Promised details from the final day of the NPRO swap.

Summary

• Measured mode matching lens positions to update PSL table as-built drawing
• Recovered both amplifiers without issue, final total output power of ~140.2 W
• Locked PMC, adjusted alignment, returned amplifier pump diode currents to previous operating levels; Amp1 out now ~68 W, Amp2 out now ~139 W, PMC Trans ~104 W, PMC Refl ~24 W
  • Have some PMC mode matching work to do, will do in future maintenance periods
• Locked ISS, had weird issue with PD values in MEDM (see details below), found it was due to NPRO noise eater needing a reset
• Locked FSS, had ~0.84 V TPD with no IMC
• Scanned NPRO temperatures for mode hopping, settled on NPRO crystal temperature of 23.96 °C
• Measured stabilization system TFs and set gains; all gains unchanged except FSS Common gain changed from 15 dB to 14 dB

Mode Matching Lens Positions

We first measured the new positions of mode matching lenses L02 and L21, I'll update the As-Built table layout with the new values.  The new positions:

• L02: 1251.5 mm
• L21: 1137.5 mm

Amplifier Recovery

With the previous day's work resulting in the NPRO being ready for amplifier recovery, this is where we started our recovery work.  Amplifier recovery was straightfoward.  As a reminder, we first measure the power into the amp and the unpumped power out, to assess our intial alignment.  Then we raise the amp pump diode operating current in 1A intervals until we get to the locking point, adjusting beam alignment into the amplifier at each step in current (so alignment follows the formation of the thermal lenses in the amplifier crystals).  For Amp1 we had 1.612 W input and an unpumped output of 1.252 W.  This is 77.6% throughput, which is above our requirement of 65% throughput before starting to pump the amplifier, so we proceeded with recovery of Amp1 (see Amp1 columns in below table).  We finished Amp1 recovery with ~70.2 W output, so we calibrated the Amp1 power monitor PD to this value (was off by a couple Watts).  We then lowered the light level in the Amp1/Amp2 path (using the High Power Attenuator (HPA) after Amp1) and checked alignment, all looked good.  We increased the Amp2 seed with the HPA to ~1.8 W and checked the unpumped output from Amp2.  This measured at ~1.5 W, which is ~83% throughput and above our 65% threshold, so we proceeded with recovery of Amp2.  This also went very well, see the Amp2 column in the below table; we had ~64.0 W output from Amp2 with an ~1.8 W initial seed power (the power was bouncing between 63.9 W and 64.0 W).  With Amp2 fully powered we then used the HPA to increase the Amp2 seed to max, which resulted in ~140.2 W output from Amp2.

Stabilization System Recovery

PMC:  After lunch we began recovering the PSL stabilization systems in order: PMC, ISS, FSS.  We began by using the HPA after Amp2 to lower the power to ~100 mW to check our beam alignment up to the PMC.  All looked good here so we increased the power to max and measured the power incident on the PMC at ~129.4 W.  We then toggled the PMC autolock to ON and it locked without issue.  We needed to use the picomotor-equipped mirrors (M11 and M12 on the layout) to tweak the beam alignment into the PMC, but were only able to get ~102.0 W in transmission with ~27.0 W in reflection.  This is 9 W more than we had after our last NPRO swap, indicating that we really need to take a look at PMC mode matching; since we still had more than enough power to deliver to the IFO we decided to defer the mode matching work to a later Tuesday and continue with PSL recovery.  The PMC Trans and Refl monitor PDs were calibrated to the newly measured values; they were pretty close to begin with, but were still 1-2 W different than our power meter was measuring.  We then returned the amplifer pump diode currents to their previous operating values (9.0 A and 8.8 A for Amp1, 9.1 A and 9.1 A for Amp2), which lowered Amp1 output power from ~70 W to ~68 W and Amp2 output power from ~140 W to ~139 W; this also changed PMC Refl to ~24 W and PMC Trans to ~104 W, indicating our beam is better matched to our current mode matching solution at these pump diode currents.

ISS:  Moving on to the ISS, we first measured the amount of power in our 1st order diffracted beam (the "power bank" for the ISS).  With the loop off and the AOM diffracting a default of 4% we expect ~5.7 W in this beam, and this is what we measured.  AOM alignment was good, so moved on to the ISS PDs in the ISS box.  A voltmeter get plugged into the DC Out ports on the ISS box and a HWP inside the box is adjusted until PD voltages read ~10.0 V.  We did this, but noticed the DC voltage reading on the ISS MEDM screen was much higher, ~12.5 V for PDA and ~13 V for PDB.  We tried to lock the loop and, as expected with PD voltages that high, the loop thought it needed to removed more power from the beam and ran the diffracted power up really high.  We immediately unlocked the ISS and began looking into what could be the problem, as the MEDM reading on the ISS PDs generally matches the voltmeter reading (I say "generally matches" because, for reasons unknown to me, the ISS does not use the DC out from its PDs, it uses a Filter out and "derives" the DC and AC PD voltages from that).  The PDs appeared to be working correctly, and we found no large dark voltages that would indicate a PD failure/malfunction.  When we unplugged the Filter output the PD reading in MEDM began to slowly climb, but when we blocked the light onto the PDs the MEDM reading went to zero.  Looking back at trends we saw the PDs behaving as expected before this most recent NPRO swap, only reading these higher values in MEDM with the relock of the PMC an hour or so prior.  I had never seen this behavior in the past, so wasn't quite sure where the problem could be.  Thinking maybe something had gone wrong in either the ISS inner loop servo box or maybe something in the CER, we called Fil and asked if he could take a look at the CER electronics for the ISS while we moved on to FSS recovery.  It was at this point we found the problem.  When the FSS MEDM screen was opened the first thing we saw was one NPRO noise eater (NE) light green, and the other red.  The green light was our NE enable monitor, indicating that the NE toggle was switched ON in the PSL software; the red light was our NE monitor, which reads the Check output from our NPRO monitor PD that indicates whether or not the NPRO's relaxation oscillation was being supressed.  So we had the NE toggled ON but it was clearly not working, so we toggled it off and on again.  The NE monitor went green and the channel monitoring the relaxation oscillation indicated it was working properly, and the ISS PD values on the ISS MEDM screen now read the correct values.  So I learned that we have another measure of if the NE is working or not, the ISS PD readings on the MEDM screen go higher.  Trending back, the NE stopped working at ~16:58 PST on Thursday, right before Ryan and I left the enclosure for the day.  We'll keep an eye on this, as right now it's not clear why the NE turned off.  At this point everything looked good for the ISS so we moved on to the FSS.

