Following on from the driven electric field meter (EFM) measurements presented in alog lho-40878, I've made a rough conversion of the EFM noise floor into meters of DARM. It looks like the EFM might just be sensitive enough to see fields which could contribute to DARM. This calibration is very dodgey and is really just an order-of-magnitude (or 3) estimate. Based on the the transfer function from volts-of-ESD-shield-drive to meters-in-DARM, in alog llo-28675 (strongly dependent on which test mass was looked at), the EFM noise floor lies somewhere between the red and purple traces in the first plot.
How I made this plot:
First I fit our measured electric field strength vs volts of ESD shield drive measurement, to get a conversion from Vshield to electric field strength [V/m] (second plot, data read off the plots in Jeff's previous log post)
I fit Anamaria's transfer functions for mDARM per Vshield as shown in llo-28675. I just did a linear fit and extrapolated it to 500Hz, since their measurements only go to 200Hz.
I converted this transfer function into mDARM per electric field [V/m], using the fit in step 1 (shown in the third plot)
Finally I took the EFM noise floor, and multiplied it by the transfer function (x 5 since we drove all 5 shields where LLO only drove 1) to get the EFM noise floor in mDARM (plot 1).
EFM measurements will continue at EX in the future, we still need to look at the frequency-dependence of the ESD shield electric field, and take measurements where we drive the ESD signal/bias, ISI horizontal actuators, and anything else in-chamber that might generate electric fields.
Estimates I made a while ago when deciding that the smaller electric field meter was not adequate are the sensitivity of the meter had to be 5 e -5 Volts/meter/sqrt(Hz) at 100 Hz to intersect the ambient field noise in the chamber. This required that the test mass had acquired a charge of about 3 e -9 coulombs.