Abstract:

I tried to lock PRFPMI with 2.8 W but failed.
When PRC flashes, ALS DARM/CARM loop lost lock.
I performed several investigation but not yet confirmed the reason.

Detail:

Today, I tried to lock PRFPMI with 2.8 W, which is the same power when trying it before O4a (klog24995).
However, lock loss happened many times between ENGAGE_PRMI_3F and PRFPMI_IR_HANDOVER like fig1.
So, I investigated why the lock loss happened.

Signals of PRMI

Though we used 3F signals to lock PRMI, arm cavity condition can affect the PRMI lock.
Figure 2 shows the several signals when PRMI was locked with 3F and arms were flashing.
As you can see, MICH and PRCL signals had gliches.
However, glitches in the feedback signals are still within actuator range and POP 90 signals were high (some drops can be sen though).
Also, These glitches can be seen even when using lower input power, so it is unlikely that these glitches caused frequent lock losses.

Signals of suspensions

Figure 3 shows the OpLev signals of XARM suspensions.
We can see large oscillation of ETMX just after aligning PRM in this lock trial.
However, it should not be due to the sensing noise like scattered light because GRX transmittion power also drops according with the ETMX motions.
So, this signals should be real suspension motions.

Signals of ALS DARM

Though ALS signals are independent from IR, strange coupling was sometimes happened such as klog11180.
Figure 4 shows the error and feedback signals of ALS DARM control.
After t=-10s (it is the timing when PRM is aligned), there are a lot of glitches.
Also, the glitch size is large and sometimes saturating at DAC outputs.
I'm not so sure why the glitches happened when PRC are resonating but it is very likely that the saturation of DARM control causes the lock loss.