sheila.dwyer@LIGO.ORG - posted 20:02, Friday 16 May 2025 - last comment - 11:33, Monday 19 May 2025(84446)
PMC left unlocked
I left 10W out of the rotation stage, with the stage de-engergized this evening and the light pipe closed. I've just now unlocked the PMC (LOCK off, RAMP off) so that we won't have 10W on the light pipe shutter all weekend.
This is a reminder that whenever the PMC is unlocked, especially for an extended period of time, the FSS and ISS should also be disengaged; otherwise, the respective autolockers will continue to try to engage these systems despite not having the means to do so (because they have no light, since the PMC is unlocked). If there are ever any questions as to how to properly and safely disengage the PSL stabilization systems, CONTACT ME. It does not matter if I'm offsite for any reason (RDO, vacation, sick, etc.).
From the 10-day trends Ryan posted this morning, neither the ISS nor the FSS were properly disengaged, so were left trying to lock all weekend. For the FSS see the plot H1:PSL-FSS_NPRO_XTALTEMP in this set of plots. It's clear from this plot that the FSS autolocker was doing a temperature search while trying to lock the RefCav, which means the NPRO PZT was also ramping. Since there was no light in the FSS path the RefCav could not lock, so the search continued all weekend.
For the ISS, see the plot H1:PSL-ISS_DIFFRACTION_AVG in this set of plots. The ISS diffraction should be completely flat if it is off, but the series of downward spikes indicate the ISS was trying to lock and failing to do so, since there was no light on the ISS PDs (my guess is it kept lowering the diffraction percentage to try to increase light on the ISS PDs, and unlocking when the diffraction hit 0% to start the process again). This can also be seen in the behavior of the PMC reflected power (1st attached picture), which was moving around substantially all weekend as the ISS was trying and failing to lock; can also see the ISS diffracted power moving between 0% and 40%. A zoomed in portion is also attached, in which it's seen that the ISS diffraction was moving from zero to ~20% (but as high as 50%) in a roughly 15 second period; note the changes in PMC Refl during this time (the small spikes in PMC Trans are not the PMC trying to lock, it's small flashes of resonance as the temperature of the PMC changes combined with the FSS actuating on the laser frequency).
While not strictly necessary, it's good practice to also close the PSL external shutter if the stabilization systems are going to be disabled for an extended period.
I don't think that this will compromise the performance of the ISS or FSS, but this does represent unnecessary wear on the mechanical components that are the actuators for both of these stabilization subsystems. I should have caught that the PSL was left in this state, but did not pay attention to the alog during my RDO weekend and therefore did not catch this. For this I apologize.