Virgo Logbook

AdV-COM (AdV commissioning (1st part) )

mantovani - 11:34 Thursday 18 May 2017 (37647)

Range vs PMC PZT

Following the discussion during the yesterday daily meeting I've tried to evaluate if the MICH noise could depend on a particular status of the Injection. Thus looking briefly to the injection signals it seemed that the low PMC PZT values corresponds more or less to a good horizon, see Figure 1 and 2 where the locks of the 16th have been analyzed.

In order to better understand if a correlation is present, to have more statistics in a quite period where no actions on the machine were applied, the C81/2 data have been analyzed collecting only the data where the final status has been reached. For these data the amplitude of the lines 30.9Hz, 62.5Hz and 101.3Hz which are related to frequency noise has been evaluated, see entry 37601, as a function of the PMC PZT as it is shown in Figure 3 and 4 and no clear colrrelation is visible.

To be better understood

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Comments to this report:

mantovani - 12:16 Thursday 18 May 2017 (37648)

This analysis is valid in the assumption that the behavior of the MICH lines is coherent with the RANGE BNS which is indeed the case, see Figure 1.

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Cleva - 13:39 Thursday 18 May 2017 (37655)

for information:
- the PMC pzt voltage is mainly triggered by the air pressure ( see attahced doc):
5 hPa leads to 411 MHz shift of the PMC cavity, which is compensated by the PMC_Pzt. The Pzt senstivity is 80 MHz/V (the associated signal monitored by datadisplay is V_Pzt/20, that is 1600 MHz/V )

- the PMC_pzt voltage might also trigger a tilt of the PMC optical axis since a longitudinal displacement of the piezo induces a small tilt of the mirror attached to it. I don't know the amount so far for the V+ PMC. The piezo actuator prototype developped for the Adv PMC which is based on an ~ equivalent design (the size is reduced by 40 %) gives ~ 2 urad/V tilt of the mirror per Volt on the piezo.

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genin - 16:03 Thursday 18 May 2017 (37657)

I gave a look to the Virgo+ data. I took 2 sets of data of similar length.

For Virgo+ the PMC cavity was operated under ultra high vacuum (10-7 mbar res. pressure).

In Virgo+ case, the PMC PZT correction drift is much smaller (see second plot Bs_PMC_Pzt) 6mV over 14 hours.

In comparison for AdV (C8 data), the correction on the Pzt fluctuates by about 0.3V over 14 hours.

Should we consider putting back the PMC under vacuum?

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cleva - 17:34 Thursday 18 May 2017 (37658)

the freq of the PMC changes due to air susceptibility to T and P according to:
d_nu/nu = dL/L = 2,74 e-4 *dP/P and -2,74e-4 dT/T (L is the length of the PMC, nu the frequency, P the pressure, T the temperature)

- putting the PMC under vacuum will reduce air effect on the frequency to zero
- closing the PMC tank valve will reduce only dP/P, it remains d_nu = 301 MHz/°C (3,76 V on the piezo)
- keeping as it is (valve open): d_nu = +82 MHz/ hPa -301 MHz/°C

The periodic temperature variations (~30 mn) in the tank are likely to be filtered by the tank and should be less than 0,1 °C (see logentry 37655)

swinkels - 0:42 Thursday 25 May 2017 (37717)

I don't find it likely that the PZT correction of the PMC should have such a large effect on the horizon. As shown in the fig 1, both the RFC (step of 0.3 V on the thermal correction of the master laser and a step of 0.06 degrees of the OMC1 temperature) and the PMC (jumps of about 0.6 Volt on the PZT correction) seem to skip to between different fringes, without any obvious correlation with the horizon.

I did a brute force search for correlation between the BNS range and almost all possible channels in trend.ffl, while considering only those samples for which the range was above zero. Among the winning channels are the demodulation phase mistuning of B2 (LSC_B2_8MHz/56MHz_DPHI) and some signals related to the MC Z position, see fig 2 and the attached list. To me, this hints at a dependency on the mistuning of the length of the IMC with respect to the modulation frequency (FMOD_ERR). Since the modulation frequency is still fixed, the length of the IMC should remain constant too. Looking at the LVDT of the top stage, there are jumps of about 100 um after unlocks, superimposed with a slow drift that is probably caused by the tides, which is not a real change of IMC length. In Fig 3, I tried to remove the slow drift and convert the length change in a frequency mistuning of the 56MHz sidebands, which gets as large as 50 Hz. Note that in the early days of initial Virgo, a mis-tuning of the modulation frequency of 4 Hz was enough to spoil the sensitivity!

Fig 4 shows the horizon vs this mistuning (note that I do not know where the real zero is, and the subtraction of the drift might not be perfect).

It should be very interesting to tune the demodulation frequency during a lock (assuming that the bug that prevented us earlier is resolved).

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correlate1179679991.txt