Aiming to identify the turbo pump which is causing the 401 Hz peak in the hrec sensitivity, today we installed 3 accelerometers on the body of 3 different turbo pumps in CEB:
Following the installation of the three accelerometers, we have checked their coherence with Hrec with respect to the corresponding sensors already present on DT, SR and IB towers. As can be seen in Figure 1, only the accelerometers on the DT (both on the turbo pump and the tower itself) show coherence at 401 Hz.
By checking the magnetometers in CEB (see Figure 2 and Figure 3), also in this case the only coherence in from the DT magnetometer, with the only exception of ENV_CEB_MAG which is placed near to the detection lab. Please remember that all towers have their own turbo vacuum pump, each one of them rotating at a frequency near 401 Hz but with a different random drift.
From a cross-check between Hrec, accelerometer and magnetometer (Figure 4) it is evident that those channels see the same signal. Another clue comes from the comparison of the time-frequency drift of the line in the channels (Figure 5).
As a final confirmation, we can thin about slightly changing the frequency of the turbo pump on DT while in LOW_NOISE_X and checking if the line in Hrec moves accordingly.
An open point is if the 401 Hz line enters Hrec through vibrational or magnetic coupling, and through which mechanism.
From a rough calculation of the coupling noise associated with the peak at ~401Hz:
where L is the arm length, CF is the coupling function value measured during the near-field magnetic sweep injection (#63039) with the small coil on the DET platform (south side) Figure 1-2, ASD is the amplitude spectral density of the magnetometer ENV_DT_MAG_* which is installed on the DET tower base (East side). This value is compatible with the strain spectral amplitude (), Figure 3.
Around 9:30 UTC we moved the ENV_DT_TURBO_MAG magnetometer closer to the DT turbo pump body, as illustrated in the attached picture. The read axis is X (radial direction).
Previous locations are described in https://logbook.virgo-gw.eu/virgo/?r=63394
Figure 1 shows a spectrum at 401 Hz of this sensor compared to the magnetometer on the DT tower base. The sensor on the pump body measures a quite intense 401 Hz line (approx 500 nT/sqrtHz). Yet, it does not see the sideband structures which are seen by the DT magnetometer and which were sensed close to the turbo cable.
This morning a setup was installed to resume the investigation of the magnetic or seismic coupling path of the 401 Hz line visible in the sensitivity, attributed to the turbo pump of the DET/INJ tower (VIR-1098A-25). The plan is to inject a seismic and a magnetic line close to the frequency of the peak.
Near the DET turbo pump, the amplifier was left powered and the small coil connected to it (the shaker will be installed later), see Figures 1 and 2. For the injections, the DAC dedicated to acoustic injections in the Central Building will be used (DAQ BOX 30, SM 21, ch 7). The corresponding cable was left on the floor, near the rack (Figure 3).
The monitoring channel for the coil current is ENV_PROBE_CURR_MON (fs = 20 kHz). The coil was cabled to the DAQ room (ADC7674, Moni0 46, ch 10). The setup was tested by injecting a 25 Hz line, confirming that the system is working properly (Figure 4).
In order to investigate the coupling mechanism of the 401 Hz peak to hrec, we injected a line at 405 Hz:
Magnetic injection: using the small coil, varying its orientation, (Figure 3, 4, 5)
Seismic injection: using the shaker at the same frequency.
Preliminary observations:
The injected line at 405 Hz is slightly visible in hrec for both magnetic and seismic injections, Figure 1 and Figure 2.
Looking at the coherence between hrec and the magnetometer (*DT_MAG*) on the DET tower base (inside lab), as well as the accelerometer (*DT_ACC*) on the flange, the magnetic coherence appears slightly higher than the seismic coherence.
The injection times are reported in the attached file.
This morning, the experimental setup (shaker, coil, and amplifier) was moved near the INJ tower in order to perform the same injections previously carried out on the DET tower.
The investigation of the 401 Hz peak was carried out at the turbo pump of the INJ tower. It is worth reminding that, unlike the DET tower, there is no bellow between the turbo pump and the INJ tower.
We injected magnetic and seismic lines at 390, 395, 401, 405, and 410 Hz.
The amplitudes of the injected seismic and magnetic lines were tuned to match the amplitude of the 401 Hz line observed in the accelerometer (ENV_IB_ACC*) and magnetometer (ENV_IB_MAG*).
From a preliminary look, we noticed:
Seismic line injections: the lines at 390 Hz (ampl = 0.2), 395 Hz (ampl = 0.4), and 410 Hz (ampl = 1.6) arose in Hrec.
Magnetic line injections: for the coil axis along the North arm and vertical directions, the line at 405 Hz (ampl = 0.8) arose in the sensitivity.
Figure 1: during the seismic injections, some lines also became visible in the magnetometer, probably because the oscillation of the tower generates a modulation of the Earth-field lines.
The details of the actions performed are reported in the attached files.
