J. Oberling, O. Patane

Today we did some alignment tweaks for the PMC and FSS RefCav.

Before going into the PSL enclosure (and disrupting its thermal equilibrium), I remotely tweaked the beam alignment into the PMC.  This was done with the ISS OFF (as otherwise the ISS sees any change in PMC Trans as something it needs to correct for, and we do not want this).  Results:

• Start
  • PMC Trans: ~108.8 W
  • PMC Refl: ~17.0 W
• End
  • PMC Trans: ~109.3 W
  • PMC Refl: ~16.4 W

So some improvement, but not quite as much as I had hoped (we've been under 16.0 W on PMC Refl in the past), so it may be time to increase the amplifier injection currents a little.  Will continue to monitor the PMC.

That done, Oli and I went into the PSL enclosure to manually tweak the on-table FSS RefCav alignment.  After a safety overview of the PSL table (as it was Olli's first time in the enclosure), we began by taking a power budget of the FSS beam path.  This was done with the ISS ON.

• FSS In: 294.6 mW
• AOM In: 288.0 mW
• AOM Out, Single Pass: 195.1 mW
  • Single Pass Diffraction Efficiency: 67.7%
• AOM Out, Double Pass: 130.2 mW
  • Double Pass Diffraction Efficiency: 66.7%
• EOM Out: 130.3 mW

Both the single and double pass diffraction efficiency are lower than usual, which makes sense as at last remote alignment I was unable to return the RefCav TPD to its normal levels.  We adjusted the AOM alignment to improve the single pass diffraction, and mirror M21 to improve the double pass diffraction.  Results:

• AOM In: 288.0 mW
• AOM Out, Single Pass: 216.9 mW
  • Single Pass Diffraction Efficiency: 75.3%
• AOM Out, Double Pass: 157.8 mW
  • Double Pass Diffraction Efficiency: 72.8%

Much better.  We then had to slightly adjust the EOM position to ensure the entrance and exit apertures were centered on the beam (to prevent any internal clipping), and then proceeded to recover the RefCav.  The alignment was far enough off that we needed to use the iris mounted in front of the RefCav to recover the alignment, then locked the RefCav and used the picomotor-controlled mirrors to tweak the alignment and improve the TPD.  At the end the TPD was ~0.94V.  To finish, we aligned the beam onto the RefCav RFPD, and measured the RefCav's visibility.

• Unlocked: 1.311 V
• Locked: 0.225 V
• Visibility: 82.8%

This is roughly where we had it last time the on-table alignment was tweaked back in May.  We then turned the ISS OFF and left the enclosure and returned it to Science Mode.

Back in the Control Room I turned the ISS back ON, at its usual RefSignal value of -2.01V, and the diffracted power % dove to almost 0%.  I had to increase the RefSignal to -1.97V to get the diffracted power % to ~2.0% (I did not accept this in SDF yet, as it will likely need to be adjusted soon as the enclosure warms up).  In addition, the PMC Refl power had increased over the course of the incursion to ~18.3 W, almost 2 W higher than when we went in.  This was all expected as an enclosure incursion disrupts the internal thermal equilibrium of the enclosure (and why we like to do these as early in the maintenance window as possible).  As the thermal environment equalizes I expect the diffracted power % to go up and PMC Refl to go down; both may need to be adjusted as a T.O.O. in the coming days, or next Tuesday, whichever comes first.  I'll keep an eye on the PSL over the coming days and adjust things as necessary (most likely ISS RefSignal adjustments, as those are quick during a lockloss recovery; any necessary PMC tweaks may likely wait until next Tuesday).  I've tagged OpsInfo so the EVE and OWL shift operators are aware of this.  This closes WP 11309.