At 157Hz there is a known resonance of SIB1, and that can be excited by magnetic injections. It would be good to compare these results to injections with the small coil next to SIB1 instead of SDB1 to see which tower is the real culprit.

SDB1 and SIB1 are structurally similar, so it could be a shared resonance, and that the excitation is indeed on SDB1. Around 30Hz there is a vertical resonance in the bench marionette if I remember well (again, a similar frequency is likely fro both SDB1 and SIB1).

The shift was dedicated to magnetic injections close to SIB1. We also repeated the injections close to SDB1 using exactly the same sweep amplitude in order to have a direct comparison.

We placed the small coil at the base of SIB1 underneath the burtington magnetometer and in turn we oriented the coil with the axis along W-arm, N-arm and Vertical. For each orientation of the coil we injected a sweep from 10 to 1000 Hz with amplitude of 5V to the amplifier and a duration of 1800s.

The log file contains details and times of the actions.

Here attached a spectrogram showing that for the SIB1 injection the effect is visible in LSC_DARM up to 800 Hz, even if fleeble. This seems not the case for the later injection at SDB1 which is visible only up to 300 Hz.

Interestingly, between 960 and 1000 Hz there is a wandering line, best seen in the second attached spectrogram.

We continued the close field injection campaign with the following:

• DET lab. small coil as in the previous injections (SDB1 tower base N-E corner, axis EW): a set of lines (600s) between 103 and 800Hz (lines above 400Hz are not at all visible even with 100s window). Couriously the injected line at 203Hz produced a wide sideband structure in DARM/Hrec: Figure 5.
• DET lab (Figure 1), small coil on SDB1 tower base but in the N-W corner (that is more distant from SDB2), we inject 1800s sweeps 10-1000Hz with the three coil axis diretions
• NI tower (Figure 2), small coil on NI tower base at bartington magnetometer location, we inject sweeps with the three coil axis directions, the first two (EW and NS coil axis) were 1800s long, the VERT coil sweep was 600s from 8Hz to 330Hz.
• WI tower (Figure 3), small coil on NI tower base at bartington magnetometer location, the EW axis sweep was 1800s 10 to 1000Hz, the NS and VERT axis sweep were 600s, 8 to 330 Hz.
• BS tower: we placed the coil. The ITF unlocked.

All the details are in the attached log file.

Thank you Romain G. for giving us 1 hour of your shift time!

Some insights on the close field injections

Comparison between injections at NI and WI towers

Figure 1  compares the averages DARM spectrum during the first 120s of the magnetic sweep injected at WI tower base (NW corner) green = coil axis along Virgo W, black = coil axis along Virgo N) and at NI tower base (NW corner) red = coil axis along W, blue = coil axis along N. (see this map for the injection locations). The injections with the coil axiis oriented ertically were significantly smaller and we have not reported them. The impact on DARM when injecting at WI tower base (axis W) is significanly larger.

Another strange effect was noticed when injecting at WI with the coil axis orineted along N (black curve in Figure 1). Seems that the injection triggered glitches in DARM which excited sidebands of some DARM lines. The peculiarity and the reason why we think the glitches were triggered by the injection is that the glitches were coincident with the sweep crossing some specific DARM lines, that are: 26.7, 77.6, 137.4, 250, 300, 350 Hz.

Figure 2 shows a DARM spectro with evidence of these glitches. Figure 3 shows the crossing of the 137 Hz line.

Figure 4 evidences the 48.6 hz peak that is exited by the WI injectecion, and the  47.7 Hz peak excited at NI.

Comparison between injections at SIB1 and at SDB1

Figures 5 to 7 compare spectral features of the sweep injections done at INJ (SIB2 base SW corner) and at DET (SDB1 base  NE corner) see this map. Figures compare the averages DARM spectrum during the first 600s of the magnetic sweep at both location, for the max impacting coil axis orientation, which for both cases is the W direction. In these figures the INJ injection spectrum is in red, the DET injection spectrum is in blue.

We notice different peaks are excited.

For INJ: a pair around 28 and 29 Hz, 118 Hz, 134 Hz, 141 Hz,  broad reagion between 160-170Hz  172 Hz 186 Hz

For DET (excited structures are less clean): 29.7 Hz, 112, 121, 123, 158 Hz, 164 Hz, strucures around 176-180, 196 Hz.

This morning we continued the near field sweep magnetic injections with the small coil :

• BS tower 
  sweep (10-1000) Hz, dur=1800s , coil axis EW
  sweep (8-330) Hz, dur=600s , coil axis NS
  sweep (8-330) Hz, dur=600s , coil axis VERT
   
• WE tower
  sweep (8-330) Hz, dur=600s , coil axis EW
  sweep (8-330) Hz, dur=300s , coil axis EW
  sweep (8-330) Hz, dur=600s , coil axis NS
  sweep (8-330) Hz, dur=600s , coil axis VERT

The details of the actions are reported in the attached file.

Thank you to Matthieu and Suzanne for allowing us to use 2h of their shif time!

This morning, we performed the near field sweep magnetic injections with the small coil :

• WI tower
  sweep (8-330)Hz, duration 600 sec with coil oriented along EW, NS and vertical direction
  sweep (8-330)Hz, duration 600 sec with coil oriented along EW, NS, vertical direction and positioned close to the tower base edge (x,y)=(53,76) cm
   
• WE tower
  sweep (8-330)Hz, duration 600 sec with coil oriented along EW, NS and vertical direction
   
• WE SAT rack
  sweep (8-330)Hz, duration 600 sec with coil oriented along EW, NS and vertical direction (see Figures)

The action details are reported in the attached file.

Some comments on near-field sweep magnetic injections at WI tower.

