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AdV-DET (Commissioning)

gouaty, menzione, ruggi - 17:35 Saturday 12 May 2018 (41361)

Low frequency line on suspended benches for evaluation of scattered light

Yesterday afternoon, we injected a low frequency line (0.1 Hz) through the suspension top stage of the benches SDB1, SDB2, SNEB and SWEB in order to displace the benches along their longitudinal axis. When the injections were performed a noise shoulder of scattered light was visible in the DARM spectrum as shown in the attached figures. This makes it possible to evaluate the coupling factor of scattered light using the noise projection for each bench (analysis to be done).

The list and times of the injections are reported below.

Bench	Amplitude (bench Z motion)	Start (utc)	Duration (s)	Comments
SDB1	+/-20 um	15:07:30	410	SDB2 LC Z control activated, FIG.1
SDB1	+/-20 um	15:14:30	250	SDB2 LC Z control disactivated
SDB1	+/-25 um	15:22:10	235	SDB2 LC Z control disactivated
SDB1	+/-25 um	15:26:15	225	SDB2 LC Z control activated
SDB2	+/-17 um	19:08:00	185	SDB2 LC Z control activated
SDB2	+/-23 um	19:11:50	440	SDB2 LC Z control activated
SDB2	+/-32 um	19:19:20	200	SDB2 LC Z control disactivated
SDB2	+/-25 um	19:23:00	315	SDB2 LC Z control disactivated, FIG.2
SNEB	+/-50 um	20:18:20	128
SNEB	+/-40 um	20:20:35	260	FIG.3
SWEB	+/-40 um	20:31:05	185	FIG.4
SWEB	+/-35 um	20:34:30	125

Data betweeen 14:50 and 15:00 utc can be used as reference quiet time for the analysis of the SDB1 injections.

Data between 18:50:10 and 19:02:35 utc can be used as reference quiet time for the analysis of the injections on the other benches.

Images attached to this report

Comments to this report:

mwas - 16:27 Monday 14 May 2018 (41372)

Evaluating the scattering coupling based on these measurements

SDB1: coupling seems to have increased from 2e-5 to 6e-5 compared to Feb 2 2018, which puts it at the same level as just after O2.
But looking at the projection MICH noise is not far below, and has a more resembling shape (figure 1). The MICH/DARM coherence is over 80%, so it is likely that the scattered light coupling is coming from the MICH loop, is there a DC normalization used for the B4 56MHz Q signal? This could be a way of introducing scattered light coupling into the MICH error signal.

SDB2 (figure 3): coupling went down from 1.1e-5 to 4e-6, no sign of MICH involvement, so the MICH coupling to SDB1 motion is not due to some PD signal on SDB2 but some more directly coupling between SDB1 and SPRB?

SNEB (figure 4): coupling went down from 6e-5 to 1e-5, but the old value is from Jul 21 2017 with larger dark fringe offset, so not really comparable. Added back B7 as an alternative way of projecting SNEB scattering.

SWEB (figure 5): coupling went up from 0.6e-6 to 1.1e-5, the old value doesn't seem realistic, and in any case, the projection using the B7_DC power (based on line injected by stepper motor from several months ago) which is a more direct measure of scattering has not changed. Interestingly SNEB and SWEB have a very similar coupling, and LSC_B7/B8_DC have also the same coupling 0.0015.

Note, all couplings are measured as a fraction of B1 power effectively coupled by the scattering object

Images attached to this comment

bersanetti - 17:15 Monday 14 May 2018 (41373)

The error signal for MICH is normalized with a low-passed version of the SPRB_B4_112MHz_mag signal.