Den, Patrick, Evan
During IX coil driver investigations yesterday, we rang up some violin modes to very high amplitude (~4×10−14 m rms in DARM, about 4 orders of magnitude higher than our optimal, damped mode height).
The existing violin mode damping settings, which consisted of a few wide filters meant to actuate on multiple modes simultaneously, were damping some of the modes while causing others to ring up.
This is not the first time we have been hoist with our own wide-bandwidth pitard. Additionally, we have more the enough filter modules per test mass to damp each first harmonic individually, which alleviates the problem of having to find a filter to damp multiple modes simultaneously.
There were five IX modes visible in DARM on rf readout. For these five modes, we constructed narrow, individual damping filters (8th order butterworths). The settings are as follows:
| Freq. [Hz] | FWHM [mHz] | Phase | Gain [ct/ct] |
| 500.054 | 100 | 60° | 100 |
| 500.212 | 100 | 0° | 100 |
| 501.208 | 30 | 0° | −10 |
| 501.256 | 30 | 0° | 10 |
| 501.452 | 100 | 0° | 100 |
Note the narrow bandwidth of the filters for the two 501.2 Hz modes.
These use the IX L2 damping SFMs, numbers 1 through 5. The corresponding OAF monitor filters have been updated as well.
That means there are three other first harmonics on IX that are yet to be damped.
The damping phase of the mode at 501.208 Hz seems to have flipped sign at the start of the most recent lock. We'll have to keep an eye on this one.