Sheila, Koji, Robert, Evan, Alexa, Dan

We have made several measurements of backscattering from the OMC.  It seems like the reflectivity of the OMC is smaller by a factor of about 20 than what was seen at LLO, and it seems that backscatter from the OMC is probably not limiting our DARM spectrum.  

Two nights ago, we measured fringe wrapping by exciting the OMC suspension in the longitudnal direction, as well as by exciting OM1.  (related alogs 17910 17904 17882)  The attached plots show the DCPD RIM, with the DARM loop supression removed, with the excitations on.  

• First, we noticed that the osem calibration must be wrong, for both OM1 and the OMC.  For the OMC our knee frequency around 45 Hz indicates that indicates that our excitation amplitude was about 35.25 um.  The osem readback  indicates that our exitation amplitude was 11.8um, if we assume that this path length is double passed the osem is underestimating the motion by about 50%.  The story for OM1 is similar, the osem indicates that OM1 was moving with an amplitude of 6.25 um, but the path length modulation must have been about 20 um based on the knee frequency.  (Tobin's thesis, p 45)
• When we excite the OMC, we see two scattring shelves, the second has a finge velocity twice that of the first path.  For the first shelf we get an amplitude reflectivity of 1e-5, for the second shelf it is 1e-6.  These are eyeball fits, but they seem to be good to within about 15%.  At LLO (alog 13607) similar measurements showed an r = 2e-4 for their first shelf.  
• One speculation is that the first shelf could come from scattering within the OMC itself, and the second from the OMC refl path. Yesterday (after these measurements were made) Koji realinged the OMC refl QODs, and the amplitude of the second shelf is reduced today.
• FOR OM1 we only see one shelf, because OM1 is not the scattering source.  With this we are only modulating the path length between the IFO and the scattering source.  However, we measure the same reflectivity as the first shelf when exciting the OMC, so the two measurements are consisitent.

Tonight Jeff made a test of turning off the HAM6 sensor correction, as was done at LLO (third attachment) (LLO alogs 16814).  The spectrum is attached, but we do not see the dramatic fringe wrapping seen at LLO.  We would expect the impact to be smaller here than in LLO because our scattering amplitude is smaller and it is also likely that the microseism could have been smaller here.  

Today we Robert Koji and I made injections into all 6 DOFs on the OMC to see fringe wrapping.  We saw nothing by exciting roll or vertical, we were able to produce shelves by exciting L, T, P and Y.  The last screenshot attached shows the sectra with the various excitations on. For the record, here are times, all excitations were at 0.2 Hz, into the test filter banks.  While I've attached spectra of these, several off these shelves were moving around durring the measurement because of some lower frequency motion. 

LVEA: Laser Hazard
Observation Bit: Commissioning   

07:00 Karen & Cris – Cleaning in the LVEA
08:04 Bubba – Starting nitrogen purge of 3IFO storage containers
08:27 Filiberto – Working on TSC-Y rack
08:35 Gerardo – Turbo valve on HAM6
08:50 Gerardo – Out of LVEA
08:52 Peter – Going to the H2-PSL enclosure
09:15 Peter – Out of LVEA
09:23 Filiberto – Out of LVEA
09:35 Bubba – Out of LVEA
09:40 Adjust ISS diffracted power from 5.5% to 7.9%
10:25 Karen – Going to Mid-Y
10:25 Cris – Going to Mid-X
12:20 Dale – School tour of control room
15:35 Alarm on H0:FMC-CS_WS_RO_alarm – Have informed Bubba

Scott L. Ed P. Chris S.

This report will include the following dates, 4/14 thru today.
Tuesday the crew completed the X-1 beam tube cleaning. Test results posted here. All support equipment was relocated around the mid station to the X-2 beam tube. Began vacuuming support tubes and spraying bleach/water solution.

Continued vacuuming and began beam tube washing, cleaning 49 meters ending 4.6 meters past HNW-4-003 at end of day Wednesday.

Today the crew cleaned 61 meters stopping 1.5 meters beyond HNW-4-006.

The Apollo crew will be out on 4/17 and 4/20 so we will not be cleaning those days.

HAM6 system is running with only the 500 l/s "main" ion pump (valved out the turbo pump), the turbo pump still running.

We'll assess progress tomorrow and determine if we can turn the turbo pump off.

As noted by Evan et al, SRCL coupling to DARM is highly non stationary. I looked into three minutes of data with the SRCL noise injection described in the cited elog entry (GPS time 1113211711 + 180 s).

In brief, the coupling is modulated exactly as the residual motion of ASC-AS_A_RF45_I_YAW_OUT. If we compute the coherence between DARM and SRCL with a gain modulated by ASC-AS_A_RF45_I_YAW_OUT,  we get values very close to one. Read the rest of this report if you want to know more.

