P. King, J. Oberling, E. Merilh

Summary

We went in to the H1 PSL this morning to touch up the FSS RefCav alignment; it had started to drift again.  We checked 4 mirrors (M24 - M27) to see if their pitch adjustment screws were locked; we found M26 and M24 to be loose.  Adjusting the pitch of M26 had no effect on the RefCav TPD voltage (and therefore we don't believe this mirror is the problem), but adjusting M24 pitch we were able to recover the RefCav TPD voltage to ~1.6V.  Both adjustment screws were locked.  We measured the power after WP4 and WP6, and measured the DC voltage of the RefCav Refl PD (RFPD) with the FSS locked and unlocked.  We are going to continue to monitor the FSS RefCav TPD to see if the loose adjustment screw on M24 was the cause of our drift.

Details

At around 11:00pm PST on 2/27/2015 (last Friday) the RefCav TPD began to drift down again.  By yesterday afternoon it was down to 1.1V (this roughly corresponds to ~11mW in the ALS beam path, see Peter's comment to alog 16887).  This time we did not adjust the RefCav input periscope mirrors, since we adjusted and tightened them last time.  We decided to look for unlocked pitch adjustment screws for mirrors in the FSS path, since the alignment adjustments we have been doing previously have been almost entirely in pitch.  We started at M27 and worked back to M24 (see the PSL table layout in D0902114 for mirror locations).

• M27: Found to be locked, no adjustment performed
• M26: Pitch adjuster unlocked.  Adjusting pitch on this mirror did not improve the RefCav transmission so we don't believe this mount is the problem.  We locked the pitch adjustment screw.
• M25: Found to be locked, no adjustment performed
• M24: Pitch adjuster unlocked.  Adjusting pitch on this mirror down (clockwise turn on the adjustment screw) drastically improved the RefCav transmission, from 1.1V to 1.63V.  This screw was locked and the RefCav transmission left at 1.61V.

We are going to continue to montior the RefCav TPD voltage to see if this loose adjuster screw on M24 was the cause of our drift.

After this adjustment we measured the laser power after WP4 (before the FSS AOM) and after WP6 (before the RefCav input periscope):

• After WP4: 64mW
• After WP6: 34.5mW (53.9%)

We also measured the DC voltage of the RefCav RFPD with the FSS locked and unlocked.

• RefCav unlocked: 280mV
• RefCav locked: 50mV
  • This gives a RefCav visibility of 82%