I've used the large temperature change over the past weekend to identify WETM mirror modes.
- - - - - -
Eigenfrequencies of the test masses are proportional to square root of Young's modulus: f ~ E1/2, where E := Young's modulus. The Young's modulus for fused silica has some temperature dependence ( measured to be (dE/dT)/E = 1.52e-4 / K ); as temperature increases, Young's modulus increases, so the mode frequency increases over a lock stretch with self heating, ring heaters, or with a strong ambient temperature increase.
With the accidental temperature change test at WE over the weekend (starting Sunday morning ~1:00 UTC, 41771), we are thus able to identify modes of of the West End mirror by their frequency shifts. The first attachment shows the rise of the room temperature on the top and the subsequent rise of the ring heater temperature sensor on the bottom, the latter lagging by ~6.7 hours.
In the rest of the attachments, I zoom into relevant frequency bands (specified in name of attachment) of the high rate V1:SDB2_B1_PD2_Audio_100k channel. We had two several hour locks on Sunday during the WE temperature change, the first lock starting shortly before the temperature change began. In the plots, purple is taken at the beginning of the first lock and blue is taken at end of second lock, at which point room temp had increased by 1 C and RH temp had increased by 0.2 C. I have tried to highlight bands expected to have the highest parametric gain according to Dan Hoak's document VIR-0525A-16, but some bands I couldn't identify.
Of note:
- 7.8 kHz (Drum modes): In 41732, Swinkels noted the missing drum mode; as he suspected, we have two test masses with nominally same drum mode frequency ~7809 Hz. WE mirror drum mode was revealed with temp change.
- 9.6 kHz: It had been thought that there was a mode group around 9670 Hz and 9685 Hz, but this indicates that the two mode groups are mixed.
- 10.1 kHz: again, two masses with almost same nominal mode frequency
- 12.8 kHz: what looks like a test mass mode with rather high amplitude, high PI gain? This would interact with LG02.
- - - - - -
Note: There are upcoming ring heater tests of the end mirrors which will further help with mode identification since the ring heaters will be stepped one at a time. keywords: parametric instability, PI, mirror modes, temperature change
