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Strange vertical 50Hz noise in one BPM

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Laura:
Hello Ji-Gwang,
Indeed we also suspect some cooling system but we have not been able to identify it. We put some accelerometers in the tunnel on the block and on the girder and in both case we see this strange frequency but we could not identify the source, moreover the frequency is there either we observe or we do not observe vibrations in the BPM data. As you suggest we can try to do some tests with accelerometers during shutdown switching off each of the element to see if the movement measured with acceleromters stop in some case.
Thanks!
Laura

Ji-Gwang Hwang:
 Hello Laura,

It is good to hear that you did the test with accelerometers. In general, the sensitivity of the accelerometers (depending on the model and frequency) is an order of magnitude better (less than 100 nm) than the BPMs (~ 1 um). This could explain the discrepancy between the two monitors.

If the vibration amplitude is changing daily (especially more in summer), the cooling system would be the source since the water flow/pressure is variating to fight against the temperature raising in the tunnel or circumstance. But I am not sure whether the cooling water could drive the 50 Hz noise. In my case, the compressor of the chiller was working with that frequency which is equivalent to the frequency of the main powerline in Germany.
 
 Best regards

Ji-Gwang Hwang

Laura:
Well, the step I observe with BPMs is 4 um so I should be able to see it with accelerometers!
Also, sometime it is a jump: it does not go back to the previous location...
I will keep you updated!

Benoît Roche:
Just an idea: maybe it could be an "impurity" which is in the vaccum chamber between the button and the vaccum chamber a create a non-infinite resistance. Sometimes the impurity creates the short-circut, sometime it does not.

It does not explain everything (why the perturbation is only in the vertical plane? why 50 Hz?), but it is easy to check with only a multimeter (without beam).

Laura:
Good idea, I will try next week (shutdown)!

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