In this afternoon we spent a shift working on the TCS tuning with CITF.

We haven't decided on the working point for the central heating yet as the interpretation of the CITF data is non-trivial.

At this point I might suggest, as a different strategy, relying on the Hartman cameras and providing the dead-reckoned thermal lens values.

[The tests]

We tried two particular CO2 settings as follows.

• Configuration 1:
  • This is to provide a 20 uD single-path lens on each arm to force the power recycling cavity stable.
  • [CO2 NI, CO2 WI] = [140 mW, 140 mW] . These numbers were derived theoretically.
  • rotation stage settings [theta NI,  theta WI] = [35 deg, -22 deg]
  • Initially locked on the carrier, then switched to the sideband in the middle of the measurement as the carrier lock wasn't able to maintain a stable lock.
• Configuration 2:
  • This to set the CO2s at the point where we had a highest build up in the past (35867).
  • [CO2 NI, CO2 WI] = [20 mW, 20 mW]
  • rotation stage [theta NI, theta WI] = [-3 deg, -40 deg]
  • Locked on the carrier throughout the measurement.

[Results]

• Conf 1:
  • Significant degradation in the build up. This was true for both carrier- and sideband-locked CITF. Consistent with the past measurement (35867).
  • B4 image showed Laguerre-type modes both for carrier- and sideband-locked CITF. This is also consistent with the past measurement.
• Conf 2:
  • No significant change in the DC signals.

In addition to these measurement, the highest build up of 15 mW at B4_DC was observed before these measurements when neither of the CO2s was activated. The B2_DC signal also dropped to 4 mW (from 13.8 mW) which is the lowest value I have seen. This is inconsistent with all the past measurements we did with CITF before. This could mean that there are uncontrolled variables somewhere affecting the build up of CITF. Note that since the arm cavities weren't available today, we set the NI alignment using the B2 and B4 cameras/DC signals as referecnes.

[Current interferometer state]

Both NI and WI central heating are switched off. The CITF locking loops are off. Also the below are the demod phases that we found (kind of) optimum for this afternoon.

• B1p_119MHz_phi0 = -0.72 rad
• B2_6MHz_phi0 = -0.67 rad