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AdV-TCS (CO2 laser projector)

nardecchia - 13:56 Tuesday 05 August 2025 (67436)

Attempt to stabilize WI CH Power using PyTCS Process

In order to stabilize the WI CH power, we attempted to reactivate the PyTCS-based process developed by Bas (see entry 52930 / vpm)

Just as a reminder: the probe and setpoint used by the loop correspond to the power measured by the signal TCS_WI_CO2_POWER_CH_PSTAB.
Note: decimals must be written with a dot, not a comma!

When the CH flip mirror closes (i.e., at CARM Offset Reduction 2 of 3), the loop goes automatically into pause.
When the flip mirror reopens, the loop automatically resumes.

To check if the servo is active or not, you have to look at the line in the VPM section (see in figure 1).

As for the gain, I kept the default value originally set by Bas.
After some trials, I enabled the WI CH servo at 09:26 UTC with a setpoint of 2.67 W (on the PSTAB power meter) in order to maintain a constant power of 0.64 W on the PICKOFF (DAS+CH). The servo’s behavior during the first two hours is shown in Fig. 2. It does not appear to have performed as expected. To be investigated.

To be monitored and evaluated whether it should be turned off or not.

Images attached to this report

67436_1754394834_Screenshot 2025-08-05 alle 13.45.52.png

Comments to this report:

nardecchia - 14:20 Tuesday 05 August 2025 (67439)

As agreed during the daily meeting, I disabled the servoloop at 12.17 UTC.

mwas - 9:11 Wednesday 06 August 2025 (67446)

Looking at the figure the error signal used for the PyTCS loop, TCS_WI_CO2_POWER_CH_PSTAB, is unaffected by the waveplate. On the other hand the total power of the DAS+CH correction, TCS_WI_CO2_POWER_CH_PICKOFF, sees the waveplate rotation. So either the actuator is wrong and it is the DAS power that was changed, which seems to me very unlikely as the interferometer in steady state looks unchanged. Or it is the waveplate that is located after the TCS_WI_CO2_POWER_CH_PSTAB power meter pickoff, which means that the error signal for PyTCS need to be changed to look at a signal after the actuator, for example change the error signal to TCS_WI_CO2_POWER_CH_PICKOFF.