Elenna, Jennie W, Sheila, Stefan, Ryan Crouch

On the weekend Stefan and company found OMC QPD offsets that increased the optical gain, and found we were close to saturating the OMC suspension 76223.  After Ryan C ran initial alignment, we went to single bounce with the AS centering and OMC ASC closed.  We stepped the OMC QPD offsets towards Stefan's values (without saturating the suspension), then introduced offsets into AS_A and AS_B pit to reduce the pitch drive to the OMC suspension.  Once the suspension drive was small, we turned off both OMC ASC and AS centering and pico'd to center the beam on the AS WFS. screenshot attached shows this process.

Mon Mar 11 10:12:22 2024 INFO: Fill completed in 12min 18secs

Gerardo confirmed a good fill curbside.

Closes FAMIS 25982, last checked in alog76035

The HAMs look fine

For the BSCs:

ITMY_ST2_CPSINF_V2 looks elevated at high frequency

I have SDFed OMC QPD offsets according to Stefan's previous alog on improving optical gain.

FAMIS 20019

Starting Friday afternoon and lasting for about a day, there was a drop in the water temperature for the powermeters' cooling loop of ~0.7 degC and a drop in the return of ~0.2degC. During this time, the signal on the PMC transmitted power PD looked much less noisy (more detailed ndscope screenshot attached). Not sure what exactly caused this, but everything is back to normal as of now.

The FSS RefCav TPD has been holding mostly steady since my alignment tweak Thursday afternoon (alog76199).

All other trends look nominal.

The 3IFO LVEA storage dew point monitors stopped updating Wed 24 Jan. This morning I restarted the EPICS IOC on h0epics, but the sensors are still unavailable.

These are serial devices, read out by the Comtrol serial-ethernet adapter h0seriall0 (10.1.3.65/16). I verified I can ping the ethernet port on this unit, suggesting the problem is closer to the sensors or their power source.

We are clipping on IM4 trans. I trended the position of IMs 1-3 over the past few months and it appears they moved significantly from their position before and after the vent. The t1 marker on these plots marks the approximate time that the positions changed which corresponds to the end of O4a and the start of the vent.

As a note, this means our PRG channel is untrustworthy right now.

TITLE: 03/11 Day Shift: 15:00-23:00 UTC (08:00-16:00 PST), all times posted in UTC
STATE of H1: Planned Engineering
OUTGOING OPERATOR: None
CURRENT ENVIRONMENT:
    SEI_ENV state: USEISM
    Wind: 14mph Gusts, 12mph 5min avg
    Primary useism: 0.04 μm/s
    Secondary useism: 0.73 μm/s
QUICK SUMMARY:

• Currently running through an initial alignment
• High winds seem to be calming down
• Microseism is elevated

Last week I scanned the SHG temperature for more data points to confirm the dip in the middle of the sinc function. A rough fit to the side lobes suggests we should have about 160mW of green output. A few days before we improved the PMC alignment we measured 230mW of 1064 input into the SHG. 160 mW would be 69.5% conversion efficiency. This number isn't too far off from the conversion efficiency measurement we took when we first installed the SHG 6 years ago. We have known for sometime that we are missing a lot of green power for the amount of 1064 we put in. This plot makes the picture a bit more clear that we have been indeed losing more than half the green power. The cause is unclear. We have been suspecting the gray tracking in the SHG cryatal to be the cause even though we have a low green finess in the SHG. 

The equation I used to create the fit was A*(np.sinc((T-T0)/width))**2 

Where A is the input 1064 power, T0 is 34.9 C, and the 'width' of the sinc function is arbituary set to 2.5. 

Sun Mar 10 10:11:07 2024 INFO: Fill completed in 11min 3secs

We reran the DARM offset step test around GPS 1394062645, similar to alogs 71913, 68870, 64974.

Our current measured contrast defect is slightly higher than in alog 71913, but overall pretty similar.
Matt will comment with the calibration of X0 offset cts into picometers.


Contrast Defect: 2.1 mW
Nominal Total Amps from DCPDs: 40 mA
Responsivity = e λ / c h = 0.858 A/W
Nominal Total Power on DCPDs: 46.6 mW
Nominal Homodyne Angle: 12.2 degrees

Using alogs 47217 and 45734 as a guide, we measured/calculated a new OMC-DCPD_MATRIX.

Note: this might have to be redone if we update the anti-TIA filtes for the DCPDs, see alog 76228.

We measured the coherent signal amplitude ratio g = PD_A/PD_B above 30Hz (i.e. above the TIA transfer function imbalance), so the matrix is optimized where we actually care.

g=1.0471

We measure the shot noise signal amplitude ratio h = PD_A/PD_B above 400Hz to avoid thermal noise and correlations from the feedback.

h=1.0247

This produced the new output matrix (see secript)

0.99861 1.00139
0.97725 -1.02328

For reference, old matrix:

0.99963 1.00040
0.98198 -1.0184

The new matrix was loaded, but not yet tested - the IFO lost lock due to 43kts wind gust.

Georgia, Trent, Elenna

We lost lock on the interferometer (most likely from 50mph gusts of wind) and had to align the arms. The y-arm proved difficult and we could not get the flashes on ALS-C_TRY_A_LF_OUT_DQ above 0.6 counts. To help the arm along in locking we changed the pdh auto-locker threshold to 0.55. We then decided to run an initial alignment.

During the initial alignment we got red dust warnings for the corner station.

After the intial alignment, the y-arm green flashes reaches 0.73 and we didn't need to change the pdh auto-locker again.

Since we have a new OMC, we need to find the best optical gain spot on the new OMC.

