Vaishali Adya, S Dwyer K Izumi

We are able to lock the interferometer at low noise but only briefly.

• The AS36 sensing for SRM is not tuned for our new alignment, so for now we are manually skipping the gaurdian states SRM_ASC_HIGH_POWER and CLOSE_BEAM_DIVERTERS (because the SRM dither uses in air signals.)
• We had some difficulty with violin modes today, and Vaishali and I spent a couple of hours damping them, they seem OK now.
• We think that we have fixed the problem at CARM 5PM by undoing a filter change from earlier in the week. 
• We managed to get to low noise with 26W input power and a recycling gain of 24 (according to POP/IM4 (LSC-PR_GAIN), but lost the lock to the csoft instability within about 15 minutes. 
• The next locking attempt, we stopped at 20 Watts to adjust the soft yaw offsets.   Using an offset of 0.12 in DOSFT Y and 0.13 in CSOFT we were able to get a recycling gain of nearly 31 with 20W input power, Now we are at 30W with offsets of 0.15 in DSOFT Y and 0.17 in CSOFT Y, and a recycling gain of nearly 29. We have been here for 20 minutes without seeing the CSOFT instability. We lost lock to the soft instability after a half hour.  
  • It seems like we are loosing lock within a few minutes of reducing the 9 MHz modulation depth in a way that looks like the soft instability. (I am not sure if it really is the soft instabilty or just an ASC loop instability caused by the modulation depth reduction.
• It still looks like the jitter coupling is better than before the vent, and the frequency noise is high. The spectrum at low frequencies was nearly back on the reference without running a2l
• In NOISE_TUNINGS, we removed the matrix change that put PRCL onto a blend of POP9 and REFL45, because this was causing locklosses (see alog 31305 for the original motivation)  This was done to improve the PRCL noise but we don't think PRCL nosie is limiting DARM.

Tasks we have not completed:

• Stop at DC readout, and try new soft offsets offsets.  If they are OK at DC readout and we can power up maintaining the recycling gain, we could make these offsets permanent by moving them to the TMS QPD offsets.
• Check if the 9MHz modulation depth reduction is what is really causing the IFO to go unstable, rather than the spot position to circulating power (CSOFT) instability. 
• To stop the soft instability if that really is the problem we could try
  • turning oplev damping back on
  • Engaging the ISS 3rd loop
  • get more gain out of the CSFOT loop 
  • Attempt to return to our old alignment (I don't think this will work, but setting soft offsets and pop offsets back would be how I would try.)
• Measuring CARM loop (we are suspicious because frequency noise is high, there is a large glitch when the IMC boost comes on in NOISE_TUNNINGS, which did not happen before, and the IMC PD was swapped).
• A2L
• Calibration measurements (instructions left by Jeff K on post-it at the old H2 operator station)
• Noise budget measurements
• Find sensing matrix that works for AS36 to SRM control