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AdV-ISC (Sensing and control implementation)

bersanetti - 19:08 Monday 27 August 2018 (42544)

56 MHz modulation amplitude increased back to 12 dBm

Following a suggestion from Michal, whose noise budget states that DARM is affected by the B4_56MHz sensing noise (either from MICH or SSFS), an additional increase of the modulation amplitude of the 56 MHz sideband was added inside the SET_MOD_AMPL state, bringing it back to 12 dBm as it was before the increase of the input power. This change was tested this both online (already when in LOW_NOISE_3) and during lock acquisition, and it seems to be working fine. Longitudinal and angular loop gains were adjusted accordingly.

The first stretch of good data is from 18:10:00 LT until the manual unlock.

Comments to this report:

mwas - 21:08 Monday 27 August 2018 (42546)

This clearly improved DARM below 100Hz and above 200Hz (also MICH and PRCL at all frequencies). As expected there is no impact on h(t), where the MICH (beam splitter motion) and PRCL (laser frequency) signals are subtracted during the reconstruction.

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mantovani - 9:39 Tuesday 28 August 2018 (42549)

The AA offsets have been retuned since they are affected by the optical gain increase due to the increase of the 56MHz modulation index:

DiffpTX from 0.05 to 0.12, DiffpTY from -0.028 to -0.04; PR TX from -0.02 to -0.026 and PR TY from -0.001 to -0.002

the improvement is visible in the range, in the darkness of the fringe and in the 56MHz sideband (figure 1 and 2).

The improvement in the range is due to the reduction of structures above 200Hz, most probably related to diffused ligth in the SDB1 bench (the lowering of B1pDC mitigates the scattered ligth) see figure 3.

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