J. Kissel

I'd collected charge measurement data this past Tuesday (4/11); here's the processed data.

Conclusions:
- ETMX continues to hover around an un-impactful 10-20 [V], because we have a relative high observing duty cycle. (see attachments 1 & 2)
We shut off the ETMX ESD's bias voltage during observation, which means stray charge in the vacuum system is only attracted by the ~10-20 [V] instead of the ~400 [V] in place when we're down or acquiring lock. As long as we continue doing so, we should be fine for the foreseeable future.

- ETMY continues to accumulate charge. (see attachments 1, 2, & 3)
While we flip the sign of ~400 [V] bias of the ETMY ESD while down and during acquisition, the high observation duty-cycle means it's essentially always one sign. That means we're accumulating charge at a relatively fast rate of 5 [V/month] -- as reported by the *angular* effective bias voltage (see 3rd and 4th attachment). What *really* matters is the relative longitudinal actuation strength -- relative to when we last reset the clock (in this case January) -- and with this we're still in the greener side of the yellow zone (see 5th attachment).

As such, my recommendations from last week (LHO aLOG 35366) are now solidified: 
Just before we vent on May 8th, during last minute preparations as we bring the IFO down, let's 
    - Turn OFF the ETMX ESD bias completely, and leave it OFF 
    - Leave the ETMY bias ON at 400 [V], with the opposite sign as in observation (Negative in NLN, switch to Positive for vent)
for the duration of the vent (i.e. until we open back up to the arms)