J. Oberling, P. King

Summary

We tweaked the beam alignment into the PMC in the horizontal direction using both mirrors M06 and M07.  Final PMC transmitted power was 22.3 W with a visibility of 91.5%.  This did not recover the FSS RevCav TPD voltage, so we transitioned to realigning the FSS RefCav.  We found the beam clipped between mirror M26 and the 21.5MHz EOM.  We corrected this using mirror M25 and the used mirror M26 and the top input periscope mirror to tweak the alignment into the FSS RefCav.  Final RefCav TPD was 1.37V.  Still unclear where the drift in the RefCav TPD is coming from.

Details

We measured the power before the FSS AOM, after the AOM (single pass), and at the base of the RefCav input periscope.  Peter also measured the power between M34 and L05 (after the 35W pickoff for the DBB)

• Before AOM: 63 mW
• After AOM (single pass): 45 mW
• Base of RefCav input periscope: 30 mW
• Between M34 and L05: 297 mW

According to Peter this is all reasonable, so we moved on to the PMC.  Motive here is to make the reflected beam shape more concentric, thereby improving the beam alignment into the PMC, and increasing the transmitted power (and hopefully the FSS RefCav TPD in the process).  We tweaked the horizontal alignment using mirrors M06 and M07, and ended with a final transmitted power of 22.3 W, an improvement of about 0.3 W.  We then had to adjust the alignment into the PMC Refl PD, PD03, by adjusting mirror M20 and thin film polarizer TFP03 (to keep the unlocked voltage reading of the PD at ~-1.5V).

• PMC Refl PD before
  • locked: -0.305 V
  • unlocked: -2.85 V
• PMC Refl PD after
  • locked: -0.132 V
  • unlocked: -1.55 V

This gives a PMC visibility of 91.5%.  Unfortunately this did not significantly increase the FSS RefCav TPD, so it seems we really do have an alignment issue.

This first thing we did was to check the beam position on various irises in the beam path.  We found the beam clipping on the iris high and to the west between M26 and the 21.5 MHz EOM.  We corrected this with M25, and then used M26 and the top periscope mirror to tweak the RefCav TPD.  While a small bit of lateral adjustment was needed, the majority of the adjustment was to pitch.  The final TPD voltage was 1.37 V, and the RefCav Refl PD read 0.060 V while the RefCav was locked.  Unfortunately we are still unclear where the RefCav drift is coming from...