Dan, Craig, Georgia, Alexei, Anamaria
We decided to try a higher power lock now that a lot of things have been smoothed out. We were able to easily power up to 36W (we did it at the POWER_30W step, before low noise ASC settings and ESD). We ran two sets of lines, low frequency and high frequency for intensity and frequency noise coupling. We also ran a 9 MHz EOM line.
- For a while we ran without ISS DC_COUPLED for unrelated reasons, but once Georgia engaged it the ISS line at 70ish MHz was reduced by a factor of 10.
- We paranoidly increased the PRCL gain from 12 to 15, reduced MICH P gain from -1.2 to -1 and reduced CHARD P from 2.0 to 1.8. Unclear if it was necessary.
- We set the SRCL length offset to what Sheila had chosen earlier for optimized squeezing, but we were only able to get 1.5 dB instead of the 2 dB she had. We didn't try to optimize anything about it.
- We raised the CM gain 3dB and saw the DARM noise at high frequency get reduced some. We could try more maybe, but we should measure the loop.
- We overall lost 4% of optical gain (says maybe-ok-kappaC) but we recovered 1% with a bit of spot move (Dan will post details). We put this into CAL DARM ERR and ran a pcal broadband to confirm the calibration was correct.
- Some numbers:
Optical gain decrease: 3%
PRG decrease: 3.3%
X arm increase: 17%
Y arm increase: 16%
AS_C increase: 16%
REFL increase: 8%
POP 18 decrease: 6%
POP90 decrease: 4%
- The ISS low frequency line increased by x1.4 but the high frequency line did not change.
- The frequency noise low frequency line did not change, but the high frequency line increased by x2.2. This is good because it means we did not make the frequency noise worse in the bucket.
- The 9 MHz line at 72.3 Hz increased by x2.4. Hopefully we can tune this coupling back down.
- The noise at the 48 Hz bump and maybe in the 20-35 Hz range is a bit worse, but the overall noise is quite comparable to the fully tuned 30W state.
We think that with some TCS tuning (we did none so far!) and squeezer re-optimization we can surpass the 30W performance.
Note on ISS 2nd loop DC coupling:
The first time we tried going through LASER_NOISE_SUPPRESSION at 36W we had an aggressive excitation on the ISS 2nd loop. This meant when the second loop was DC coupled the output of the AC coupling servo was held at a large value and pushed the ISS diffracted power to zero, causing almost-instant lockloss. Lesson learnt: do not try to DC couple the ISS with a big excitation.
The second time time we went to ISS_DC_COUPLED we lowered some gains to compensate for the extra power:
- PSL-ISS_SECONDLOOP_REFERENCE_IN_MTRX_1_4 was 125 (from 150 @30W)
- lscparams.ISS_FinalGain was 5 dB (from 7dB @30W).
DC coupling was seamlessly engaged. For future commissioners: I have returned the parameters to their 30W values in guardian, and the has been loaded.
DARM plant flips back to antispring detuning with 36 W and SR3 heater on at 5W. This could be why our range did not improve as much as hoped. See 47604
After some spot moves we managed to max out the arm powers with 181kW and 167kW in Y and X respectively, up ~2kW.
| Mirror | P2L start | P2L end | Y2L start | Y2L end |
|---|---|---|---|---|
| ITMX | -3.98 | -3.6 | 0.26 | -0.02 |
| ITMY | -3 | -2.6 | -0.2 | 0 |
| ETMX | 4 | 4.98 | 4.0 | 5.1 |
| ETMY | 4.5 | 4.7 | 2.5 | 2.18 |
The largest change was in ETMX yaw position, which moved slightly further away from the point absorber there. This is where we gained the most. The spots moved down slightly on the ITMY point absorber. I thought I had some nice HWS images of the ETMX point absorber reducing, however it was being ruined by a dead pixel.
I also tried a brief SR3 heater test. For the whole 36W test the heater was at 5W. I dropped it to zero for 20 minutes but the RF18 dropped very quick, from 47 to 44. This was preceded an ADS instability at 0.012Hz, it was stopped by just switching off all the ADS channels, however switching them back on results in the the instability ringing back up. Given that I didn't tune any of the TCS for 36W yet, it's no surprise that 9MHz is affected by SR3 due to too much cross coupling from PRCL to SRCL.
Hopefully these new spot positions work at the usual 30W settings tomorrow. I have left ADS off, some residual of the 0.012Hz oscillation is still visible, we'll see if it survives the night...
Since the beam spot 'positions' in Dan's alog are quoted in A2L gain numbers, here I tabulate what that means for spot position in mm. Note that I am quoting today's current values for the A2L gains, which differ very slightly from those quoted above.
Sign convention for spot position in mm: up (+Vert on SUS screens) is positive for pitch and farther to the left (+Trans on SUS screens) is positive for yaw.
| Pit P2L gain | Pit [mm] | Yaw Y2L gain | Yaw [mm] | |
| ITMX | -3.76 | 20.8 | -0.17 | -0.6 |
| ITMY | -2.6 | 15.6 | 0.0 | 0 |
| ETMX | 4.85 | -18.3 | 4.95 | 18.2 |
| ETMY | 4.55 | -16.9 | 2.03 | 7.5 |