J. Kissel

Executive Summary: Driving TMSX in YAW from the top mass (M1) OSEM coils, and using the same stage M1 OSEM sensors as my response channels -- thus far, I can only report null results for finding any excitable mechanical features 102.12833 Hz. I found a feature at 103.585 +/- 0.004 Hz, but it's only a single feature, so I suspect it's the M1, "lower" blade bending mode rather than any evidence for violin modes. But, given the arrangement of sensors, actuators, and the wires on the transmon, I retrospectively am not surprised to not have found violin modes.

I'm looking to confirm whether the frequency that the IFO has been ringing up recently, 102.12833 Hz, is a violin mode of TMSX, as is currently "the most likely suspect" for what this giant feature is that rings up for the first ~3 hours of a lock stretch, after "apparently" being "rung up" by the turn on process for the LSC feed-forward (see, among others, LHO aLOG 72064, 72214, 72221).

Recall the TMTS is a double suspension -- see D0901880 for top-level assembly drawing.
Recall that TMSX is a "production" TMS, rather than a "First Article" TMS, but I can't find a single source link that tells us the difference clearly and concisely.
   - From the ISI, there are two blades (no drawing! Only an excel spreadsheet of characterization under D1200116), and two wires suspending the top mass (M1). 
   - From M1, there are also two blades (no drawing! Only an excel spreadsheet of characterization under D1200117), and *four* wires suspending the optical bench and telescope assembly. The TMS is unique in that it's the only 4-wire suspension clamped from 2 blade tips (see D1101163).

So, if we drive the M1 stage (the only place we can), and with no IFO all we have are M1 stage sensors, then we would hope to see 
   - two violin modes from the Sus. Point to M1 "upper" wires, *maybe* each split into two orthogonal DOFs if the Q is high enough and the bending points move asymmetrically enough
   - four violin modes from the M1 to M2 "lower" wires, again, *maybe* split into eight.
T1300876, table 3.11 for the "production" TMTS predicts a single violin mode frequency from first principles, the properties of the wires, and the load on the wires, to be 
   - Upper -- 331.7 Hz
   - Lower -- 104.2 Hz

Looking at the OSEM arrangement for the TMTS (see E1200045) -- which is *like* the QUAD, but rotated 90 deg such that only one SD OSEM senses / actuates in Longitudinal -- I chose to start the exploration for violin modes by driving in Yaw around 100-110 Hz using awggui, and measuring the responses, transfer functions, and coherence simultaneously in DTT. I slowly narrowed in my 6th order elliptic band pass on the region, such that I could drive more and more power. I had plenty enough SNR in the M1 OSEM to M1 OSEM transfer function at these frequencies that I could get coherent transfer functions.

I did find a feature at 103.585 +/- 0.002 Hz, but since it's only one feature rather than four (or eight), I can't claim that "this is *the* violin mode(s)" but I guess instead that it's a blade bending mode of the "lower" M1 to M2 blades. Why? 
    :: These TMTS M1-to-M2 blades are much like the QUAD's M0-to-L1 and L1-to-L2 blades -- copies and pastes of the UIM blades -- which have bending modes at around ~110 Hz (see analysis for the QUAD blade conditions in T1300595).
    :: while I expect energy is getting into the violin modes from the M1 Yaw drive, I guess I'm not surprised that the energy of those violin modes does not couple back through the blade clamps, the blades, and the M1 mass to the OSEM flags enough to see above the OSEM sensor noise.

Here's some of the data:
    - 1st attachment: The transfer function between M1 Y drive to M1 Yaw and Transverse OSEM response, with an FFT length of 128 seconds for a frequency resolution of 1/128 = 0.0078125 Hz (and an effective noise bin width of (1/128)*1.5 = 0.01171875 Hz given that I'm using the default Hanning window, which has a NENBW of 1.5 bins). This was the first clue that I wouldn't find anything at 102.129 Hz, and that there was something at 103.585 Hz.
    - 2nd attachment: Yaw Drive with a tighter band-pass filter from 102 to 103 Hz, and measured with the same 0.0078 mHz resolution, we see nothing in this band.
    - 3rd attachment: Yaw Drive with a higher frequency 1 Hz band pass, from 103 to 104 Hz, and measured with a much higher frequency resolution -- FFT length of 512 seconds, for a resolution of 1.95 mHz, and effective noise binwidth of 2.92 mHz.