Reports until 04:10, Wednesday 23 March 2016
H1 TCS
kiwamu.izumi@LIGO.ORG - posted 04:10, Wednesday 23 March 2016 - last comment - 13:18, Wednesday 23 March 2016(26211)
common CO2 test with the interferometer locked

I did a common CO2 test with the fully locked interferometer. Unfortunately the interferometer could not stay locked due to the SRC1 and 2 pitch loops running away.

The interferometer was fully locked with 8 W PSL (10 W was not stable and in fact it lost lock once due to the angular instability 26208). ASC loops were all closed. No LSC feedforward engaged. DARM line was injected at 331.9 Hz monitoring the cavity pole and optical gain. The line was provided by the LSC lockin oscillator and not by Pcal Y.

Since it lost lock, I set the CO2s back to nominal. I will check the data later.

Comments related to this report
kiwamu.izumi@LIGO.ORG - 13:18, Wednesday 23 March 2016 (26218)
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Inconclusive. The test lasted only for 17 minutes and this is too short to say something conclusive.

The goal of this measurement was

  • To let the common substrate lensing sweep across the predicted optimum lensing point
    • In order to experimentally find an optimum point for the common lensing.
  • Also, to see what kind of badness it would introduce to the ASC loops.

The first goal was not achieved because the interferometer unlocked before reaching the predicted optimum. Regarding the second goal, it seems that the common lensing affected the SRC loops. This is something we kind of expected.

[The test]

  • 10:19 UTC, the interferometer locked with 10 W PSL. ASC fully engaged.
  • 10:34 UTC, I decreased the PSL power down to 8 W in order to avoid angular instabilities.
  • 10:36 UTC, I started injecting the line at 331.9 Hz for tacking the cavity pole.
  • 10:46 UTC, the common CO2 laser was introduced as written in the above entry.
  • 11:03 UTC, unlocked. Turned down the CO2s back to nominal.

The first two attachment show the trend of various channels during the measurement.

[Cavity pole]

With the line injected at 331.9 Hz, I was able to track the change in the cavity pole frequency. It is shown in the third attachment together with the simulated substrate lensing of ITMX and ITMY. The time scale of this plot is the same as those for the first two attachments. The cavity pole measurement is only valid in the time band roughly between t = 2100 sec to t = 3800 sec. The cavity pole increased at the beginning and went down afterwords. One might think that the highest cavity pole frequency is the point where the SRC couples to DARM maximally, but I can not support that hypothesis. If one looks at the lensing of ITMs, they were slowly settling to 10 and 7.5 uD for ITMX and ITMY respectively in 2000 - 2500 sec while the cavity pole kept increasing. Immediately after that the CO2 lasers started dominating the lensing and pulled the lensing upward rapidly. It does not look like that the ITM lensing went across the optimum SRC coupling point given the behavior of the lensing. It rather looks as if it was caused by some other effect (perhaps some ASC loops ?).

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