J. Oberling, R. Short

This afternoon, Jason and I started to look into why the FSS has been struggling to relock itself recently. In short, once the autolocker finds a RefCav resonance, it's been able to grab it, but loses it after about a second. This happens repeatedly, sometimes taking up to 45 minutes for the autolocker to finally grab and hold resonance on its own (which led me to do this manually twice yesterday). We first noticed the autolocker struggling when recovering the FSS after the most recent NPRO swap on November 22nd, which led Jason to manually lock it in that instance.

While looking at trends of when the autolocker both fails and is successful in locking the RefCav, we noticed that the fastmon channel looks the most different between the two cases. In a successful RefCav lock (attachment 1), the fastmon channel will start drifting away from zero as the PZT works to center on the resonance, but once the temperature loop turns on, the signal is brought back and eventually settles back around zero. In unsuccessful RefCav lock attempts (attachments 2 and 3), the fastmon channel will still drift away, but then lose resonance once the signal hits +/-13V (the limit of the PZT as set by the electronics within the TTFSS box) before the temploop is able to turn on. I also looked back to a successful FSS lock with the NPRO installed before this one (before the problems with the autolocker started, attachment 4), and the behavior looks much the same as with successful locks with the current NPRO.

It seems that with this NPRO, for some reason, the PZT is frequently running out of range when trying to center on the RefCav resonance before the temploop can turn on to help, but it sometimes gets lucky. Jason and I took some time familiarizing ourselves with the autolocker code (written in C and unchanged in over a decade) to give us a better idea of what it's doing. At this point, we're still not entirely sure what about this NPRO is causing the PZT to run out of range, but we do have some ideas of things to try during a maintenance window to make the FSS lock faster:

• Reduce delay turning on temploop (state 2; currently 1.0 sec)
• Increase wait time for mode to be centered (state 1; currently 0.1 sec; LLO uses 5.0 sec)
• Allow more voltage to PZT (requires using newer version of TTFSS box)
  • Current TTFSS limits PZT voltage to 13V-15V max
  • HV capable of supplying up to 65V
  • PZT capable of running up to 65V-100V