ryan.short@LIGO.ORG - posted 12:23, Monday 25 April 2022 - last comment - 12:16, Tuesday 26 April 2022(62782)
ITMX Absorption Measurement
Building off of Georgia's alog 62468
Camilla and I have fitted the HWS spherical power on ITMY from a lockloss on April 12 and on ITMX from a April 22 lockloss using the
hws_absorption_fit script to create the attached plot. Using the arm power in channel H1:ASC-X_PWR_CIRC_OUT16 from the ITMX April 22 lockloss, 250kW, and seeing that we have 108mW absorbed on ITMX by comparing to Aidan's comsol model, we have a 432ppb absorption on ITMX.
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Since the last attempt at this measurement (alog62600) we improved the alignment of ITMX HWS by picoing and steering beam onto CCD (alogs 62745 and 62641).
Using Aidan's instructions' and numbers in alog25984.
PRH = Pself x ( - Gself / GRH )
Surface curvature:
For the ITM self heating, the surface (SURF) defocus gains per absorbed Watt is Gself = -3.65E-5 D/W.
For the RH, the SURF defocus gains (per Watt applied power) is GRH = +1.07E-6 D/W.
So you can multiply the ratio of these two numbers by the absorbed power Pself (ITMX = 432ppb for 400kW circulating arm power = 172.8mW absorbed; ITMY = 369ppb for 400kW circulating arm power = 147.6mW absorbed).
This gives 5.90W ITMX and 5.02W ITMY applied ring heater power PRH to keep the surface curvature constant. Total for top and bottom so half for each segment.
Lensing/ Material Substrate effect (This is negligible for our IFO absorbed power in the substrate):
For the ITM self heating, the substrate (SUB) defocus gains per absorbed Watt is Gself = 4.8709E-4 D/W.
For the RH, the SUB defocus gains (per Watt applied power) is GRH = -9.0E-6 D/W.
Multiply the ratio of these two numbers by the absorbed power Pself (ITMs = 0.02ppm/cm* x 40cm double pass for 1.65kW ITM transmitted power (for 400kW circ, ~66W at PRC x 50 PR_GAIN x 0.5 at BS) = 1.32mW absorbed). *check this value
This gives 0.07W ITMX and ITMY applied ring heater power PRH to keep the substrate deformation constant. Total for top and bottom so half for each segment.