC SUS master out channels as an indicator of when this peak appeared. I noticed it was not present April 7th and then appeared on April 8th, UTC time. More specifically, it appeared in the Lownoise ASC guardian state. This was around the time I was working on removing RPC completely from the ASC control and working with Dan and others on increasing the power. It appears that the peak pops up right when I turned off the RPC and transitioned all the HARD loops to the new high power controllers.
The fact that the peak height doubled when we went down in power also bolsters this theory: if there was something a bit marginal in the loop at higher power, the shift in the radiation pressure plant could have worsened the marginality. To further confirm, I looked at spectra of the HARD and SOFT loops, and the 2.6 Hz peak becomes prominent around the time I changed the loop control and turned off RPC.
As a test, Gabriele and I would like to make small changes in the HARD loop gains and see if we can pinpoint which loop is marginal. If we're lucky, the peak could be fixed with a gain change. If we're less lucky, maybe we need to redo one or more loop controllers.
First attachment is an ndscope screenshot. I plotted the guardian state, RPC gains, and the SWSTATs for all the HARD loops. The cursor is on the time when I hard turned off all the RPC gains and switched the loop controllers. GPStime: 1364955781. This also corresponds to when the peak appears in the OMC suspension channels.
Second attachment is a dtt screenshot of a plot comparing the OMC sus master out spectra now (blue refs) with the spectra on April 7 before the ASC change (red live).
Edit to add: Gabriele and I tested raising and lowering various ASC loop gains but we saw no difference in the peak.
We also hypothesized that this is related to the reaction chain tracking and/or damping. We turned off the reaction chain tracking of ETMX for L, P and R and saw no change in the peak.