craig.cahillane@LIGO.ORG - posted 21:21, Tuesday 09 April 2019 - last comment - 16:11, Monday 15 April 2019(48366)
Calibration group, Hall, and Ward DARM plants are not sufficient to explain the phase behavior seen in the DARM plant at frequencies below 10 Hz
We've measured the DARM plant to have a prospring at 6 Hz and a Q of about 4. About the spring frequency, the phase goes through around -90 degrees, and not the expected +180 degrees alog 48083 This +180 degrees is not typical for causal control systems, but it is expected from SR IFOs as discussed in Section II B from BnC. Using the Ward DARM model (Eq 3.83) I was unable to achieve a satisfactory fit of the H1 DARM plant at low frequencies, so I made some sliders to see if I could get a heuristic match. I was not able to for reasonable IFO parameters. I discovered that our Q is far too low given our optic transmissions. One can lower the Q of the optical spring by increasing the SRM transmission or reducing the ITM transmission. However, changes to these parameters also change the frequency of the optical spring and the DARM pole. The pictured plot shows the best Ward model I was able to come up with to explain the current plant, featuring detuning of -0.5 degrees. DARM Plant Measurement I began questioning the measurement itself, but the procedure is pretty simple. A PCAL to DARM measurement gives C/(1 - G), where C is the DARM plant, and G is the DARM OLG. Then a DARM OLG is taken to get 1/(1 - G), and these two measurements are divided to give the DARM plant C. The PCAL calibration into meters is just two real poles at 1 Hz. This has phase of -160 degrees at 6 Hz, i.e. there is some dynamic phase rotation happening at LF due to the calibration which may not be real.
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Does it change with arm power? dc offset power? Does AS45 see the same feature?
March 13 antispring DARM plant at 30 W Input Power and 0 W on SR3 heater: 47493 March 18 antispring to prospring at 30 W Input Power with 0 W to 5 W on the SR3 heater: 47604 March 20 prospring at 35 W Input Power with 5 W on SR3 heater: 47728 April 12 prospring at 35 W with 5 W to 3.5 W on SR3 heater: 48453 There have not been tests for the following: - Spot positions (for L2A2L effects) - SRCL offset - DARM offset Based on the above results, I think that higher arm power and higher SR3 heater power both push the DARM optical prospring to higher frequency. Danny Vander-Hyde tells me that the SRC gouy phase goes like around ~1 degree/watt of SR3 disk heater power, so we probably change the SRC gouy phase by ~5 degrees on March 18.