Abstract:

One possible reason of the sensitivity breathing observed at 60–100 Hz is glitches affecting the MN or IM vertical actuators.

Details:

I investigated the cause of the sensitivity breathing at 60–100 Hz, because this frequency range is the most critical for the BNS range.

Fig.1 presents examples of DARM sensitivity spectra taken during two different periods.
The spectra were obtained with an 8-second time span, 75% overlap, and 10 averages.
When a large glitch occurred, several violin modes were excited, as shown in the bottom panel.
Similar glitches, which excite violin modes, were also reported in klog:32506.
Given that violin modes are most easily excited by IM pitch motion, this suggests that the glitch signal was injected into the IM or MN and excited the pitch motion.

Fig.2 provides another set of examples, where the red spectrum is slightly noisier than the blue one.
At the time of the red spectrum, the vertical, pitch, and roll modes of the IM (CuBe resonances), which are excited by the vertical actuators of the MN or IM, also appear to be excited.
Although this relationship is not yet definitive, it is worth further investigation.

With today's upgrade of the DGS system, we expect fewer glitches in the Type-A suspension.
We will soon measure the frequency and amplitude of the breathing to confirm this improvement.