One strange thing is that it seems that the main contribution to this motion comes from ETMY PIT, even though the AS signal is YAW... See the last plot to see how AS_A_RF45_I_YAW moves in a very similar way to ETMY_L3_OPLEV_PIT...

Spectrogram, coherogram and tf-gram

The first attachment shows a spectrogram of DARM during the SRCL noise injection. The non stationarity behavior of the noise is very evident. The second attachment shows a coherogram, which is basically the same thing as the spectrogram, but showing how coherence changes over time. Again, coherence can go up to values of one, but clearly if you average over the entire time, you lose a lot since the coupling is non stationary. The third attachment shows the transfer-function-gram, which again shows how the transfer function from SRCL to DARM changes over time. You can see from the phase plot that the sign of the coupling flips multiple times. The 4th attachment is an animation of the transfer function between SRCL and DARM over time, which makes even more clear how the coupling changes amplitude and sign.

Band limited RMS

During the noise injection, I repeated the my standard BLRMS correlation with angular signals. I use the frequency band between 50 and 100 Hz. The correlation with angular signals (using all ASC error signals) is quite good, as shownm in the 5th attachment. I didn't continue further the analysis of which channels are more relevant, since the analysis described in the next section gives a better understanding.

Coupling variation over time

Looking at the transfer function over time (see for example the animation in the 4th attachment) it is clear that the shape remains mostly the same, and only gain and sign changes over time. I therefore computed the average of the TF around 93 Hz, which corresponds to a point where the phase is close to zero (this is just for convenience, in this way the real value of the TF is a good estimate of the gain variation over time). The TF gain varies a lot over time, and as expected changes sign many times. In the 6th attachment I show in the blue trace how this gain varies over time. The red trace is the best fit obtained using my algorithm and all ASC error signals. The green is the residual, which shows hoiw the reconstruction is very good. Using my channel ranking algoritmh I found out that the main contributor is  H1:ASC-AS_A_RF45_I_YAW_OUT_DQ is anticipatedm above. The 7th plot shows indeed that the fit is very good even if I use only this signal: so the conclusion is that the SRCL coupling is modulated completely by angular fluctuations visible in the AS port.

Coherence maximization

The 8th plot shows in blue the coherence between DARM and SRCL, if averaged over the entire 3 minutes of data. Clearly the coherence is kind of low, as explained above. The green trace is instead the coherence between DARM and an 'improved' SRCL signal, built as SRCL * (1 + gain * AS_A_RF45_I_YAW_OUT_DQ), with the gain obtained from the previous fit. The improvement is clear, showing that indeed I was able to recover most of the coupling gain and sign changes.

To get an even better estimation, I did an optimization based directly on the coherence. Basically, I considered the averaged coherence in the band 50-300 Hz, between DARM and a signal given by SRCL(1 + gain * AS_A_RF45_I_YAW_OUT_DQ). The optimal gain is similar to the one obtained with the previous analysis. However, the improved coherence is very close to one, as shown in the 9th plot. This is very efficient way to prove that indeed most of the SRCL coupling is modulated by the above angular signal.

Following up on the TTFSS work in alog 17885, we remeasured the input mode cleaner open loop transfer function.

The ugf is 50 kHz. With 3 dB of additional gain we can push this to 100 kHz and engage the second boost stage. The gain margin will be a little marginal at 1-2 dB, so.

For the measurement the common gain was at 18 dB, the fast gain at –6 dB, the compensation and one boost stage was engaged.

The small feature at 750 kHz is still there but of little relevance. This now closely resembles the iLIGO configuration and behavior.

The night before last, OMC injections suggested that our scattering problem did not involve a back-reflection from the OMC. While looking at the data, I noticed that the injections seemed to excite a 0.05 Hz oscillation (servo?) that did produce scattering up to 100 Hz. The time series in the figure show the 85-90 Hz band of DARM against vertical and horizontal OMC OSEMs. The longitudinal injection is between about 1400 and 1800s and is visible on the T3 OSEM but not on the vertical OSEM. The 0.05 Hz oscillation starts near the end of the injection.  The level of motion that produced shelves to 100 Hz was not that much greater than normal motion. This suggests that we should try exciting the OMC in other degrees of freedom, in case the scattering is in another direction besides the main beam direction. I think vertical injections would be the place to start because vertical seems to correlate the best; perhaps light is scattering back from the shiny table top just below the upside down OMC. 

