Reports 1-1 of 1 Clear search Modify search
AdV-PAY (NE and WE Payloads)
sorrentino - 22:54 Sunday 15 June 2025 (67001) Print this report
Mechanical losses of new WE mirror

The mechanical quality factor of the drum mode around 7.8 kHz for the new WE mirror in much larger than with the mirror used until March. In the attached plot the resonance at higher frequency is due to the WE mirror. The quality factor is similar to the one measured when the same mirror was mounted as NE TM. This excludes that the mechanical losses in the old NE mirror were due to surface contamination. 

The thermal noise due to the mechanical losses in WE mirror can possibly explain why the sensitivity at 100 Hz is slightly worse after WE mirror replacement, though the optical gain has improved. 

Images attached to this report
Comments to this report:
mwas - 8:27 Monday 16 June 2025 (67004) Print this report

I expect that what is meant is that the quality factor of the WE mirror used in June is much lower than the one of the WE mirror used in March, and that consequently the drum peak is wider. It would be good to quantify that statement, what was the estimated quality factor  mean value and error bars in March and what it is in June, and also what it was in April/May with the new mirror that was temporarily installed and then removed becauce of a large point absorber on it.

Figure 1 compares one of the best sensitivites in March, to one of the best in June. There is no significant changes between the two in the mid-frequency band. These spectra are calculated using the median-mean average spectrum method, which should reduce how much the spectrum is affected by step glitches, that have a 1/f^{2/3} spectral shape very similar to the stationary noise floor. However, with the high rate of glitches (several per minutes) even the median-mean spectrum may be affected by glitches. The standard BNS range calculation (based on the Welch method) is more affected by glitches.

As for changes in noise level with optical gain, these do not necessarily translate into noise improvements. The study of the mystery 1/f^{2/3} (VIR-0967A-24) states that the noise level doesn't depend on the input power, which also mean it doesn't depend on the power stored in the arms.

Figure 1. The power in the arms has increased by 10%, the power on B1 has been kept constant, which means the dark fringe offset has been reduced and the optical gain should have increased by 5% solely due to the increase in the power stored in the arms. The optical gain has not been very stable in June, but its increase is about 5%. At least comparing the average optical gain for the times of the two curves on figure 1 the increase in optical gain is exactly 5%. So the contribution of the 1/f^{2/3} noise to h(t) is expected to be unaffected by this increase in optical gain.

Images attached to this comment
direnzo - 16:41 Tuesday 17 June 2025 (67025) Print this report

I performed a quantitative estimate of the quality factor variation using Lorentzian fitting. The model used is a sum of a power-law baseline and four Lorentzians, each convolved with an f^−2 response to account for interferometer response. The model is as follows around the frequencies of the drum modes:

Figure 5 (LaTeX equation)

where f_i is the frequency of the drum mode 3 for each test mass, and Q_i their quality factors. 

Figure 1 shows the PSDs of the V1:SDB2_B1_PD2_Audio channel around the frequency of drum mode 3 of the test masses, across three different periods, late March (with the original WE mirror), mid-May (with the replacement WE mirror), and early June (with the old NE mirror reinstalled at WE). Besides a slight drift in frequency, it's evident that the current WE mirror exhibits a lower quality factor, as discussed by Fiodor and Michal in the previous entries.

Fit Results Summary:

Baseline: A = 11.

Mass 1: A_1 = 19e3, Q_1 = 32e5, f_1 = 7798.708,

Mass 2: A_2 = 52e3, Q_2 = 93e5, f_2 = 7805.979,  

Mass 3: A_3 = 48e3, Q_3 = 16e5, f_3 = 7806.684,

Mass 4: A_4 = 75e3, Q_4 = 65e4, f_4 = 7814.724,

Note: A frequency drift is observed for some masses, likely due to temperature stabilization effects, e.g. in Fig. 2. This may have caused an underestimate of the corresponding Q_i. I attempted to reduce the data interval to minimize this drift, but the reduced resolution of the spectral estimate resulted in nearly identical fit outcomes accounting for uncertainty.

A similar fit repeated for mass 4 in the other period resulted in:

March 30: A_4 = 32e3, Q_4= 71e5, f_4 = 7814.531,

May 15: A_4 = 30e3, Q_4 = 715e4, f_4 = 7817.553.

This means that the quality factor for mass 4 (WE) is about one order of magnitude lower than before.

 

Images attached to this comment
mwas - 17:41 Tuesday 17 June 2025 (67026) Print this report

This is a nice analysis, it would mean that the quality factor of EM01 has increased compared to the last measurement VIR-0407A-23. Back in 2023 the quality factor was measured to be ~100e3, and now it has increased to ~650e3, which is comparable to the measurement of that same mirror in 2020.

Note that most of the other measurements were done using the ring down of a kick of the mirror. I don't know if the result of the fit of the steady state width of the line can bias the result to be an under or an over estimate.

 

 

Search Help
×

Warning

Error

The present report has been modified outside this window. Please check for its integrity in the main page.

Refreshing this page will move this report into drafts.

×