Peter F. and Kiwamu,

Related logs: 36106 and its comments

We looked into several (randomly chosen) lock loss examples to study how much the spot position can typically move on the test masses immediately after lockloss.

Summary:

• The arm powers decay in ~1 sec to get to 10% of the max powers.
• The test mass can get a kick vertically by 5 urad (corresponding to a spot position change of ~120 mm on the test masses) within 1 sec
• On the other hand, the horizontal kicks seem consistently smaller than the vertical ones by a factor of a few.

Locklosses:

We randomly picked the following lock losses:

• Apr 04 2017 13:02:26 UTC (Pin = 30 W)

• May 05 2017 23:48:06 UTC (Pin = 30 W)

• Dec 08 2016 06:09:25 UTC (Pin = 30 W)

• Nov 12 2015 18:11:16 UTC (Pin = 22 W)

• Jul 27 2016 08:15:22 UTC (Pin = 50 W)

See the attached pdf for the actual oplev signals and arm power signals for those times.

Angle motion to spot position:

Because of the negative g-factor design in aLIGO, the effective lever arm for the light in the arms is amplified by 1/(1 - gi * ge) = 5.8 for the spot positions where gi and ge are the g-factors for ITM and ETM, respectively. Therefore the lever arm is L/(1-gi*ge) = 23.5 km. In order to bring a beam with 5 cm radius to the edge of a test mass (with 34 cm diameter), the beam spot must move by 340 mm /2 - 50 mm = 120 mm. To produce such a large excursion in the spot position on an optic, one of or both test masses needs to tilt roughly by 120 mm / 23.5 km = 5 urad.

Some observations:

Usually ETMY PIT experiences the fastest change in the first 1 second after lockloss. It can easily reach 5 urad within 1 sec. Other pitch signals also showed a similar behavior in that they move relatively fast and typically reach 3-4 urad within 1 sec. All the pitch motions tend to go negative for some reason.

In contrast, the yaw motions didn't move as much as pitch motions. They typically jump by less than 1 urad at the beginning and later drift by a few urad relatively slowly.