Virgo Logbook

In the past days the dark fringe has been reached several times permitting to search for the mirror modes. Since the OMC is not locked the only channel available for this search is Pr_B1p_ACp, acquired at 10kHz; hence only the first (“butterfly") mode of the main mirrors is correctly detected. Despite what I stated in the entry New input mirrors resonant modes, the frequencies of the new mirrors are expected to be slightly higher with respect to the old ones because the thickness of the new mirror is slightly larger: (NI: 96.8 -> 100.4; WI: 96.8 -> 99.25; NE: 95.6 -> 99.95; WE: 95.7 -> 99.90. units mm, source LMA datasheets). The expected resonance frequencies, roughly computed with the F. Bondu code Cypres (approximate solution that neglects the mirror flats and lateral ears, but should predict the real frequency within few Hz of range) are NI:4015Hz, WI:3985, NE:3991, WE:4002.
In the first figure the Pr_B1p_ACp spectral amplitude is shown in the frequency interval of interest and 5 resonances appear. At the present status it is impossible to say if all these are corresponding to the mirrors (why 5?) and who is who. The figures 2-5 show, for 6 different locks, the behavior of the power spectrum peak for these 5 frequencies; 4 of these peaks (4026, 4015, 3980, 3969) relax after the lock; if this is due to the mechanical excitation of the mirror mode caused by the lock procedure, it is possible to evaluate the quality factor. A rough evaluation is shown in the figures and it ranges from 5E6 to 14E6, very good numbers, if compared to the old ones (about or less than 1E6). If all these considerations are right, this could be a good sign for the expected thermal noise of the new mirrors. More news when the Pr_B1_ACp channel will be available to investigate the drum modes.