J. Kissel

Now that (we think) we trust our model to predict the open loop gain transfer function at all frequencies in the current H1 SUS ETMY damping loop configuration with an EPICs gain of 1.0 (see LHO aLOG 6969), I've made a model of how increasing the gain in L and P by 3.0 compares to a gain of 1.0. The gain of 3.0 configuration is what the ISC commissioners run to reduce the spot motion to what they desire (on a camera alone), and some how it is stable. (And yes, they still run with the ISIs only damped, because there is too much low-frequency motion generated by the isolation loops -- also a camera alone "measured" statement).

The first attachment are the loop figures of merit. Dashed lines are for a gain of 1.0, and solid lines are a gain of 3.0. We see that the Longitudinal loop should have a upper unity gain phase margin at 3.9 [Hz] of 9 [deg]. This causes a factor of 10 gain peaking at 4 [Hz]. In addition, the (originally) 0.43 [Hz] mode is no longer damped, but distorted to 0.35 [Hz]. In Pitch, although the loop is not as unstable as L, there is still a sharp gain peaking feature at 5.1 [Hz] because the gain margin is so small. Similarly, the lowest (originally) 0.56 [Hz] mode is distorted down to 0.43 [Hz].

We have yet to measure the open loop gain of this exact L/P Gain = 3 configuration, but we were able to get some passive measurements:
(1) A comparison of the in loop OSEM sensors, which show similar distortion, and 
(2) Optical lever spectra of the G = 3 configuration alone to quantify what a "good" level of motion for the cavity locking is. 
These are the second and third attachments. Confusingly, the L in-loop sensors seem to follow the model behavior, but we don't see similar modification on the middle frequency resonances in Pitch. Also, the optical lever spectra show none of the expected gain peaking.