TVo Aidan Danny Georgia

Following on from alog 43788. I had a look at the ITMY Hartmann wavefront sensor data after out final lockloss on Saturday, where we were locked at 8.3 W. I used Aidan's fitting script and method from LLO alog-14634, and fit the spherical power (thermal lens) as the test mass cooled down (second attachment). From this measurement I calculate the absorbed power, and from the circulating power estimate the absorption to be 394 +/- 107 ppb.

Details

The first attachment shows a ndscope screenshot of the ITMY HWS spherical power, prism values and the PSL power, after lockloss on Saturday. By fitting the spherical power decay time we can estimate the power absorbed in the optic. The initial kick in the spherical power is possibly due to the ITM misalignment after lockloss, this makes it tricky to tell when the fit of the ringdown should begin, I've just done it by eye. The fitted absorbed power is 17.1 +/- 4.6 mW, as shown in the second attachment with data in blue, fit in red, uncertainty in yellow. The uncertainty looks a bit excessive on this plot but depending on when I start the data to fit, the absorbed power can move around within these error bars.

The circulating power can be calculated:

P_circ = P_in * recycling gain * BS reflectivity * arm cavity gain

= 8.3 W * 37 * 0.5 * 282 = 43.3 kW

From this and the thermal lens the absorption is calculated. The script used to generate the plot and calculate absorption is here:

/ligo/home/georgia.mansell/TCS/20180904_ITMYAbsorption/LHO_ITMY_TL_20180901.m

A similar measurement on ITMX was not possible with this particular lockloss due to the bright ghost beam on the HWS, Danny has since closed the iris on this beam.

When we lost lock the ALS beam became unshuttered, as set in ISC_LOCK down state, saturating the end-station HWSs. I've since moved the unshuttering to LOCKING_ARMS_GREEN. Though if we want to take a meaningful measurement on the ETMs we will need ALS shuttered for ~5000 seconds after lockloss.