J. Kissel

I've gathered this week's effective bias voltage measurement which provides an alternative confirmation that the actuation strength of the electrostatic drive (ESD) systems is not varying significantly over time from the accumulation of charged particles. This is no longer an issue because of the regular sign flipping of the ~400 [V] of requested bias that we began in November of last year (see LHO aLOG 31929 for ETMY and LHO aLOG 31172).

Attached are plots showing the effective bias voltage trend over the past year for each quadrant of control, as measured by the optical lever, in both pitch and yaw. The first is for ETMX, and the second for ETMY. One can see the marked change in effective bias voltage accumulation starting in November, as the trend turns over and becomes flat over the past few months. The last two plots re-cast this trend in terms of relative actuation strength of the ESD systems. One can see that the relative actuation strength change caused by the accumulation of charge remains small and static in time, at the level of a few percent. 

We can use this confirmed static angular actuation strength as a proxy to confirm that the longitudinal actuation strength remains static, as shown in LHO aLOG 24241. That means the actuation strength we use in our model of the interferometer's differential arm length control system when it was updated last in January (see LHO aLOG 33004) is still valid, and time-independent filters we used to calibrate the real-time output of the detector -- which are based on that model -- are accurate to within these limits. (Any remaining small time-dependent change has been corrected for in the almost-real-time, low-latency correction algorithm since the start of O2; see LHO aLOG 31732.