Sheila, Camilla, Robert

A change in the ETMX measured charge over the O4a-O4b break (e.g. 78114 ) suggested that there might be a change in electronics ground fluctuation coupling. This is because the hypothesized mechanism for ground fluctuation coupling depends on the test mass being charged so that, as the potential of electronics (such as the ring heater and the ESD itself) near the test mass fluctuate with electronics ground potential, there is a fluctuating force on the test mass.

We swept the bias (see figure) and found that the minimum in coupling had changed from an ESD bias of about 150 V in August of 2023 ( 72118 ) to 58 V now, with the coupling difference between the two setting a factor of about ten (in other words, if we stuck with the old setting the coupling would be nearly ten times worse).  Between January of 2023 and August of 2023, the minimum coupling changed from about 130 V to about 150 V, with the coupling difference between the two settings being less than a factor of two. The second page of the figure is from this August alog, showing the difference in the coupling between then and now. I checked the differences across the break for ETMY, ITMY and ITMX and the coupling differences across the break were not much more than a factor of two, so the change in ETMX, about a factor of ten, seems particularly large, as might be expected for a significant charge change.