Since last Monday night we have an increase of noise coming from the INJ side.

Since then it is very difficult, almost impossible to lock. The CITF is lockable but it is much more difficult to engage its control, and we rarely manage to go further than DARM to IR step.

The noise is present, apart from the INJ channels, on LSC_CARM, LSC_PRCL, LSC_MICH and on the Z corrections of the input mirrors.

In order to understand if the coupling mechanism was passing trough the beam injected from the terminal benches (doubling of the main beam) and then on the CITF trough the Input mirror correction, we have changed the gain of the ALS fast loop.

Figure attached:

- purple: before the EIB failure

- red: today with standard gains for fast ALS

- green: NEB increased from 0.4 to 0.6

- blue: NEB @ 0.6 and WEB from 0.75 to 1.1

No improvement are visible, thus this noise should be coupled in the central area (this includes the parts of the green signals generation that happens at the EIB and Atrium).

NB it seems that the red curve is less noisy on LSC channels, but in that period also the INJ_IMC_TRA is less noisy