To that end I closed the OMC ASC dither loops iwth a simple diagonal matrix (OM3-->OM3 and OM1-->OMC_SUS). It did converge with very low gain, but to a spot with not much headroom on the OMC_SUS (we are at 115000ct out of 131000cts on T2 and T3).

In that setting I reset the H1:ASC-OMC_[A/B]_[PIT/YAW]_OFFSET offsets, so right now we can lock to that spot with either dither or QPD.

Next: hunt for best optical gain around this spot.

This is a summary of information that is spread across different alogs in the last few days.  When recovering from a vent, in which the green camera reference may have been lost when the gate valves are actuated, these are some useful steps and guardian changes.

• Avoid having ISC_LOCK send the ALS ARM guardians to increase flashes.  This can be done by adding a weight of 5 to the edge between READY and LOCKING_ARMS_GREEN (line 6442 right now).  Ryan Short took care of this this week, he may have done this in a different way. 
• Avoid shuttering ALS in the CARM offset reduction sequence, so that we can check and reset green references when we do lock.  This can be done by changing the weights of     ('IDLE_ALS', 'CARM_OFFSET_REDUCTION', 5),   ('PARK_ALS_VCO',  'SHUTTER_ALS', 1). 
• If the alignment of the green is so far off it doesn't stay locked we need to run the green QPD offset scripts (see 50968), we can start this as soon as the ALS_XARM and ALS_YARM guardians are in RED_LOCKED, and before DHARD is engaged.
• Remove DRMI ASC by setting the flags in the ISC_DRMI guardian ENGAGE_DRMI_ASC (usePRC1, ect) to false, except for MICH.
• Depending on what has been happening and how SDF has been kept up to date, one could make CHECK_SDF the default to skip SDF_REVERT.

We have made and reverted these changes this week, I'm not sure if we've reverted the change to the INCREASE_FLASHES timer.

This log follows up some previous work that's been done to understand the LSC, in particular MICH, noise coupling that we have observed. The most recent work done on this front was by Dana, in alogs 74477 and 74787.  Notably, the MICH coupling follows an interesting shape here, especially below 20 Hz. For MICH, we expect a flat coupling depending only on the finesse, Gm = pi/2*F. Evan Hall made some great measurements of this coupling years ago and you can see the results in Fig 2.17 of his thesis(link to thesis)- without any feedforward the magnitude of the coupling is flat in frequency down to 10 Hz. However, Dana's measurements clearly show a frequency dependent coupling below 20 Hz. Also, Dana mentions that the MICH coupling is about 40% too high than we expect based on finesse.

I'll add here a quick note that currently the UGF of MICH is about 8 Hz and the UGF of SRCL is about 11 Hz, so I think it's safe to assume that we are not seeing any significant closed loop effects from either of these loops down to about 12 Hz in these measurements.

Something to remember here is that when we measure the MICH coupling we drive the beamsplitter which does induce a change in the MICH length, but it also changes the SRCL and PRCL length. Therefore, we should expect to pick up SRCL coupling along with the MICH coupling when we measure. Evan's thesis also includes the form of the SRCL coupling in Eq 2.29: Gs = 0.012 [m/m] * Pa/750 [kW] * dL/10 [pm] * F/450 * (10 [Hz] / f)^2 (I am only including the first term that dominates at low frequency). The SRCL coupling results from radiation pressure in the SRC due to the DARM offset (dL), power in the arms (Pa), and finesse (F) and follows a 1/f^2 slope (again, I am ignoring the second term here that rises like f^2 at high frequency). We are operating at much higher power and double the DARM offset, Pa=370 kW and dL = 20 pm (and F = 440).

Regarding the 40% excess MICH coupling, if you divide Dana's result by an additional sqrt(2), it lines up exactly how you would expect. Is there a chance the MICH calibration is missing a rt2 factor related to the fact that we measure from the beamsplitter?

Assuming I'm correct about the sqrt(2) factor, and plotting the magnitude of the expected MICH and SRCL coupling along with Dana's calibrated measurement, our expected O4 SRCL coupling lines up almost exactly with the response at 20 Hz and below (see first attached plot).

I think this effect has always been there, but not easily observable because the SRCL coupling was much lower due to lower operating power and smaller DARM offset. Comparing Evan's MICH coupling measurement (Fig 2.17) with his SRCL coupling measurement (Fig 2.18), the SRCL coupling at 20 Hz was close to an order of magnitude lower than the MICH coupling.

I think this understanding of the coupling better explains why we need to put so much work into the LSC feedforward. Specifically, we measure the coupling for MICH and SRCL, and then usually tune MICH first and then SRCL. Then, to do our iterative tuning, we redo the MICH coupling. It's likely we could do a better job first just tuning SRCL and then measuring and tuning MICH, since we need to subtract the SRCL coupling well enough to get a "true" measure of the MICH coupling.

Furthermore, the SRCL coupling is dependent on our DARM offset. When we make changes to OM2, for example, we change the mode matching at the output. Since we servo to keep the amount of light on the DCPDs constant, we could be changing the required length to achieve this amount of light slightly. In principle, this should only effect the SRCL coupling, but since SRCL shows up in our MICH coupling measurement, it effects both.

Finally, it appears in Dana's measurement that there is a rising trend above 30 Hz, which I have no explanation for. I don't think that can be explained by the SRCL coupling. A look at that second term in the SRCL coupling equation: Gs = 3e-5 [m/m] * phi_s/10 [deg] * dL/10 [pm] * F/450 * (f / 100 [Hz])^2 (Evan's thesis Eq 2.29), I estimate that with a SRCL detuning of about 1 deg, around 30 Hz the SRCL coupling term with an f^2 slope is about 5.3e-7 m/m. This is both too small and the wrong slope to explain that feature.