J. Kissel, Operators
Where we left off on Friday:
- We were battling 2 (actually 4) modes,
ETMY: 1008.4502 & 1008.4938 Hz (Mode separation: 0.0436 Hz)
Unknown: 1463.0974 & 1463.1005 Hz (Mode separation: 0.00309 Hz)
For the the later unknown modes, I tried the following:
- Driving ITMX or ITMY, and even Both Simultaneously and for each of those configurations:
- Multiple bandwidth sizes
- Driving in Pitch, Length, and Yaw
- Gains from very little to about to saturate the DAC
- The usual phase rotation game
I attach a StripTool that shows the control signals of the damping filters all I was ever able to get was the beatnote between the two frequencies, with a period of 5.3 minutes. That extra long beat-note envelope makes it extra time-consuming to try new configurations, because one has to wait for one, if not several peaks of the envelope to assess whether a positive (or negative) change has been made.
- We also has somehow managed to ring up > 1.5 kHz harmonics, especially some 6 kHz mode
Where we pick-up today:
See attached spectra.
These same modes mentioned about continue to dominate the RMS, along with another set of modes (1475.2520 & 1475.0984 Hz) that has rung up again over the weekend, where the latter of the two (1475.0984 Hz) is also one for which we've not yet identified a filter and/or settings.
We're otherwise keeping the Violin Mode Table as up-to-date as possible with any success we have.
I have edited the Violin mode tables you linked to which it had already the mode 1463.097 identified as ITMX, I can confirm that the other pair mode 1463.1005 is also associated to the same mass actually to the same fibre. UPDATE: I have been asked to provide some evidence of this affirmation, so here it is. A couple of year ago (during O1) Evan Hall and myself were allowed to drive the PUMs of each test mass individually with the damping filters turned off for fundamental and higher harmonics, while the detector was locked (see original alog here). Next a reminder of times of injections:
Injection on ITMX: from 2015-10-28 20:39:44 to 2015-10-28 20:44:13, channel H1:SUS-ITMX_L2_DAMP_MODE5_GAIN
Injection on ITMY: from 2015-10-29 20:22:51 to 2015-10-29 20:31:17, channel H1:SUS-ITMY_L2_DAMP_MODE8_GAIN
Injection on ETMX: from 2015-10-29 20:37:20 to 2015-10-29 20:45:37, channel H1:SUS-ETMX_L2_DAMP_MODE5_GAIN
Injection on ETMY: from 2015-10-29 20:52:36 to 2015-10-29 20:59:11, channel H1:SUS-ETMY_L2_DAMP_MODE6_GAIN
Then I monitored change of amplitude and phase of the violin modes and harmonics before and after the injection (for each injection). I attach the plots for the harmonic lines at 1463.097 and 1463.1005 Hz. The injection starts at about second 1200 on these plots. It is unquestionable the huge difference observed when the injection was applied to ITMX respect to all the other cases.
UPDATE2: While I am 100% sure that 1463.097 and 1463.100 are associated to ITMX however my confidence of them being associated to the same fibre is not that high, I just made an obvious conclusion based on the fact that they are the closest pair of modes for the 3rd harmonic, by an order of magnitude!
They are 0.003Hz apart while the next adjacent pair of modes for the 3rd harmonic are 0.05Hz apart. Maybe actually the fact that they are much closer than any other pair shows the opposite conclusion, that they actually correspond to different fibres. In a way that one of the modes of each pair happens to be incredibly close.
And actually looking to the original identification of the fundamental mode to actual fibres (https://alog.ligo-wa.caltech.edu/aLOG/index.php?callRep=17610) this may actually be the case. Look at the identification of modes 501.208 (associated to ITMX FR) and 501.254 (associated to ITMX BL), they are 0.046Hz apart, closer than any other pair of modes on ITMX. A difference in inharmonicity of the 2 fibres may explain why their 3rd harmonics are much closer than their fundamentals.
It may be worth trying to damp these modes by acting on those 2 fibres.
The other 2 modes you are having problems with; 1475.2520 it is unquestionable ETMX, and 1475.09 most probably is also associated to the same mass (probably a pair mode) but notice that this mode did not have a noticeable change in amplitude when driving any of the PUMs associated to QUADs, which means is weakly coupled explaining why you are having issues with damping filter settings.
