In a previous report I showed that the beam can wander on the IMC WFS by something like 0.1 in QPD normalized units. To understand how much this can affect the optimality of the IMC alignment, I moved the beam on WFS using the picomotors, and tracked the change of jitter to RIN coupling.
I injected a 11 Hz pitch line on the PZT (amplitude 0.5) and a 17 Hz yaw line on the PZT (amplitude 2) and looked at the amplitude of induced RIN, both in the second loop signals and in IM4_TRANS. All signals gave consistent result, so here I'm quoting only one value.
Here are the raw results. The first four columns give the deviation of the beam position with respect to the initial one (which was not centered)
| WFS A pitch | WFS A yaw | WFS B pitch | WFS B yaw | 11 Hz (pitch) RIN x1e-6 [1/rHz] | 17 Hz (yaw) RIN x1e-6 [1/rHz] |
|---|---|---|---|---|---|
| 0 | 0 | 0 | 0 | 8 | 8 |
| +0.1 | 0 | 0 | 0 | 6 | 13 |
| 0 | -0.1 | 0 | 0 | 10 | 8 |
| 0 | 0 | +0.1 | 0 | 5 | 24 |
| 0 | 0 | 0 | +0.1 | 8 | 8 |
The effect is not so small, in particular a pitch miscentering of the beam can increase by as much as a factor 3 the coupling of jitter to RIN.
So it seems that the normal wandering of the beam on the long period can give some IMC misalignment that will cause a significant worsening of intensity noise. One solution would be to implement the beam size modification already done at Livingston.