J. Kissel

I've measured the effective bias voltage of the H1SUSETMX and H1SUSETMY ESDs in the traditional fashion (as measured by the zero in actuation strength [rad/V_S] from each quadrant reported by the optical lever). 
Excellent news: All Quadrants of ETMX ESD report 0 +/- 3 [V] effective bias voltage. AWESOME.
where, via the same, single-number metric (the mean and standard deviate of the weighted mean in all four quadrants, over two angular degrees of freedom), ETMX used to have 17 +/- 19 [V] effective bias voltage. Of course, you should look at the plots, since this single-number metric only works when all the weighted means of quadrants are very similar.

Note, however, this is a bulk measurement of all the ways that the ESD can be effected by charge. As Leo has discussed (see T1500467), and Sheila has recently resurrected (see LHO aLOG 38387), the force on the test mass is determined by 

    F_ESD = A (V_B - V_S)^2 + B (V_B - V_S) + C (0.5*(V_B + V_S) - V_R)^2 + D (0.5*(V_B + V_S) - V_R) + E 


where V_B, V_S, and V_R are the applied bias voltage, the applied quadrant control "signal" voltage, and the voltage of the surround cage components, respectively. A, B, C, D, and E are coefficients that depend on the geometry of the ESD pattern, the distance between the test mass & reaction mass, as well as the surrounding cage components (ring heater, EQ stops, fiber guards), and the polarizability of the test & reaction mass. This effective bias voltage measurement is a measure of a collection of all but one of these coefficients,
    V_eff = 0.25 (D - 2 C V_R - 2 B) /  (C - 4 A)

Once we recover the interferometer, Sheila and I will remeasure each coefficient independently, a. la LHO aLOG 38387, and BSC ISI excitations as in LHO aLOG 38132. Let's hope this has some impact on the sensitivity.

ETMY's effective bias voltage remains high (using the above metric, 51 +/- 34 [V]), but we're attacking the charge on that chamber next week. May we have the same amounts of success there!

Three cheers for Rai and the systems design team!