Summary:

I analyzed the HWSY measurements from a ~28 hour period around the 18th of May during a long lock segment and lock/unlocked transitions. There are systematic shifts in the measured defocus which can only be coming from some alignment coupling. There is evidence that the delivered CO2 laser power may be more than the CO2Y power meter suggests.

Our attempts to maintain the total lens using the CO2 laser power during locked/unlocked transitions are currently ineffective because the CO2-induced thermal lens response is not calibrated correctly.

Analysis:

There was as a period of a 7 hour lock last week. I extracted the RAW HWS images corresponding to this period (t0 = 1147500000 to t0 + 1E5s) from the directory: controls@h1hwsmsr:~/framearchive/h1hwsmsr/ITMY/1147500000/. These were centroided offline to determine the measured defocus (a proxy for H1:TCS-ITMY_HWS_PROBE_SPHERICAL_POWER which should be running online). The offline calibration included:

• dividing by a factor of 2x to convert from double-pass to single-pass
• dividing the raw spherical power measurement by the nominal magnification (7.5) squared to convert to the spherical power at the ITM.
• The zero value was reset such that defocus = 0 around t = 70 minutes

I also extracted the SIM estimates of defocus (defocii?) from the self-heating (H1:TCS-SIM_ITMY_SUB_DEFOCUS_SELF_OUTPUT) and CO2 laser power (H1:TCS-SIM_ITMY_SUB_DEFOCUS_SELF_OUTPUT). All three of these time series are plotted below.

And a zoomed version of the first 600 minutes is shown below:

A few points to note:

• There are unexplained (and unphysical) jumps in spherical power after the system loses lock around 550 minutes. My working hypothesis is that alignment changes during attempts to reaquire lock are somehow coupling to this HWS measurement.
• There are transient glitches in the spherical power every so often. I don't know the cause of these
• The measured drop in spherical power from the HWS is much larger than the expected drop in the SIMulation (from lowering the CO2 laser power)

If I double the expected CO2 response and create a new channel that is the sum of the double CO2 lens and the SELF heating (and reset the zero value of this channel), I get a result that is much closer to the measured lens in the HWS.

Additionally:

As you can see in the images attached to aLOG 27047,  that CO2Y table is capable of delivering a maximum of ~2.3W off the table (when the CO2Y laser is running at 57W). The CO2X table is capable of delivering a maximum of ~5.5W off the table (when the CO2X laser is running at 59W). There is no functional difference in the design of the two tables (expect that Y is a mirror image of X). The possibilities for this difference are:

• Optical (we may be clipping the beam somewhere on the CO2Y table)
• Electrical (We're somehow losing signal between the H1:TCS-ITMX_CO2_LSRPWR_MTR power meter and the ADC input electronics).
• Software (we have an unexpected calibration difference between the tables). Scanning L1TCSCS.txt and looking at the gain settings in the filter banks for the X and Y power meters and comparing the EPICS GAIN channel for these filter banks rules out this possibility (barring some hardcoded difference in the SIMULINK model).

And: the maximum power that could be delivered from this table on 26-Jan-2016 was around 5.5W.

Therefore: Given all this information, I suspect that the CO2 laser is somehow reading back less power than it is delivering (or, if you like, delivering more power than we're expecting).

TO DO:

• Measure the CO2 laser power directly on the table with an independent power meter
• Measure the CO2 central heating thermal lens with the HWS with the IFO unlocked and no other changes to the IFO alignment
• Determine why we see systematic changes in the HWSY defocus apparently with IFO alignment changes.