Robert, Koji, Sheila

We checked the coefficient between the RIN and the PZT of the laser. We swept from 1 kHz down to 10 Hz, but we only got good coherence below 20Hz. The measured value for H1:PSL-ISS_PD[AB]_OUT (Volts) / H1:IMC-F_OUT (kHz) at 10 Hz was –40 dB. Dividing by 10 V (ISS PD DC level) and multiplying by the measurement frequency of 0.01 kHz we get

RIN/rad = 10^–5.

The ISS inner and outer loops were off during the measurement.

The HAM6 Vent and Table payload changes caused a change in the Free Hanging Position of the Optical Table.  This resulted in tripping of the ISI during Isolation, 17816, and subsequently, a resetting of the CPS Target Locations.

With a couple nights of DC Readout Locking, I've gotten the go ahead from DanH, Koji, Sheila, and Kiwamu. I've now updated the safe.snap and the HAM6 SDF is now dull again.  Attached is the change to the Targets we've been running with for almost a week now.

Koji, Evan

Summary

MICH feedforward seems to be doing its job, although there is room for improvement by implementing a frequency-dependent subtraction.

SRCL coupling into DARM seems to be very nonstationary. Consequently, the feedforward is not working.

Details

We injected band-limited white noise (elliptic bandpass, 10 Hz to 1 kHz, 6 ct amplitude) first into MICH, then into SRCL, to test the feedforward that was implemented a few weeks ago.

For MICH, frequency-independent subtraction is fair to middling (red) compared to no subtraction (blue); at best we get 20 dB of subtraction around 150 Hz. Note that the TFs in this plot use the whitened DARM channel. The whitening is undone for the spectra in the fourth pad.

For SRCL, the 1/f² feedforward via ITMY L2 gives no subtraction at all. The attachment shows the TF of SRCL control → DARM with the feedforward off and with broadband noise injected into the SRCL error point. Unlike MICH, appearance of this excitation in DARM is highly nonstationary, fluctuating by a factor of 2 or so in a frequency-dependent way. Additionally, the coherence is poor above 20 Hz, despite the excitation elevating the DARM noise by more than an order of magnitude from 20 to 100 Hz.

The shape of the excess noise is more or less the shape of the 100 Hz elliptic cutoff that we put into SRCL a few weeks ago. Is it possible that the SRCL control noise explains the nonstationary, 100 Hz "scattering" shelf that we've seen in the DARM spectrum this past week?

Alexa, Dan, Elli, Evan, Koji, Sheila

• With the youngest parametric instability on the continent tamed by the ring heater, we locked a few times tonight, each time for an hour or more.  At the start of each lock the OMC DCPDs were either saturating or approaching their nonlinear region, due to an excited violin mode harmonic around 1009.62Hz.  We believe this is an ITMY mode but haven't been able to damp it yet.
• A quick transfer function of the OM1 suspension showed that the mode splitting we saw in the yaw d.o.f. last week is no longer there, see the first image attached.  This is good, but it means that this suspension is very close to some kind of mechanical contact that changes the plant.  I have reverted the yaw damping gain to the value from before, and committed new L and Y TFs to the SUS svn.
• The green light transmitted to the corner through the X arm has decreased by ~15% in the past day, see the second plot.  Sheila renormalized the X_TR signal so that the guardian will be satisfied with the X arm alignment.  Changes in the alignment of PR3 didn't recover the power (so we're not misaligned on the PD), and the power level at the endstation hasn't changed.  Baffled.
• We studied the OMC SUS excitation data from last night to measure the scattering noise from the OMC, Sheila will post more information later.  For now, compare the first PDF attached, which shows a spectrogram during the large longitudinal excitation of the OMC SUS from last night, with the second PDF, which is the same plot from tonight with no excitation.  The quiescient motion of the OMC does not appear to generate scattering noise.  There is clear nonstationary noise around 100Hz and also around 40Hz, but this does not have the arch-shaped features from scattering, and isn't correlated to the OMC suspension motion.  There's still a turbo pump running outside of HAM6.

Sheila, Gabriele, Evan, Koji, Dan, Alexa, Elli

Yesterday Koji, Evan and Sheila locked at 15W, and after an hour and a half the lock became unstable and they reduced they power due to suspected PI. A spectrogram ('darmspectrogram3_1.png ') of last night's lock shows a 844Hz line grow in the DARM spectrum (see attached spectrogram from 2015-04-15 09:30:00 UTC).  This corresponds to a PI at frequency 15540Hz, which is the first PI LLO saw (LLO alog 15934).  The line appears about an hour after the 15W lock began and the power was reduced after 1hr25min.  Once the power was reduced to 10W the line got smaller.  

This evening we locked at 15W at 2015-04-15 23:40:00 UTC and we saw the same line grow at 843.4Hz, corresponding to a PI at 15540.6Hz (see spectrogram 'spectrogram16April_843HzPI.png ').  This line grew untill we lost the lock at 1hr55 mins later ~ 2015-04-14 01:35:00 UTC (not due to the PI).  We measured the DARM spectrum around 15.5kHz using the SR785.  Attached is a plot '16AprilDARMSpectrum.jpg ' of the growing 15.54kHz line.

