craig.cahillane@LIGO.ORG - posted 04:49, Tuesday 05 February 2019 - last comment - 03:50, Wednesday 06 February 2019(46781)
Nominal Low Noise tonight
- There is an unsolved problem somewhere around DARM_TO_RF. I believe it has to do with the DIFF PLL OFFSET stepper servo not quite working fast enough to keep H1:ASC-AS_A_RF45_Q_SUM_NORM near enough to 0 before the switch happens. I am able to slowly step through the state successfully, but not request beyond it with the guardian without losing lock. - I removed REDUCE_RF9_MODULATION_DEPTH from the guardian path because this seemed to trigger the 1.4 Hz DHARD instability. I have been able to sit at NLN for 6 hours without problems. - We are still controlling DARM with L1 and L2 on ETMY and L3 on ETMX. Because of this the calibration is a complete mess at low frequency. At high frequency it is good to one percent (assuming we trust the PCAL at this point, unclear if we do because the force coefficients are changing.) I fiddled with the DARM calibration screens and switched the calibration back to ETMY L1 L2 such that the DARM calibration should be good to 15% according to PCAL. Some notes about the noise with the actuator switch: - MICH FF would need retuning, since MICH is coherent with DARM. - The ISS coupling to DARM was reduced at high frequencies. However, after two hours of sitting locked at NLN, the ISS is back to limiting high frequency DARM. - The scatter shelf is back and happens pretty consistently every two minutes. I did a couple of ISS broadband injections and TFs to test the arm power and intensity noise. I attached the TF template. Some notes: - Our ambient input RIN is ~10-8, according to ISS OUTER RIN. - DARM is limited by the ISS at high frequency (~2k on). DARM is not limited by the ISS anywhere else. - Arm RIN/Input RIN goes like f-1 as expected. - DARM/Input RIN goes like f-5 according to my TF measurement, and not like f-3. I do not yet understand this, I would have predicted -3, with -1 from the CARM pole filtering of the Input RIN, and -2 from the compliance of the quads coupling due to radiation pressure effects. - DARM/Arm RIN goes like f-4. - All the DARM/Arm RIN TFs seem to be nearly the same magnitude. Some fits must be applied to be able to really tell. - To calibrate I used the DARM FOM calibration for DARM meters, and found the average value of the Arm QPDs and divided by them. I assumed that ISS OUTER RIN was already calibrated. While doing some of these injections I excited ETMX mode 9. I ran ENGAGE_ALL_DAMPING in the violin mode guardian, this worked.
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I've found the error in the above results. The DARM calibration stored in the DTT templates was incorrect, or at least very different from Keita's stopgap calibration from January 2019. Using Keita's DARM calibration I got sensible results, i.e. DARM/Input RIN goes like f-3, and DARM/Arm RIN goes like f-2. Attachment one shows DARM/Input RIN. The results agree pretty well with Gabriele's simulation, within a factor of two (At 30 Hz, my measurement gives 1.1e-12 m/RIN while Gabriele predicts around 7e-13 m/RIN) I am not sure how Gabriele has defined DARM, I have used Keita's stopgap which assumes DARM = Lx-Ly. If we trust the equation from alog 46759, and my by-eye fits here of Arm RIN/Input RIN and DARM/Input RIN, we can calculate the differential arm power: Δ Parm = 2 π2 m c α / β = 11.9 kW where α is the DARM/Input RIN f-3 fit coefficient = 3e-8 m/RIN, and β is the Arm RIN/Input RIN f-1 fit coefficient = 5.9e-1 RIN/RIN. If we assume that my arm power calculations were correct of 143.0 kW of average arm power, and that the 8% difference is real, we would get 143.0*0.08 = 11.4 kW
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