FSS:  For the FSS, we first tried to see if the RefCav would lock with the autolocker; it would not.  We had to manually tune the NPRO temperature to find a RefCav resonance, one was found with a slider value of ~ +0.06.  The temperature search ranges were adjsuted to this new value and we tried the autolocker again.  While we could see clear flashes the autolocker would not grab lock for some reason.  The FSS guardian was paused so it would stop yanking the gains around upon lock acquisition, but this did not help, the autolocker refused to hold lock for some reason.  So I did it manually (from the FSS manual screen, manually change NPRO temperature until a resonance flashed through, then really quickly move the mouse up to turn the loop on; if the loop grabs go back to the FSS MEDM screen and turn on the Temperature loop, if not then turn the loop off and try again), which worked.  With a locked RefCav we measured a RefCav TPD voltage of ~ 0.84 V.  The RefCav Refl spot looked pretty centered so we did not do any alignment tuning.  This completed our work in the enclosure so we cleaned up, turned the computers and monitors off, left the enclosure, and put it into Science mode.  Outside, we scaned the NPRO for mode hop regions and measured TFs of the stabilization loops.

NPRO Temperature Scan

Now outside the enclosure we set up to scan the NPRO temperatures to check for mode hopping.  We took the HV Monitor output from the PMC fieldbox to trigger an oscillscope on the PZT ramp and used the PMC Trans PD to monitor the peaks.  We set the PMC's alignment ramp to +/- 7.0 V and a 1 Hz scan rate, and monitored the peaks as we tuned the NPRO crystal temperature.  We used the slider on the FSS Manual MEDM screen, which gives us a total range of approximately +/- 0.8 °C (0.01 on the slider changes the NPRO crystal temperature by roughly 0.01 °C and the slider goes from -0.8 to +0.8).  Since we were close to zero on slider, sitting at ~ +0.07, we started by moving lower (which reduces the NPRO crystal temperature); our starting crystal temperature, as read at the NPRO power supply front panel, was 24.22 °C.  We got all the way to the negative end of the slider, which gave a crystal temperature of 23.38 °C, and did not see any evidence of mode hopping on the way down.  Heading back up we finally started to see early evidence of mode hopping near the top end of the slider; we could clearly see a new forest of peaks show up in the PMC PZT scan and one of them started to grow noticebly as the temperature was further increased.  This mode hop region began at a crystal temperature of 24.76 °C, and the slider maxed out at 24.91 °C.  At this point we still had not fully transitioned through the mode hop region, but we did have a peak starting to grow very large indicating that we were almost there.  Since we saw no evidence of mode hopping by making the temperature colder, we went back to our starting place of 24.22 °C and then reduced the temperature further to the next RefCav resonance below that; this resulted in a crystal temperature of 23.96 °C at a slider value around -0.17.  Again I had to lock the RefCav manually, as the autolocker did not want to grab and hold lock.  With all of the stabilization systems locked we moved on to TF measurements.

Transfer Functions and Gains

We started with the PMC.  With the current settings we have a UGF of ~1.6 kHz and 60° of phase margin, see first attachment.  Everything looked good so we left the PMC alone.

For the ISS, we have a UGF of ~45 kHz and a phase margin of 37.5°, see second attachment.  Again, everything look normal here so we left the ISS alone.

For the FSS, we started with a Common gain of 15 dB.  Everything looked OK, but since we had seen some potential zero crossings that like to hide in the longer range scans we did a "zoomed in" scan from 100 kHz to 1 MHz.  Sure enough, there were a couple peaks in the 500 kHz to 600 kHz range that were pretty close to a zero crossing.  We lowered the Common gain to 14 dB to move them away from the potential crossing; the third attachment shows this zoomed in area with the Common gain at 14 dB, and the peaks in question are clearly visible.  With this Common gain we have a UGF of ~378 kHz with ~60° of phase margin, see fourth attachment; we took this TF out to 10MHz to check for any weirdness at higher frequency and did not see anything immediately concerning.  To finish we took a look at the PZT/EOM crossover (around 20 kHz) to set the Fast gain.  The final attachment shows this measurement (a spectrum of IN1) at a Fast gain of 5 dB; this looks OK so we left the Fast gain as is.

At this point the NPRO swap was complete, the PSL was fully recovered, and we handed things over to the commissioning team for IFO recovery.  We still need to look at PMC mode matching, and will do so during future Tuesday maintenance periods.  This closes WP 12210.