I set up a simple prototype system to monitor the beam tube between the corner station and mid-Y for impulsive events, like stick-slip events, that might generate beam tube particulate glitches in DARM. The idea is to monitor this for a few months and, if we see events, propose a full deployment. If we don’t see events, we may move the prototype system on to another of the 4 sections of beam tube.
I originally used a microphone in the beam tube enclosure because there were many bellows that would attenuate the signal travelling on the tube. But the air signal also moves the beam tube locally, which, because it is similar to the beam tube at the impact location, likely resonates at the same frequencies that the impact excited, so I tried an accelerometer on the beam tube. This turned out to be about a factor of 2 better than the microphone at detecting distant taps when focused on a beam tube resonance at about 200 Hz. With this system I could detect light taps 2km away. My standard tap is made by holding a plastic and metal scissors 6 inches above the beam tube, then letting the scissors rotate freely around a finger so that the tip of the scissors hit the beam tube (see Figure 1). A standard tap in the control room elicited no reactions and is not as loud as dropping a scissors from the same 6 inch height. This standard tap is somewhat softer than the taps that generated DARM glitches; if it can generate glitches, it does so only rarely. At 2km this tap is just visible so I think that we are in good shape to detect anything that has a high probability of generating a glitch over the entire 2km stretch. The signal from the taps travelled at an average of just under 400 m/s.
The prototype system also includes a microphone set up just outside of the beam tube enclosure. This microphone is used to discriminate local from distant events. For example, the LN2 dewar stick-slip events that can often be heard would produce small beam tube motions, as would passing cars; these motions might, without the microphone, be confused for distant large impulsive events.
Figure 2 shows the signals from standard taps at various distances, as well as rattling the beam tube enclosure door nearest the accelerometer (happens during high wind) and impacts on the large LN2 dewar. The door and dewar signals were easy to differentiate from the BT taps because large microphone signals were present.
