Following the Qs measurements done by Terra and I a couple of days ago (alog 45812), we wanted to measure the max PI gain for the 15kHz, 15.5kHz and 18kHz (aliased from 47.5kHz) mechanical modes. This is done by ringing up and down these modes for the different test masses during a self heating thermal transient and 2.4 W ring heater transient. When the HOM3 and the mechanical mode frequencies overlap, the maximum PI gain is achieved.

Carl's python script was used to excite the mechanical modes with the ESDs every 300 seconds, similarly to what has been done at LLO (see alog 41630). By fitting the ringdowns, the Qs are then estimated. I've used a homemade matlab script to do the tau fits, manually rejecting bad or fishy data.

We managed to have a ~6hours long stretch (see Dec11.pdf) and ~1h stretch (see Dec12.pdf) both at 20W. During the first lock, we monitored the 15kHz and 18kHz modes (unfortunately, we didn't monitor the 15.5kHz). The measured Qs are shown in Dec11.pdf. relative to the simulated HOM3 spacing (x3). We only observe an evolution of Q (apart for statistical deviation) for mode 14.98 IX, 14,98 IY, 15.01 EY, 15.08 EY and 17.94 EX. Unfortunately, this doesn't seem to correlate with the HOM spacing. Could the simulation be off?

On the following days, the longest lock that we manage to have was an hour. The measured Qs as we go through self heating are shown in Dec12.pdf. The self heating goes fast and we don't have a lot of data point, but I put it there if people are interested. No obvious change in Qs.

To conclude, I would say we have some interesting data that are worth to analyze a little more, as we did see some nice Q changes the first night. By comparing these measurements with what has been done at LLO, we should come up with some interesting numbers... We will work on that in the coming weeks.