Virgo Logbook

AdV-ISC (Commissioning up to first full interferometer lock)

bersanetti, ruggi - 3:39 Friday 22 June 2018 (41936)

New MICH filter, improved DIFFp_TY filter

Today we worked on the general performance of the ITF:

we tested a new MICH filter for science mode, with a lower correction w.r.t. the old one and a general improvement w.r.t. the current one (see Figure 1); this is not automated yet, as we had a few issues when engaging it, that I guess are related to the order of the filter in Acl: we managed to use it only by getting rid of a few notches to save a few orders (this is not related to the line tracker analysis as, unfortunately, the BS DRUM/BUTTERFLY are kicked earlier in the lock acquisition, and also when using the notches);
the final gain for MICH was adjusted to cope with the different sensing coming from B4_PD2 w.r.t. PD1;
a new set of longitudinal noise injections has been performed with the new MICH filter: looking at Figure 2, the coherence between MICH and DARM has increased around 10 Hz (Figure 3), we will tune the Alpha subtraction once the filter is stably implemented, as some changes may still happen. In Figure 4 there is the current longitudinal noise budget; the injections are in /virgoData/NoiseInjections/LSC/LSC_Injection-1213640748.txt;
the DIFFp_TY filter has been tweaked a bit (see Figure 5);
the F7 dampers for the 0.4 Hz mode have been improved, so its effects are less impacting on the alignment loops of the cavities; this effect could be even more reduced by locking again CARM at low frequency using the RFC error signal;
both the BS loops and the DIFFp_TY loop have been calibrated, hence the change in the values of the loop gains in the ITF_LOCK.ini file.

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Comments to this report:

ruggi - 11:32 Friday 22 June 2018 (41938)

Some additional plots to better see the effect of a reduced MICH correction on DARM:

Fig 1: rms at 38-44 Hz of MICH_CORR superposed to the rescaled DARM_ERR. With the new filter the correction is lower and consequently the noise coupled to DARM is reduced. The comparison of the jumps says that with the new filter MICH_CORR is not limiting DARM anymore.

Fig 2: the same computation performed at 91-95 Hz. Even in this case MICH_CORR is reduced, but there is no change in DARM. We can conclude that MICH_CORR was not limiting DARM also with the old filter.

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ruggi - 12:25 Friday 22 June 2018 (41939)

Something more about the issues when engaging the new filter:

Fig 1: the input and the output of the filter are not coherent at low frequency when the new filter is running;

Fig 2, Fig 3: the problem seems to be due to the input, rather than the output;

Fig 4, Fig 5: during the adjustment of gain and phase the low frequency behavior of the error signal was changing in a pathological way.

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