SudarshanK, TravisS, RickS
Yesterday, we went to Xend to assess and optimize the Pcal beam locations on the ETM surface.
Since O1, we have known that the beams were clipping somewhere on the receiving side.
Unsing an updated method of assessing the beam locations that is based on the work that Miranda Chen (REU student from UTRGV) did this summer (basically using the ESD electrode pattern rather than the edge of the ETM) we found that the Pcal beams were quite far from their optimum locaitons (see first image attached below). The offests from the optimal locations (111.6 mm above and below the center of the face of the optic) were U [ 1.71, 12.27], L[10.02, -7.27] where U and L denote the upper and lower Pcal beams and the coordinates are [x offset, y offset] in millimeters with the usual conventions, positive x to the right, positive y up. Because we use 2" diameter optics in the Pcal periscope structure and because the receiveing-side mirrors are twice the distantance from the ETM, an offset of 12 mm at the ETM would correspond to an offset of ~24 mm at the periscope, likely the cause of the clipping we have observed. The reflected power measured by the power sensor in the receiver module increased by about 50% from about 4 V to 6.2 V.
We adjusted the beam locations using the steering mirrors in the Pcal transmitter module and ended up with positions that are within a mm of the optimal locations (see second image attached below): U [ 0.15, 0.43], L[-0.18, 0.05].
The third image below shows the Pcal spots on the ETM surface (inside green circles), visible when the chamber illuminator is off.
We are in the process of modeling the expected calibration errors resulting from bulk elastic deformation induced by the Pcal forces being so far from the optimal locations, but we expect errors as large as 20-40% at frequencies above 4 kHz for offsets as large as what we had before correcting the positions yesterday.
J. Kissel, S. Dwyer First attachment -- a screen cap the currently functional Bounce and Roll mode damping settings that work as Jenne describes above. One potential cause Shiela and I explored: we're using DARM_CTRL as our error signal for the bounce mode damping. We've always assumed that the DARM open loop gain is large enough that the open loop gain of the bounce mode damping loop is unaffected by changes in the DARM filter, D, or the DARM plant, or sensing function, C. Given recent, uncompensated issues with SRC cavity detuning cause optical spring changes in the plant, the DARM open loop gain near 10 [Hz] has been questionably low. We've been struggling with interferometric SRC alignment signal, and therefore leaving SRC angular control essentially off recently. This implies the SRC detuning can be changing from lock-stretch to lock-stretch, which means the DARM optical plant is changing from lock-stretch to lock-stretch. And if the overall DARM open loop gain is low enough, then the phase impacts of this change in optical plant will change the phase necessary to damp the bounce modes. The second attachment compares Kiwamu's DARM OLG TFs taken on 2016-09-16 vs 2016-09-21, and indeed, the open loop gain magnitude is only ~4 to 5 at 10 [Hz], and once can clearly see substantial phase differences between the two measurements. The third attachment shows the loop math to aide the following proof of how the two loops are coupled: C = DARM IFO plant, or sensing function D = DARM filter bank A = ISC L to TST L DARM super-actuator TF V = Bounce mode damping filter alpha = TOP V to TST L actuation function xi = coupling from TST L to DARM Delta L_{res} = Delta L_{V} + Delta L_{ctrl} = Delta L_{V} + A D C Delta L_{res} d_{ctrl} = D C Delta L_{res} --> Delta L_{res} = d_{ctrl} / (D C) d_{ctrl} / (D C) = Delta L_{V} + A D C d_{ctrl} / (D C) d_{ctrl} = D C Delta L_{V} + A D C d_{ctrl} G_{darm} = - A D C (1 + G_{darm}) d_{ctrl} = D C Delta L_{V} d_{ctrl} / Delta L_{V} = D C / (1 + G_{darm}) --> G_{V} = alpha xi beta D C / (1 + G_{darm}) --> IF G_{darm} << 1, then G_{V} ~ alpha xi beta D C and we have dependence on the changes in the optical plant of DARM. --> IF G_{darm} >> 1, then G_{V} ~ alpha xi beta D C / G = alpha xi beta / A and we have the scenario we have always assumed was happening. Now that Jenne's just restored old damping, this is less of a suspicion, but I wanted to write it down so that we keep this sort of loop inter-dependence in mind.