Today's ISC shift focused on reallocating the low frequency components of the ALS Green Laser on the North Arm cavity mirrors, namely the NI, and NE, as performed for the west arm in 50463.
The gain has been initially tuned using a derivative filter, resulting in an optimal gain of 8. After the tests with the derivative filter, the same integrator used on the West arm was succesfully applied to the North arm too.
Figure 1 shows a stretch of the reallocation with the integrator, during the first lock the acquisition has been done only with with the mirror while in the second mir + mar.
Figure 2 shows the engagement of the mechanical loop with mirror actuation only, and the corresponding reduction of the amplitude of the correction sent to the ALS VCO.
Figure 3 shows the amplitude variation of the correction signal oscillations with respect to different gains applied to the mechanical actuation. A maximum gain around ~20 was found, beyond which a ~7Hz oscillation started. Lowering the gain below ~2-3 also led to an unstable behaviour.
Figure 4 Shows the engagment of the loop with both marionette and mirror actuators.
Figure 5 shows the spectra of the Errror, VCO correction and MIRROR correction signals before (purple) vs after the reallocation (blue).
A noise injection on the mechanical actuation path was also performed to estimate the low frequency OLTF, further results are still under investigation.
After the replacement of the West Arm ALS VCO, we also performed noise injections to check the resolution of the 90kHz noise issue (50391), as shown in figure 6.