J. Oberling, R. Savage, J. Bartlett

Summary

We went into the PSL to tweak the beam alignment into the FSS RefCav and PMC.  The FSS TPD was left at 1.67 V with a RefCav visibility of 85% and we found that the RFPD DC monitor on the FSS MEDM screen is not reading accurately; will have to fix this.  We still are not sure what is causing the drift.  Due to time constraints we did not touch the PMC, currently planning on doing that during the next maintenance period.

Details

Here is the FSS as we found it upon entering the PSL enclosure:

• TPD: 0.556 V
• RFPD DC: 0.015 V
• Power incident on L10 (meas with small thermal head): 65 mW
• Power incident on lower periscope mirror: 30 mW

We performed the following steps to realign the beam into the RefCav:

• We first adjusted the beam on the Reflection PD (RPD) to better center it, using the RFPD DC output on the FSS MEDM screen as a guide
  • Raising beam on rfpd by 1/4 turn of mirror in front of rfpd increased dc level 60-70 mv
• Then we adjusted pitch on the bottom periscope mirror.
  • Backing off vertical got the TPD up to 0.85 V
• Using the upper and lower periscope mirrors, we walked the beam up the RefCav input.
  • TPD was up to 1.50 V with only this vertical walk
• Maxed the TPD using a horizontal tweak on the top mirror, then a small walk to the west using both periscope mirrors
  • TPD up to 1.575 after horizontal walk
• At this point the RF PD was reading 0.133mV
• We touched up RFPD alignment
  • RFPD to 0.130mV
• Final TPD reading of 1.581V

At this point we wanted to measure the visibility of the cavity.  We plugged a multimeter into the DC port of the RPD and noticed that the reading on the multimeter was different from the reading on the RFPD DC monitor on the FSS MEDM screen.  Rick then used the multimeter to center the beam on the RPD; this was done while the loop was locked.  There was no change in the TPD voltage.  We then measured the RefCav visibility:

• RPD w/ FSS Locked: 38 mV
• RPD w/ FSS Unlocked: 255 mV
• RefCav visibility: 85%

Finally, we set the UGF of the FSS loop.  To maintain the UGF at 500 kHz we set the FSS common gain at 26dB.  This gave a UGF of 500 kHz and a phase margin of 59°

After all was said and done the RefCav TPD had a final reading of 1.67 V.  In the interest of time, we then left the PSL without adjusting the beam alignment into the PMC so Robert could begin his work with the IO periscope.  We plan on returning to the PMC alignment next maintenance period.