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AdV-TCS (CO2 laser projector)
casanueva, lumaca, magazzu - 23:46 Tuesday 31 January 2023 (58656) Print this report
DAS alignment tuning

The goal of the shift was to investigate the ITF behavior by moving the CO2 actuators on the ITMs.

As suggested by the commissioning coordinator, after the work performed on the ITMs/YAG alignment of the past days (58580, 58595, 58611), we started by investigating the vertical direction, moving separately the NI DAS first and the WI DAS after. Here are the details:

  • 15:27 UTC: CARM NULL 1F

  • 17.07 UTC: STEP (1) NI CO2 actuators moved by 5 mm along vertical direction (TX) upward (backward)

  • 19.23 UTC: STEP (2) WI CO2 actuators moved by 5 mm along vertical direction (TX) upward (backward)

We observed that (Fig.1):

  • B7 and B8 started to separate in between STEP (1) and (2)

  • 12 MHz have bumps corresponding to the STEPs done and then stabilized at a level slightly improved wrt the previous one; similar behavior for 112 MHz

  • B1p raised a bit up, but the drift was already ongoing before doing STEP (1)

  • A small improvement of sidebands/carrier overlap is visible on PosX, starting form STEP (1): 56MHzLSB raised up, together with CAR.

The plot in Fig. 2 shows the evolution of the main figures of merit from the controls point of view. Starting around 18.00 we can see a differential variation of the arms power that doesn't seem related to the TSC activity. Also during the whole lock the SRCL SET and the DCP seem to have a decreasing trend not correlated to the TCS activity. Regarding the sidebands and B1p DC it seems to be a small increase of the power at the same time as the movement of the TCS actuators. It is not clear the effect on the CMRF. 

Overall, the impact of these actions is not very big.

We decided to leave the CO2 actuators in this last alignment position, to further investigate its effect on ITF signals.

At the end of the shift, at 21.46 UTC the CH flip mirrors were opened one per time to raise up the powers, to improve the lock acquisition (issues highlighted here 58631). The following table summarize the changes (bold):

 

 CH [W]

INNER DAS [W]

OUTER DAS [W]

W

0.082 → 0.089

0.141

1.92

N

0.126 → 0.133

0.286 + 0.08 (cold lens)

2.277+1.8 (cold lens)

During the N CH power change, the ITF unlocked. We tried some unlucky attempts to relock and then Julia and Magazzu put the ITF in autorelock up to CARM NULL.

Images attached to this report
Comments to this report:
flaminio - 8:03 Wednesday 01 February 2023 (58657) Print this report

According to Fiodor one of the goals of this work was to see if it is possible to change the interferometer contrast by changing the DAS position.

The Figure attached to this plot shows that indeed B1p_DC changed by 10% during the shift. For comparison, this is the same amount of change that was obtained this weekend by changing the SR CHRoCC.

The problem is that this change in the dark fringe power does not seem to relate to the changes in the DAS.

There are at least three other changes in the alignment going on at the same time:

1) The alignment of the input beam wrt to the arm cavity is changing as witnessed by the B4 quadrants.

2) There is a new process that changes the relative alignment of the two arms by moving DIFFp offset and BS offset every 5 to 10 minutes

3) Not visible on the plots, but there is also the usual shift of the beam in the laser lab for which we do not have an indipendent monitor

In conclusion, it is not possible to conclude on the effect of the DAS because the alignment is changing too much for other reasons.

By the way, it seems that the new process acting on DIFFp and BS is having a non negligible effect on the erratic changes of the dark fringe and producing regular spikes on COMMp.

Images attached to this comment
flaminio - 8:31 Wednesday 01 February 2023 (58659) Print this report

This change of DAS position might have changed the position in Y of the sidebands wrt carrier in the recycling cavity (see Figure 1).

Figure 2 and 3 shows the same signals in two other locks on Jan 28th and Jan 31st.

In the first case the ITF was already hot (had been locked previously).

In the second case it was cold.

The change in the DAS position done yesterday seems to trigger some change in the Y position of the carrier wrt to sidebands.

As usual the 56 MHz LSB tends to follow the carrier while the other SB's don't.

 

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