At the start, I clarify that these above measurements that Rahul regularly takes are measures of *absolute* *angular* actuation strength of each ETM ESD system, in terms of effective bias voltage in Volts on the ESD bias electrode (i.e. an imagined *extra* bias voltage on top of the regularly *requested* bias voltage [think ~100s of Volts] that would bring this absolute angular actuation strength to *zero* when the requested voltage is set to zero).

I re-clarify all this terminology because conversations are happening about whether we'll need to
    - launch a Test Mass Discharge System campaign any time soon, 
    - begin alternating the requested bias voltage 
    - do nothing
during O3.

I answer the question with additional plots of another, different metric of the charge accumulated: the *change* in *relative* *longitudinal* actuation strength for the ESD system on ETMX (alone) between 
    - April 18th 2019 (during O3, after we think we finally got the calibration tracking system right), 
    - May 18th 2019, and 
    - June 18th 2019. 
Remember that the *absolute* *longitudinal* actuation strength for the ESD was measured to be 4.739e-12 N/ct by the calibration group on Mar 28th 2019 (see LHO aLOG 47941), and this relative longitudinal actuation strength is with respect to that. Note, you can find these plots on any day, readily made under the "CAL > Time Varying Factors" tab of the individual IFO summary pages, and these are plots of the real part of the "\kappa_TST" factor (e.g. ).

The message: the *longitudinal* actuation strength for ETMX has not changed since Mar 28th 2019 (about 4 months) by more than 1%. (Probably less than that, but difficult to quantify rigorously from the plot alone since there are so few tick marks on the y-axis.)

Recall, in O1 and O2, we were seeing changes on the order of 0.5% *per week* in this relative longitudinal actuation strength (e.g. LHO aLOG 24241) -- which motivated us to develop a campaign of requested bias voltage sign flipping (see LHO aLOG 33985 and references there-in).

Also recall that, during O1 and O2, we were not only concerned about relative actuation strength changing -- which impacts the calibration of the detector -- but also the noise contributions of charge to the detector sensitivity. This was identified by shaking BSC-ISIs and measuring the direct coupling the sensitivity in the 10-50 Hz region (when one would naively expect any nominal such excitation to be filtered aggressively by the QUAD suspension with a slope of 1/f^8). We indeed found such coupling -- see e.g. (LHO aLOG 37752). 

However, finally recall that, we then successfully deployed the Test Mass Discharge System at each end station for at LHO (ETMX LHO aLOG 38457, ETMY LHO aLOG 38524), and clearly successfully reduced the effect on noise on ETMX (LHO aLOG 38507) and successfully reduced that same test masses estimate of effective bias voltage (as measured by optical levers LHO aLOG 38604). (At the time, we were using ETMY as our DARM actuator, and it's absolute longitudinal actuation strength was remeasured, thus resetting the relative longitudinal actuation strength back to 0%).

My suspicion is that now that we've mitigated sources of free ions, e.g.
    - Moving ion pumps 250 m away from the test mass
    - Installed chevron baffles around those ion pump intakes
    - developed a program of discharging the test masses upon exit from the chamber,
that we've successfully created a system which no longer charges up, even though we're still regularly operating with the ETMX ESD system in use at 70% duty cycle and a ~400 [V] requested bias voltage.

We have not yet remeasured the BSC-ISI to DARM coupling in O3, so we don't yet know of any direct noise coupling, so this assessment is positive, but a bit incomplete until those measurements are redone. However, for now, I think we can take option 3: 
    - We do not need to do or change anything thus far in O3; charge is not a problem.
We will, of course, remain vigilant and continue regular measurements, as we all know that as soon as we stop measuring we'll have another 6.7 Mag earthquake in Montana.