J. Oberling, P. King

Summary

Attempted to test a spare TTFSS box that Peter had modified to see if it would increase the bandwidth of the FSS.  Unfortunately the FSS would not lock with the modified box so we installed the original and moved on.

Noticed that Front End (FE) diodes 3 and 4 were reading ~95.5% of ideal power (diodes 1 and 2 are still good).  This is odd as we just increased the diode currents 3 weeks ago.  We raised the current on diodes 3 and 4 to 52 A (from 51 A) and adjusted the temperature on both diodes to 18 °C.  Power out of the FE is now 33.1 W.  Will keep and eye on this.

Tweaked the beam alignment into the PMC using mirrors M06 and M07 and adjusted the position of PMC mode matching lenses L02 and L03.  PMC transmitted power is now 23.7 W and has a visibility of 90.7%

Since we swapped the FSS box we tweaked the FSS RefCav alignment.  FSS RefCav TPD is now reading 1.49 V and the RefCav has a visibility of 70.9%.  While doing this we began to suspect that the bottom mirror of the RefCav input periscope (PRS01) might be a source for the RefCav drift issues we've been dealing with.  It has been suggested that we move the bottom mirror from the periscope post and mount it directly on the table.  Will discuss this with the larger PSL group.

Details

TTFSS Box

As noted above, we attempted to test a spare TTFSS box that Peter had modified to hopefully give more bandwidth to the FSS.  Unfortunately the FSS would not lock with this modified box, so we reinstalled the original box and moved on.

Front End Diodes

Peter noticed that FE diodes 3 and 4 were reading ~95.5% of ideal power, which is odd as we just adjusted the currents on the FE diodes 3 weeks ago; they should not degrade this fast.  Diodes 1 and 2 are still reading 99% of ideal.  We raised the current on diodes 3 and 4 to 52 A from 51 A, and then tweaked the diodes' operating temperature to 18 °C from 19 °C.  The power out of the FE is now reading 33.1W.  We will keep an eye on this to see if this quick degradation continues.

PMC Alignment

We tweaked the beam alignment into the PMC as well as the position of the mode matching lenses in an effort to increase the power transmitted by the PMC.  We started by adjusting the lens positions to maximize transmitted power, and then adjusted the beam alignment.  The old and new lens positions, as read from the micrometers attached to the lens mounts, are:

• Lens L02
  • Old position: 11.3 mm
  • New position: 13.68 mm
• Lens L03
  • Old position: 4.15 mm
  • New position: 6.29 mm

We then proceeded to adjust the beam alignment into the PMC.  The majority of the adjustment was in yaw, pitch yielded very little improvement.  After alignment:

• PMC Trans
  • Old: 21.7 W
  • New: 23.7 W
• PMC Refl:
  • Old: 2.9 W
  • New: 2.1 W

This is a marked improvement, as previously the reflected power was 13.4% of the transmitted and now it is only 8.9%.  Finally we measured the PMC Refl PD (RPD) voltage while the PMC was both locked and unlocked to calculate the visibility:

• Locked: -0.143 V
• Unlocked: -1.54 V
• Visibility: 90.7%

FSS Alignment

Since we had changed out, then changed back in, the TTFSS box we tweaked the beam alignment into the FSS RefCav.  A small pitch tweak in the up direction on the top periscope mirror increased the RefCav TPD from 1.35 V to 1.5 V.  We then tried to tweak the bottom periscope mirror.  As soon as Peter inserted the adjustment knob into the pitch screw of the mount, the TPD dropped from 1.5 V to 1.34 V.  What's odd about this is that no adjustment was performed.  Simply inserting the adjustment knob caused the TPD to decrease.  What is even more odd is that Peter could not recover the alignment; could make it worse but could not get better than a TPD of 1.34 V, regardless of which periscope mirror was adjusted.  I then checked that the periscope mirror mounts were tight on the post, which they seemed to be.  All of a sudden Peter was able to recover the TPD back to 1.49 V with a small pitch tweak on the top periscope mirror.  Related?  Not sure...  At any rate, we started pressing and tapping on the bottom mount in various places to see if we could influence the RefCav TPD.  Turns out that we could slightly change the TPD, especially when manipulating the knob that secures the mount to the periscope post; when the same was done to the top periscope mirror mount and the periscope post, no change was observed.  We will discuss moving the bottom periscope mirror from the periscope post to the PSL table, in the hope that this would help alleviate the RefCav drift issue we've been seeing.

We then measured the RefCav RPD DC voltage and calculated the visibility of the RefCav:

• Locked: 75 mV
• Unlocked: 258 mV
• Visibility: 70.9%