Seeing the increase in PMC Refl variation after our small PMC alignment tweak last week, I took a look at PMC things this morning.  Chiefly, I slightly tweaked PMC alignment to see if I could change the variation seen in PMC Refl, and then revisited the amplifier pump diode injection currents after our previous measurement of the pump diode slopes.

PMC Beam Alignment Tweak

I first tweaked the beam alignment to the PMC using our pico mounts.  With the ISS OFF PMC Refl was hovering around 23.6 W, and after a couple of small tweaks PMC Refl was hovering around 23.4 W; this was the lowest I could get PMC Refl.  PMC Trans increased by roughly the same amount; it began hovering around 104.9 W and ended hovering around 105.1 W.  Not a large improvement, but also not unexpected.  I turned the ISS back ON and let things sit and collect data for 15 minutes or so, then took a look at the variation in PMC Refl before and after the tweak.  Before today's tweak PMC Refl was moving roughly 0.45 W peak-to-peak, and after it was moving roughly 0.27 W peak-to-peak.  This is closer to the variation before last week's alignment tweak so I left the alignment here.  Will check in on this later to see if it holds.

Amplifier Pump Diodes

That done, I took a look at the injection current for the amplifier pump diodes.  From our diode slope measurements a couple of weeks ago the pump diodes are outputting more power than they were at install.  Case in point, at install we were seeing ~68 W out of Amp1 with 9.0 A of injection current to all 4 pump diodes; now with 9.0 A for diodes 1 and 2 and 8.8 A for diodes 3 and 4 we have ~71.5 W out of Amp1.  Keep in mind, the pump diodes are powered in groups of 2.  So for Amp1, power supply 1 supplies pump diodes 1 and 2 and power supply 2 supplies pump diodes 3 and 4 (power supply 3 supplies Amp2 diodes 1 and 2, and power supply 4 supplies Amp2 diodes 3 and 4); this means that whenever we change the current of one power supply we are changing the injection current for 2 pump diodes.

Amp1 first.  Looking at the slope measurements, to get close to our pump diode output powers at install the injection currents need to be set at 8.7 A for all 4 diodes.  To see how this affects our PMC Trans/Refl relation, I turned OFF the ISS again and set the Amp1 injection current to 8.7 A for both power supplies.  This dropped Amp1 output to ~69.2 W (from ~71.5 W), Amp2 output to ~138.3 W (from ~141.0 W).  PMC Refl dropped to ~22.8 W and PMC Trans dropped to ~104.0 W (from ~23.5 W and ~105.2 W, respectively).  PMC Refl remained ~22% of PMC Trans after this change (21.8% vs 22.3%), so this implies a straight power drop with no real change in the beam quality.  Since this is as close to the install output power as I can get, I recalibrated the pump diode power monitors for Amp1 at this point, as based on the slope measurements we're clearly not outputting between 85% and 95% of our install output power (which is what the Amp1 pump diode monitors read at this injection current).  I set both Amp1 power supplies to 8.8 A as this gave a good balance of output power, PMC Trans/Refl ratio, and pump diode output power.  At this point Amp1 was outputting ~70.2 W, Amp2 was outputting ~140.1 W, PMC Refl was ~23.1 W, and PMC Trans was ~104.7 W.

Next I took a look at Amp2.  We had tested putting Amp2 back to its install injection current (8.6 A) when we recovered the PMC after the pump diode slope measurements and this was not great for the PMC; Refl increased and Trans decreased by large amounts.  So even though Amp1 is happy with similar pump diode power vs install, Amp2 seems to want more.  Now this was putting the Amp2 injection current back to the install level, but not looking at the actual pump diode output power.  To get close to the Amp2 pump diode output powers at install the injection current would have to be lowered even more (8.3 A for diodes 1 and 2 and 8.4 A for diodes 3 and 4).  The table below summarizes the results I saw when changing the injection current for Amp2; I've bolded the row that represents the closest I can get to our install pump diode output powers, based on our diode slope measurements from a couple weeks ago.

First things first, we are currently overpumping Amp2, as seen by the behavior of Amp2 output with the changes in injection current.  However, as the injection current of Amp2 is decreased the PMC gets less and less happy, with the transmitted power dropping quickly despite there being little change in the power output from Amp2, and a slight increase in injection current reduces PMC Refl while keeping Amp2 output the same.  I can think of 2 reasons for this off the top of my head: these lower pump powers result in more power in higher order modes vs the fundamental mode, or our Amp2-to-PMC mode matching is somewhat less than ideal when the amplifier is not overpumped (I'm leaning towards the latter, we've suspected for a while that PMC mode matching isn't ideal).  To test this we would have to take beam propagation measurements on the beam at these different injection current levels.  IFF there is enough time during the upcoming break in O4 (we would probably need 2 or 3 days, likely after the vent work) we want to revisit PMC mode matching, so we would take a look at this then.  I also want to note that at the install pump diode output power (bolded row), Amp2 output was very noisy, varying by roughly +/-1.5 W.

I also tried 9.3 A injection current on all 4 pump diodes, but this yielded very little improvement over 9.2 A (~0.2 W better for both PMC Trans and Refl).  Despite slightly overpumping Amp2, I left the Amp2 injection currents at 9.2 A as the PMC is performing better at this current setting; we will revisit this if/when we take a look at PMC mode matching.  I then recalibrated the Amp2 pump diode monitors to more accurately reflect the amount of power they are outputting now vs install.  Before and after pictures of the pump diode monitor calibration factors are attached (looking at the PumpPD column).

To end, I turned on the ISS and adjusted the RefSignal to -2.0 V so it was diffracting ~4% (RefSignal adjustment necessary since PMC Trans increased by ~1.5 W); at 12:30 PST, when I finished, PMC Refl was ~22.3 W and PMC Trans was ~106.4 W.