• Figure 1 and Figure 2 show the Hrec and DARM spectrograms during the injection before and after the removal of the WI mirror magnet, respectively.
   
• In Figure 3,  we report the coupling function values (m/A) during the magnetic sweep injection before (blue dots) and after (red dots) the magnet removal. Looking at the north and west orientation of the axis coil during the sweep injections, we observe a change in the slope of the coupling function.
   
• In order to estimate the effect of the distance coil position-mirror magnets on the coupling function calculation, we moved the coil approximately 60 cm toward the edge of the tower, Figure 4. Moving away from P1 position, the coupling (red dots) has reduced of about a factor two but the slope has not changed, Figure 5.

The ISC team left us a locked interferometer at about 5pm (LT). The ITF proved very sensitive to people moving around NE tower, infact we unlocked twice when changing the orientation of the small coil positioned on the NE tower base (SW quadrant). Also the ITF was very difficult to be relocked. Thanks to Piernicola, Ilaria, Diego and Nicola Menzione for their help in locking and attempting to lock the ITF.

We could perform (only) these injections with the small coil at NE tower base:

• coil axis oriented along EW direction (sweep 8-330 Hz, 600s, 5V)
• coil axis oriented along NS direction (sweep 8-330 Hz, 600s, 5V)
• coil axis VERTICAL (sweep 8-330 Hz, 600s, 5V)

Attached is the log file with details and time of the performed actions.

The shift was dedicated to WEB. We injected with the small coil in these locations:

• WE tower base (North-East corner)
• same X,Z position, Y position at about  beam level
• same X,Z position, Y position on scaffolding first level
• in front of SAT rack, Y position is between Sc and Sa crates

in each location we repeated the injection for 3 coil axis orientation: EW, NS and VERT. We injected a sweep 8-330 Hz, 5V and 600s.

During all injections the MICH, SRCL and PRCL lines (respectively at 21.7 Hz, 26.6 Hz and 64.4 Hz) were switched off (Thanks to Diego, Manuel, Piernicola, and Francesco Berni!)

We installed one fluxgate magnetometer at the WE tower base, North-East corner, same X,Z position and beam height: ENV_WE_MAG_N,W,V

Far field magnetic injection in the CEB:

• we injected a sweep 8 - 500 Hz, 9 V and 1800s duration

Times and details are in the attached log file.

Note: after the lines switch off the hrec spectrum was much cleaner in the 20-40Hz range. We noticed a sort of recurring change in the noise floor between 20 and 40 Hz, as can be seen in the attached plot.

Some plots with the measured couplings to hrec normalized to the current in the injection coil

• Fig.1 shows the 3 locations of the injection coil around the WE vacuum chamber, with indication of the distance from the closest mirror magnet and from the closest marionetta magnet (thanks to Tatiana!).
• Fig.2 another view of the 3 locations around the WE tower
• Fig. 3 measured coupling in all locations (the three mentioned above, and one location in front of SAT electronic crates about 0.5m away - black curve), coil oriented with axis along EW
• Fig. 4 measured coupling in all locations, coil axis along NS
• Fig. 5 measured coupling in all locations, coil axis along VERTICAL 

We observe that all the measured couplings have more or less a similar frequency slope, which approximately goes as f^(-2.5).  A hint of a change of slope, with the slope becoming steeper below ~20Hz, is present for the platform position with coil axis oriented NS (Figure 4 - yellow curve).

In principle we would expect a steeper slope (1/f^2 steeper) if we move the mirror acting from the marionette. But, actually the induced mirror motion from each location depends on 1) the relative distance from the mirror or marionet coil ... and we see from Fig. 1 that all locations were more or less equally distant from both; 2)  the ratio between the magnet strenght (mu) and the mass (iniertia) of the body (marionette or mirror). Thus is indeed possible that the magnetic force we exerted on the marionette (also in the top location, which as can be read in Fig. 1 is 45% closer to the marionette magnet than to the mirror magnet) was not sufficient to move the mirror in a measurable way.

From the top location (coil positioned onto the platform' 1st level) we could excite more effectively peaks at 48 Hz and 70.6 Hz, particularly when the coil was oriented vertically (Figure 5). These frequencies should correspond to resonant modes of the F7 crossbar.

We also observe that the coupling to SAT electronics (from the 0.5m far location) is negligible. Also, we find no evidence of an increased coupling at high frequency as it would result from induced currents into cables.

Magnetic injections at WE

We positioned the small coil at the platform level (marionetta level)  North side. We injected the usual sweep (8-330Hz, 5V, 600 minutes) with the 3 orientations of the axis: EW, NS and vertical.

details of the times are in the attached logbook.

For all injections we observed a large excitation of the 48 Hz (F7 crossbar) and, while injecting between roughly 100 and 200Hz, a significant increase of the Hrec noise in the range 20 to 60Hz. Probably our injection was disturbing some control.

Unplug of NE F7 motor crates

1379707207  25 Sep 2023 19:59:49 UTC  (25 Sep 2023 21:59:49 CEST)
no visible effect in Hrec is observed.

Thank you Diego for your the help by locking the ITF above 33 Mpc.

*** Near-field sweep magnetic injections at NE tower ***

We performed near-field sweep magnetic injections with a small coil on the tower base.  We injected a sweep in the frequency range (8-330) Hz, 5V and duration of 600s .
Below, are listed the times of the actions:

Quiet: 14:48:00 UTC (300 sec)

Coil axis EW
Start: 14:57:09 UTC - Stop: 15:08:23 UTC

Coil axis NS
Start: 15:27:01 UTC - Stop: 15:37:34 UTC

Coil axis Vertical
Start: 15:48:23 UTC - Stop: 15:59:22 UTC