Background information:

• Coarse tidal correction actuation disabled during E2
  
  
• Expect periodic lock loss as dynamic range of longitudinal actuation exceeded
  
  
• Rate of lock loss determined by linear combination of common-mode and differential-mode tidal slopes (steep slope --> short locked stretches)

• Pitch compensation reduces effective dynamic range
  (must push harder with upper coils than with lower)

• Residual imbalance in mirror suspension requires significant DC offsets in some coil currents.
  
  
• Asymmetry leads to saturation sooner in one direction than the other
  
  
• Data taken in recombination configuration (no recycling) and with each arm individually
  
  
• Actuation configuration:

• L₊  -->  Equal push on ETMX and ETMY

• L_-  -->  Opposite push on ETMX and ETMY

• l_-  -->  Opposite push on ITMX and ITMY (and on ETMX and ETMY!)

• Lock durations

In recombination locks lasted typically an hour with expected (qualitative) correlation of length with tidal slope inferred from L+ control signal. Lock durations especially short when common-mode tidal slope falling, suggesting asymmetric sensitivity. [Figure: pdf eps]
 
• Y arm behavior [Figure: jpg (scanned figure)]
  
  

Transmitted light through Y arm drifted linearly with time during each lock, as if misalignment steadily worsening

Optical levers for both pitch and yaw also showed linear drift


Interpreted as mistuning of pitch compensation and yaw balance

ETMY output matrix retuned Saturday morning (R. Schofield). 
Smaller drifts in both pitch/yaw seen subsequently

Despite these drifts, we don't believe the Y arm was responsible for most lock losses



• X arm behavior

Transmitted light through X arm relatively flat during most of lock, but rapid and non-linear degradation seen in minutes preceding lock loss

Yaw of ETMX showed modest, linear drift

Pitch flatter, but same rapid pre-loss degradation seen as in transmitted light

Two coils on ETMX with large DC offsets

Symptoms suggested "premature" saturation in one or more ETMX coils, causing mirror twist
 

• Coil currents

Examined long-term trends of ETMX and ETMY coil currents and found anomalous behavior in upper left (UL) coil of ETMX:
 

• "Software rails" seen in data acquisition ADC at expected +/- 2 V for all ETMX and ETMY coils, but for periods preceding lock loss , evidence of "hardware rail" seen in ETMX UL --> Odd, symmetric "exponential decay" of min/max envelope, suggesting internal electronic saturation in UL circuit. [4-day trend (pdf eps), 6-hour trend (pdf, eps)]

• During one such "decay", checked monitor points on ETMX controller in X mid stations with portable oscilloscope. Observed severe, asymmetric railing at +2 V on UL coil, with moderate railing on UR and very little railing on LL and LR. Eyeball estimate from scope traces indicated a strong 2-2.5 kHz component. During and after subsequent lock reacquistion, railing nearly absent, but envelope of signals uncomfortably close to rails --> perhaps a long-term noise concern.

• Power spectrum [pdf, eps] of ETMX UL coil current also peculiar: anomalous peak at about 30 Hz (aliasing of higher frequency  (>2 kHz) oscillation?)

• Necessity of coarse tidal actuation amply confirmed!
  
  
• ETMX UL coil probable culprit in most lock losses

-->  saturation
-->  mirror twisting
-->  degradation in one or more servo gains, with growth of higher order modes

Open questions (still to be investigated):

• Is premature UL saturation strictly result of initial DC offset?
  
  
• Or does a faulty electronic circuit in the ETMX controller contribute too?

• What is the exact mechanism by which gross misalignment leads to lock loss?
  (which servo gives way first?) Calibration lines may be helpful here for understanding gain loss.

• Create a complete list of E2 locked stretches longer than, say, 1 minute, from replay of data. 
  (Our lock loss monitor wasn't up and running until Friday, and database triggers weren't generated until Saturday.)

• Online tools in control room (data viewer and diagnostic test tool) extremely valuable
  
  
• But data viewer unavailable outside control room, even to other Hanford computers
  -->  Hampers off-site analysis

Aside from "official" lock loss investigators, many other physicists contributed ideas to investigate and helped to carry out studies. A partial list includes S. Whitcomb, D. Sigg, R. Schofield, R. Drever, D. Ugolini, R. Flaminio, R. Adhikari.