Reports until 11:40, Friday 14 October 2016
H1 PSL
peter.king@LIGO.ORG - posted 11:40, Friday 14 October 2016 (30527)
noise eater and FSS behaviour
IMC_F.png shows the 4 occasions from last night when the noise eater oscillated.  Anecdotally this tends to happen whenever
the IMC is trying to lock.  My conjecture is that rapid transients in the FAST actuator cause larger than normal changes in
the stress-induced refractive index change in the NPRO crystal, which in turn steers the beam out of the crystal a little
differently.  Since the photodiode used for the noise eater is relatively small, the beam could steer off the photodiode.
The reduction in photodiode signal then causes the noise eater to oscillate.  This does not happen all the time however and
I cannot explain why.  Zoomed_IMC_F.png shows that the IMC was not trying to lock at the time.

    Looking at the first time the noise eater misbehaved.  FAST_v_NoiseEater.png indicates that the noise eater oscillation
coincides with the third rapid increase in the FAST actuator output.  The long stretch where the FAST actuator is pegged at
~11.7V indicates that the FSS was not locked at the time.  SLOW_v_RCTPD.png shows that the FSS did not acquire lock at this
time but acquired lock during the second burst group of activity.  Of note is Kiwamu's observation that the FSS acquired lock
at a point outside the range where the SLOW actuator is limited to in software +/- 0.1V.  The FSS acquired with a SLOW
voltage of ~-0.25V.  The delay in settling down is the time taken for the so-called "temp loop" to be activated, presumably
by the autolocker.  Why it acquired outside this range might be due to the noise eater oscillating.

    Perhaps since the autolocker did not acquire on two flashes through the reference cavity, Travis disabled the autolocker
and began the hunt for the fringe manually.  Not realising that ALS likes having the SLOW voltage between -0.1V and +0.1V,
the crystal temperature kept being adjusted until a fringe was found.  The next fringe was found outside this range and the
FSS acquired at -0.25V, at which point the autolocker was re-engaged.
Images attached to this report