In light of the violin mode stuff over the weekend, I wanted to make sure we aren't going to be caught unawares by other harmonics ringing up.

We have paid considerable attention to damping the fundamentals and second harmonics of the violin modes, but in general we leave the higher harmonics undamped. (We have had some isolated cases in which we have had to temporarily apply damping to high harmonics; see, e.g., LHO#18599).

I checked the heights of the third harmonics during the first long lock (25 hours long) that we had after the power outage. I chunked the DCPD sum timeseries into 1000 s segments and then took ASDs of each. To identify the violin modes, I simply looked for peaks in the DCPD spectrum that were higher than some threshold above the noise (in this case, the noise floor is about 1×10⁻⁷ mA/Hz^1/2, and the threshold I chose was 5×10⁻⁶ mA/Hz^1/2). This resulted in 25 peaks being identified. For each ASD, I integrated under each peak to get an rms mode amplitude. Since the violin modes are anharmonic, I believe that these peaks (which range in frequency from 1456.18 Hz to 1484.67 Hz) are truly third harmonics, rather than upconverted first harmonics (which range from 498 to 509 Hz, and hence we would expect upconversion to appear from 1496 to 1527 Hz).

All 25 peaks appear to be ringing down, with time constants on the order of 2×10⁵ s (≈50 hours). I fitted each ringdown to an exponential, which resulted in Qs ranging from 0.55×10⁹ to 1.28×10⁹. [For similar results on lower-order mode Qs, see, e.g., Keiko's measurements on L1 IY.]