aLIGO LHO Logbook

H1 PSL (DetChar, IOO, Lockloss, OpsInfo)

jeffrey.kissel@LIGO.ORG - posted 18:57, Friday 14 December 2018 - last comment - 08:50, Saturday 15 December 2018(45954)

PSL FSS Oscilliations are a good fraction of Lockoss Recovery Time

J. Kissel

A big time suck after lock losses this evening, and last night, has been the FSS going into oscillation after a full IFO lock loss. This has been reported before (45551) but the loop parameters appear to only have been checked in upon last in late November (LHO aLOGs 45547, 45541). 

I'm not enough of an expert to commission the thing better... we've tried only the few superstitious things we know of -- namely, turning on and off the autolocker. We're typically waiting for the time scale it takes for the temperature loop to scan the entire cavity -- 5 to 10 minutes.

Comments related to this report

peter.king@LIGO.ORG - 08:50, Saturday 15 December 2018 (45967)

Link

I believe part of the problem lies in the fact that either the front end model, or guardian,
ramps the gain down and up as the FSS is trying to acquire lock, based on the oscillation
threshold (top, centre of the MEDM screen) being set too low.  As the laser approaches
resonance, the common gain gets ramped between 0 and 20 as the servo is trying to lock instead
of staying fixed.  The gain ramping should only really take place when the loop is locked
and oscillating, not when it's trying to acquire.