I traced the source of the small peaks in DARM around 280 Hz that were coherent with the EX ESD power supply voltage monitor channels (https://alog.ligo-wa.caltech.edu/aLOG/index.php?callRep=45241 ) to the fans in the IO chassis at EX. This coupling is present even after the switch to the new dedicated 18 and 48 V power supplies for the ESDs.

The DARM coupling is consistent with fan contamination of test mass actuation: Sheila and I found that the fan peaks were in DARM when the control was at ITMX, but only if the ETMX bias was on. The coherence went away when the ETMX bias was turned off (I should note that this test was before the ESD power supply switch).

Figure 1 shows the fans in DARM, after the power supply switch, during E13. The peaks were identified by individually manipulating the fan frequency by partially blocking the fan ports. All 8 fans from the 4 I/O chassis (h1iscex, h1susex, h1susauxex, h1seiex) appear in DARM, the ESD power supplies, and a monitor of the voltage difference between the field rack and the vacuum enclosure. The presence of, for example, the SEI chassis fans in DARM, and in the voltage monitor channels that pass through the ISC IO chassis, suggests that the fan coupling mechanism is not the previously identified mechanism where the fans for a particular IO chassis contaminate the channels in that chassis by producing ripple on the chassis power supply ( this mechanism has virtually been eliminated in the new IO chassis that are starting to be installed (https://alog.ligo-wa.caltech.edu/aLOG/index.php?callRep=43425 ). Instead, the fans are contaminating external power, even the ESD supplies; I think that the most likely mechanism is that the fans are contaminating grounds.

There appears to be significant a significant current and voltage difference between the grounding bar for the ESD HV power supply and the bar for the 48 and 18V ESD supplies. It may be that moving ESD power supplies (and ring heater supplies also) to the same bar would reduce the coupling. But Im not yet convinced that this is the main problem and Id like to do some more grounding investigations when I return.

Robert, Sheila