Grabriele saw that the 843Hz line was coherent between between the X-arm ASC QPDs and Darm, so we turned on the ETMx ring heater (see alog 17899).  The ring heater is requesting 1W total, or 0.5W each on the upper and lower segments.  We locked for a second time tonight at 15W at 2015-04-16 05:18:10 UTC. With the ETMX ring heater running, we didn't see any growth in the 15.54kHz line, as measured by the SR785.  See 2nd plot '16AprilDARMSpectrum_withETMXRH.jpg '.

Measurement notes:

-Spectrogram generated at LigoDV https://ldvw.ligo.caltech.edu/ldvw using channel H1:CAL-DELTAL_EXTERNAL_DQ.

-Spectrum meaurement taken with SR785 connected to OMC DCPD readout.  There is a script in /ligo/home/eleanor.king/netgib/SR785/SPSR785omcdcpds.yml that is used to take the spectrum (use the command <    ./SRmeasure SPSR785omcdcpds.yml > to take a measurement).

-Dan points out there are also OMC DCPD 64kHz  testpoints which are channels H1:IOP-LSC0_MADC0_TP_CH12 and H1:IOP-LSC0_MADC0_TP_CH13, which correspond to DCPD_A_INMON and DCPD_B_INMON.

The numbers are relative to the power delivered to HAM6. We still need single bounce OMC lock data in order to infer the OMC throughtput.
Obviously the systematic error was larger than the statistical error noted...

Sheila, Gabriele, Elli

ETMx ring heater turned on at 15:04:16 1:56:00 UTC requesting 0.5 W in upper and lower segments.

The ring heater is on because it looks like we have been watching a parametric instability build up at 15540.5 Hz over the last hour or so of lock.  There is coherence between the X-arm ASC QPDs and Darm at this frequency, which is why we are trying using the ETMX ring heater.  There is a script  in /ligo/home/eleanor.king/netgib/SR785/SPSR785omcdcpds.yml that we have been using to track the amplitude of this mode (use the command <    ./SRmeasure SPSR785omcdcpds.yml > to take a measurement).

For some reason all of the gains on the ETMx upper and lower segments were set to 1, so I have changed them back to what they were in February (which is also what the ITMs are currently set to).

H1:TCS-ETMX_RH_SET UPPER&LOWER DRIVECURRENT_GAIN =12.5

H1:TCS-ETMX_RH_UPPER&LOWER VOLTAGE_GAIN =3.5

H1:TCS-ETMX_RH_UPPER&LOWER PCB_GAIN=3.5

H1:TCS-ETMX_RH_UPPER&LOWER CURRENT_GAIN=-0.08

Continuing on from yesterday's TTFSS work.

All notches were removed, and the transfer function was measured (see NONOTCHS.tif).
The peak at ~760 kHz is of no big concern.  However the peaks at around 1.77 MHz are.
These were notched out with C50 (1-65 pF) and series with L2 (220uH).  C50 was adjusted
to suppress the two peaks.  The transfer function was re-measured (see OLTF.tif).  The
unity gain frequency is around 450 kHz.

Currently the common gain is set to 27 dB and the fast gain to 5 dB.  Should the
loop break out into oscillation, reduce the fast gain to 0 dB, wait until the oscillation
stops and then increase it back to 5 dB.





Rick, Peter

Koji, Evan, Sheila

Tonight we saw the shelf from 50 up to 100 Hz, however we are back to a range of around 35 Mpc.  We saw this last week and hoped that it was related to the HAM6 cleanroom.  We have done several things:

• It seems that the peak at around 250 Hz is related to the OMCR path.  The peak was reduced after the vent and replacement of the BS with a 90/10, and closeing the OMC refl beam diverter reduces it below the DARM noise floor.)
• closing POP and AS air beam diverters didn't seem to have any impact on the spectrum, the DHARD roll offs we added ddin't do much either.  
• We measured fringe wrapping by exciting the OMC and OM1.  Analysis coming tomorow, but at first it looks like the OMC backscatter should not be limiting the sensitivity.  
• We were locked at 15 Watts for a few hours, after this time we saw broadband noise in DARM and tried to use the 785 to check for PI.  We had used the 785 earlier in the lock, but the second time we weren't able to connect to it.  By reducing the power we stayed locked.  (An incorrect calibration around this time lead to a sensmon range of around 40 Mpc)
• ALS scanning is faster this week, since we added a fast scan durring the vent that checks the two most commonly found frequencies.