Correlation with 1-3 Hz seismic noise

• Commissioning experience before E4 indicated that large seismic noise in the 1-3 Hz band contributed to difficult lock acquisition and retention.
• Using the output of Ed Daw's band-limited rms seismic DMT monitor and Dave Chin's lock transition DMT monitor, we confirmed a strong correlation between 1-3 Hz seismic noise and lock livetime fraction over the course of the 72-hour run.
• Lock was consistently lost during the passage of trains early in the morning. Steve Penn reported observing a dramatic rise in power at 1.2 Hz (BSC stack mode) in the DC power on symmetric and antisymmetric port photodiodes as one of these trains approached Monday morning. He suggests this indicates overdamping on large optics, perhaps leaving us overly susceptible to seismic noise at stack frequencies. A subsequent search by KR through data channels for large and small optics coil currents at the moment lock was lost during this train passage failed, however, to find any smoking guns (currents saturating before lock loss), but it should be noted that only the end mass coil current readouts were DC coupled.

Coil saturation after tidal drift

• Because of other lock retention difficulties, straightforward saturation of coils after long tidal drifts was less common than in the E2 run, but here is one example of this phenomenon, which should not be a problem once feed-forward tidal correction is implemented. It should be noted that several of the end mass coil currents routinely saturate during lock acquisition.

Apparent servo instability

• The saturation of the ETMX LR coil in the preceding example was seen in a large number of lock losses, including a class in which all three active ETMX coils appear to undergo an unstable oscillation immediately preceding lock loss. This plot shows a 3-minute second trend of the six active end mass coils (1st column = X, 2nd column = Y) for an example lock stretch Saturday morning, along with the transmitted arm powers (3rd column). ETMX LR (ETMX_CAL) appears to saturate first at about the time of lock loss. A magnified view of the six coil currents confirms this guess. For reference, the transmitted arm powers do not drop significantly until well after saturation ensues, as shown in this plot with the same time-zero offset but larger range. The source of the apparent instability which causes an exponential growth of the ETMX coil currents with time constant of about 5 ms is not yet known.
• As a side note, we looked at all DMT triggers that were recorded in the metadatabase at the time of lock loss. Although we didn't find any triggers that gave us further insight into the lock loss cause, we did find two unexpected and interesting triggers generated by Masahiro Ito's GlitchMon monitor. These were caused by a transient in the microphone in the X end station at the onset of coil saturation and by another in the beam splitter OSEM LR sensor at the first significant drop in X arm transmitted power. The microphone transient may indicate acoustic noise pickup from the end mass coil (switching) power supply as it approaches and reaches saturation, or there may be a coupling through a common AC power circuit. The OSEM sensor transient seems to indicate that the BS LR sensor is more exposed than the other BS sensors to scattered laser light. Both types of transients were observed to coincide with other lock losses too.

686 Hz oscillation in length servo

• Rana Adhikari spotted an unusual oscillation in the arm length servo control signals. This plot shows a gradual growth in  the envelope(min/max) of both signals, a growth that suddenly comes to an end as the transmitted power in the X arm dramatically increases(!). The turn-off in envelope growth and increase in X arm power turned out to coincide with saturation of the ETMX UL coil (GDS-EX_TO2), as shown in this plot, where the other two active X end coils are unsaturated, but where the yaw and pitch suddenly change. The natural interpretation is that after a long lock period  where tidal correction and residual coil matrix imbalance caused a drift away from optimum ETMX mirror alignment, the saturation of an upper coil caused a twisting of the mass that (temporalily) brought it back to better alignment.
• What is not so easy to understand is the source of the original oscillation. This plot shows the power spectra of the two arm control signals and the Mode Cleaner's fast control signal at a time when the envelope above was growing. There is a strong (and new) peak at 686 Hz in the arm signals that does not appear to originate from the input optics. This plot shows the spectra of the differential mode control signal at 20-minute intervals covering the absence of oscillation and at its full strength.

Information on locked stretches

• The cumulative live-time fractions for arm locks during E4 were

X arm     - 51%
Y arm     - 49 %
Both arms - 48 %
• List of 2-arm lock stretches at least 1 minute long