Bias
Late in my last visit I found that there was a lot of common mode rejection of voltage noise on the CS building electronics ground when the ITMX and ITMY biases were set to similar values and I suggested leaving them at -428V and -428V (https://alog.ligo-wa.caltech.edu/aLOG/index.php?callRep=67627 ). This trip I have investigated combinations of biases in more depth. Figure 1 shows a substantial reduction in coupling of existing lines relative to the -428V, -428V settings with 0V (ITMX ) and 60 V (ITMY – an offset of 1.3). I was able to tell that this was better than ITMX: 0V, and ITMY: 0V but not able to tell if this was better than ITMX: 0V, ITMY: 90V.
Grounding
In addition, I found that I could also reduce the coupling by better grounding of the Beckhoff chassis in the CER (Figure 2) and Fil is planning on improving grounding as was done at EX and EY.
Witness
I set up three magnetoemters that I moved around to see if I could find a better ground current witness than the current clamp and magnetometers. I placed magnetometers on the beam tube, on the BSCs and in cable trays. I did not find anything better than the original current clamp. The magnetometers have a somewhat better SNR but also detect non-grounding currents. So I have left my temporary current clamps set up at each station:
EX: H1:PEM-EX_ADC_0_09_OUT_DQ
EY: H1:PEM-EY_ADC_0_14_OUT_DQ
CS: H1:PEM-CS_ADC_5_26_2K_OUT_DQ
In addition, the ebay magnetometers are also good but not exclusive witnesses of ground currents.
Low frequency coupling
I previously noted that low frequency (say 5-30 Hz) ground injections were not minimized by adjusting bias (https://alog.ligo-wa.caltech.edu/aLOG/index.php?callRep=67627 ). This trip I tried to minimize low frequency coupling by improving grounding of PUM coil drivers, satellite amp racks, and other SUS chassis. I was not able to reduce the coupling and will have to get back to it when I return. Injections so far suggest that noise from ambient low frequency ground currents is, at 11 and 22 Hz, roughly 1/5 of the DARM level.
