Today's ISC shift focused on completing the reallocation of the low frequency components of the ALS Green Laser on the West Arm cavity mirrors, namely the WI, both mirror and marionette.
This involved the following tasks:
- Improving the alignment of the green laser in the WARM cavity by tuning the BPC set point;
- Exporting the following new channels to the DAQ to monitor the locking process (ALS_NArm_TRIG, ALS_NArm_REL, ALS_NArm_ERR, ALS_NArm, ALS_NArm_OUT, ALS_NArm_CORR, ALS_WArm_TRIG, ALS_WArm_REL, ALS_WArm_ERR, ALS_WArm, ALS_WArm_OUT, ALS_WArm_CORR);
- Obtaining a rough estimate of the relative gain between the ALS VCO actuator and the mechanical actuation, by using a simple derivator (see figure 1) to filter the VCO correction (which is the input of the reallocation loop). Using a gain of -8 allowed to keep the lock for ~5 minutes and to notice a reduction in the amplitude of the green laser correction signal (ALS_WEB_Corr) when the mechanical correction is enabled (see Figure 2). A higher gain generated a ~4Hz oscillation yet to be completely understood;
- Perform a stable reallocation using a simple filter, which uses an integrator, a zero at 150 mHz and a double pole at 20 Hz; the test was successfull, and the actuation reallocation was also soon splitted between mirror and marionette, using the usual strategy; we had a long lock stretch (25 minutes, starting from 18:23:00 UTC, see Figure 3);
- Inject noise in the new loop to have a first estimate of the OLTF of the mechanical part of the loop locked with the simple filter (10 minutes, starting from 19:09:00 UTC, Figure 4). For (self)-reference: we used the already present MICHband noise filter with amplitude 2e-2; quick tests showed that at 4e-2 the actuation on the marionette was close to saturation. In order to have a better estimate of the low frequency region of the OLTF a different noise filter should be used.
Wthe interferometer was left with both arms locked to the IR laser using the automation.
