Yesterday I did a cavity scan with IR while holding the Yarm with green.

ALS servo tuning:

• C1ALS-BEATY_FINE_PHASE

             The gain of the loop is set such that BEATY_FINE_Q_ERR x GAIN = 120k. This is a kind of "empirical low" in order to have the UGF around 1kHz. 

• C1ALS_YARM

             Start with FM5 [1000:1] enabled, determine the sign of the gain increasing it in small steps and making sure that the mirror doesn't get a kick. Then gradually raise it while looking at the BEATY_PHASE_OUT power spectrum.

             Enable FM7 [RG16.5], FM6 [RG3.2], FM3 [1:5], FM2[0:1], FM10 [40:7].

Plot 1 shows the power spectrum of BEATY_PHASE_OUT (calibrated in Hz).

1. blue curve - ALS disabled
2. green curve - in loop measurement, ALS enabled and servo tuned as described above
3. grey curve - RMS of the in loop measurement
4. red curve - out of loop measurement (arm locked with IR)
5. pink curve -  RMS of the out of loop measurement

Offset setting and cavity scan

The C1ALS_OFFSETTER2 was used to set an offset for ALS scan.

• LPF30m enabled
• Ramp time set to 150s
• Offset set to 1500 (approximately 3 FSR in this interval)

Many scans have been done to find the optimal offset conditions, I only attached one (Plot 2).

I also misaligned the END mirror in pitch to enhance the HOMs peaks, but it turned out that it was not enough, because I didn't see a very big difference between the "aligned" and the "slightly misaligned" measurements (Plot 3). 

NEXT STEPS

Increase the cavity misalignment both in pitch and in yaw and repeat the measurement.