[Jeff K. Arnaud P.]

B&K hammer measurement on the ITMX suspension cage was completed yesterday while the ISI was floating. The corner cube left from alignment was removed before starting the measurement, and replaced by the protecting plate after. The corner cube as well as the screws are now on the floor in the chamber (cf 2nd picture attached). The protecting plate of the reaction chain test mass was removed and replaced after the measurement.

The tri-axis accelerometer used has been mounted on the side of the structure, with the axis oriented as described in the picture attached. As the previous measurement of ITMX, two locations were chosen for the hammer hit, one on the front (labelled as the Y axis of the accelerometer, corresponding to the sus axis "longitudinal"), and the other one on the side of the optic (labelled as the X axis, or "transverse" for sus axis)

The measurement was taken with the usual Pulse software, using the templates from the last ITMX measurement (cf alog 7762).

Results in the pdf attached are described page by page below :

(1) X-Y-Z response to X excitation, suspension unlocked, isi unlocked, in chamber, vibration absorbers on

(2) X-Y-Z response to Y excitation, suspension unlocked, isi unlocked, in chamber, vibration absorbers on

(3) X Resp to X excitation : comparison between b&k ISI locked (LLO ITMY (red) + LHO ITMX test stand (green)) and ISI unlocked (LHO ITMX in chamber (blue))

(4) Y Resp to Y excitation : comparison between b&k ISI locked (LLO ITMY (red) + LHO ITMX test stand (green)) and ISI unlocked (LHO ITMX in chamber (blue))

Note : even though it hasn't been exported, the coherence looks really good in the pulse raw data.

The results from yesterday afternoon are showing a lot of high q resonnances and noisy data in Y direction, which is not the response we would expect to see, cf (2). Also we can see the overall magnitude of the measurements with the ISI unlocked is below the ones with the ISI locked by an order of magnitude cf (3) and (4). The exported data was compared to the pulse data, to make sure it's not a calibration factor issue in the process.
It seems unlikely that those differences are due to the state of the ISI, but since we don't have other data comparing b&k ISI locked/unlocked (or at least I didn't find any), it is hard to tell. Tomorrow, we will double check the status of the ITMX suspension cage, to check if we didn't forget to remove something that won't stay permanently, that might have caused the peaky resonnances. Also, it will be interesting to reproduce the test when we will have a chance at ETMX (seems like next week, when all the work will be done on the suspensions after discussing with Keita), see if we get the same drop in amplitude.