jeffrey.kissel@LIGO.ORG - posted 10:47, Saturday 02 May 2015 - last comment - 12:15, Saturday 02 May 2015(18173)
H1 DARM 8 [Hz] Oscillations After Beginning to DC_READOUT_TRANSITION
J. Kissel, J. Warner We're trying to recover the IFO after the epic 10.5 [hr] lock stretch, but we're having trouble getting past the DC READOUT transition. I attach the last four lock losses, 2015-05-02 16:36:53.342000 ISC_LOCK LOWNOISE_ESD_ETMY -> LOCKLOSS 2015-05-02 16:56:35.161000 ISC_LOCK DC_READOUT -> LOCKLOSS 2015-05-02 17:10:12.335000 ISC_LOCK DC_READOUT_TRANSITION -> LOCKLOSS 2015-05-02 17:36:44.730000 ISC_LOCK DC_READOUT_TRANSITION -> LOCKLOSS in which all but the first (which is the end of the 10.5 [hr] stretch) show a ring up of some 8 [Hz] oscillation. In the DARM ASD this appears as a sharp non-stationarity at 6-8 [Hz] with harmonics at 12 and 18 that show up as soon as the OMC starts to look for the carrier. Is this back scattter? Is this the DARM offset being incorrect? I don't know... we'll keep lookin'; any help is appreciated.
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J. Kissel, J. Warner Another example. This time we were able to hold in DC_READOUT_TRANSITION, but after ~10 minutes the 6-8 [Hz] non stationary would keep popping up, and eventually we lost it. I tried the following: - Reducing the DARM gain from 600 to 500 (perhaps because the IFO optical is different/better/worse the loop is on the edge of stability) -- no effect - Reducing the OMC's input ASC QPD servo gain from 0.2 down in 0.025 increments (the DCPD camera shows angular fluctuations, the ASAIR camera looks pretty solid) -- no effect Maybe this is something to do with some new back-scattering? The non-stationarity only begins to appear once I start to engage the OMC locking, but I don't really understand why locking the OMC would have an effect, given that it should all be back-reflection from the OMC's input coupling mirror...
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