After the tuning of the injection system (#47259), we started working on the ITF.
Before reaching LN1, a check of the SSFS demodulation phase was performed, but it was off only by ~0.05.
In order to compare the SSFS OLTF before and after the INJ intervention, a noise injection on SSFS was performed at PRITF MICH offset 0.5, and the two measurements are compared in figure 1. No change in the delay was observed, but only a small change of gain.
Then we locked to LN1 and we scanned the OMC1 in order to look, as an additional figure of merit for the TCS working point tuning, the sidebands amplitude balancing, the amplitude of the CAR order2 mode and the relative amplitude of the order2 mode of the sidebands.
The first step was performed with the initial tuning set by Ilaria, which was
WI --> 162 mW INNER, 172 mW OUTER
NI --> 353 mW INNER, 2.36 W OUTER
The first scan (figure 2, GPS = 1255288220) shows that in this configuration the sidebands are well balanced, and there are 0.61mW in the CAR TEM02 mode.
Then Alessio peformed another step on the WI INNER DAS, bringing it to 220mW. The second scan (figure 3, GPS = 1255291518) shows that there is an unbalance of the sidebands of about 30%, that's why we decided to go back to the previous point. However, in this configuration the CAR TEM02 is reduced to 0.43mW.
We finally went back to the initial configuration for the DAS, and from the scan (figure 4, GPS = 1255295868) we got an unbalance of 13% and 0.51mW in CAR TEM02. Notice that the thermalization could be not yet completely finished when the scan was performed.
It is possible to get some information also from the relative amplitude between the SB TEM02 and TEM00. For instance, the amplitude of LSB TEM02 with respect to LSB TEM00 in the three scans is 17%, 20% and 16% in the three scans, respectively. However, this latter measurement is not really accurate, as the TEM02 peak is in the shoulder of the USB TEM00 peak, and needs a deconvolution to get a precise information, although we can still look at the trend between one scan and the next.
After few unlocks in LOCKING_OMC1 happened at the beginning of the scan, the fast retuning of the working point was performed, and the ITF reached LN3. However, since the lock was not really robust and the relock failed many attempts, ITF was left in LN1 autorelock failsafe.