sheila.dwyer@LIGO.ORG - posted 14:06, Monday 31 July 2017 - last comment - 15:16, Monday 31 July 2017(37905)
ALS lasers off
at around 18:00:17 UTC today both ALS lasers turned off, within seconds of each other. Just now (around 20:10 UTC) I caused a lockloss. Heading to end stations to restart.
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J. Kissel, for S. Dwyer, T. Vo, and J. Bartlett
Recovery Note / Instructions:
- The lasers did fail at the same time (within a second).
- Drove to end stations (Thomas to one, Sheila to the other), walked into X/YVEA and hit the "reset" button on the laser controller (Thomas cycled the interlock key out of good practice, but Sheila did not, as it was not needed), laser turned on without problems.
- Once starting lock-acquisition again, found that arms would not lock on green. Chased the Beckhoff errors on the ALS screen to find that the X arm PLL was not locking.
- Upon turn on, one has a 50/50 chance of the ALS laser frequency (H1:ALS-X_LASER_HEAD_CRYSTALFREQUENCY) being on the "wrong side" of the PSL laser frequency, making the PLL unable to to lock the beatnote (H1:ALS-X_FIBR_LOCK_BEAT_FREQUENCY) at 40 MHz. We were unlucky on ALS X. To fix:
- hit "disable" on the PLL locking (H1:ALS-X_FIBR_LOCK_LOGIC_ENABLE),
- manually move around the crystal frequency until the beat note is within 10s of MHz of 40 MHz,
- enable to the PLL.
If it doesn't work, you're likely on the wrong side of the PSL frequency.
- Try bringing the beat note to zero and "come up from the other direction" with the crystal temperature, as the beat note as recorded is an absolute value.
- enable PLL again.
Still no evidence for why the lasers turned off, likely won't investigate.
FRS Ticket 8645 corresponds to this issue.