jeffrey.kissel@LIGO.ORG - posted 19:20, Wednesday 30 November 2016 - last comment - 09:34, Thursday 01 December 2016(32050)
Campaign to Reduce 2nd Harmonics (~1000 kHz) of QUAD Violin Modes
J. Kissel, E. Merilh, J. Warner, T. Hardwick, J. Driggers, S. Dwyer Prompted by DetChar worries about glitching around the harmonics of violin modes, Ed, Jim, and I went on an epic campaign to damp the ~1kHz, 2nd harmonic violin modes. These are tricky because not all modes had been successfully damped before, and one has to alternate filters in two filter banks to hit all 8 modes for a given suspension. We've either used, or updated Nutsinee's violin mode table, with the notable newly damped entries being 994.8973 ITMY -60deg, +gain MODE9: FM2 (-60deg), FM4 (100dB), FM9 (994.87) 997.7169 ITMY 0deg, -400gain MODE9: FM4 (100dB), FM6(997.717) VERY Slow 997.8868 ITMY 0deg, -200gain MODE10: FM4 (100dB), FM6(997.89) Also, we inadvertently rung up modes around 4735 Hz, so we spent a LONG time trying to fight that. We eventually won by temporarily turning on the 4735Hz notch in FM3 of the LSC-DARM2 filter bank and waiting a few hours. I had successfully damped the ETMY mode at 4735.09 Hz by moving the band-pass filter in H1:SUS-ETMY_L2_DAMP_MODE9 's FM10 from centered around 4735.5 to centered around 4735 Hz exactly, and using positive gain with zero phase. However, there still remains a mode rung up at 4735.4 Hz but it's from an as-of-yet unidentified test mass, and we didn't want to spend the time exploring. These 4.7 kHz lines have only appeared once before in late October (LHO aLOG 31020). Attched is a before vs. after ASD of DELTAL_EXTERNAL. I question the calibration, but what's important is the difference between the two traces. Pretty much all modes in this frequency band have been reduced by 2 or 3 orders of magnitude -- better than O1 levels. Hopefully these stick through the next few lock losses and acquisitions. Thanks again to the above mentioned authors for all their help!
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Thanks to all for your efforts! You can really see the dramatic decrease in the glitch rate around 21:00 UTC in the attached plot. The glitch rate in the lock after you did this work (which ended around 5 UTC today) looks much more typical of what we know the glitch rate at LHO to be.
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Comparing yesterday before damping to today the high frequency effect of the damping seems to be the removal of glitchy forests around 2, 3, 4, and 5 kHz (base frequency 2007.9 Hz but wide). Great! Not sure of the mechanism to get these frequencies yet, seems to be more than double the modes you damped. As noted above the 4735 is pretty large.
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Attached is a spectrogram showing how the 2000 and 3000 Hz bands go away as the 1000 Hz violin modes are damped. You can also see that the bursts in these bands correspond with places where the spectrogram is 'bright' at 1000 Hz. Having two violin modes very close at 1000 Hz is like having one mode at 2000 Hz with a slow amplitude modulation. Probably that is getting turned into bursts in DARM by some non-linear process, modulated by that effective amplitude variation. The 1080 Hz band is bursting on its own time scale, and does not seem to be related.
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