Transient is considerably reduced when IFO is kept at 25W with annualr CO2s for 40minutes, see plot. Is this caused by annular CO2s being on or from the 25W circulating arm power for 40minutes before final power up to NLN?
Dan's next suggested test is to not turn off CO2s on lockloss and try to lock with them.
- Left plot is a power up after we waited 40 minutes at POWER 25W (with both annular CO2s on) before powering up to 60W (due to violin damping).
- Right plot is a normal power up with 3 minutes at 25W.
- CO2s had not been on in the 2 hours before either lock or for periods of >15 minutes in the 12 hours before.
Leaving the CO2s on all the time should get rid of this 5+ hour transient we are seeing when we get to max power, as Camilla's plots are nicely hinting at. The uniform heating time constant is much faster, so we'd expect to see the arm powers reach a steady state in about 1-1.5 hours.
Plots attached for the surface and substrate spherical power change over time, ring heater included for reference.
Plots show the spherical power change from 1W of uniform (scaled down by 0.05) and annular CO2. There are two longer time constants when switching the CO2 on due to the CP eventually radiating on to the ITM AR surface. One is from the ITM substrate eventually developing a thermal lens which oppposes the annular CO2 CP lens, hence the overshoot. The other is the thermo-elastic deformation of the ITM HR surface from the radiating CP. Both effects are still changing the IFO over about 10 hours, the thermal lens taking the longest to level off.
As can be seen in the steady state, watt for watt, the CO2 actuates about 40% of the RoC actuation that 1W of RH does. In terms of substrate lensing the annular CO2 generates a lens that is about 30% more than the RH.
Today we are applying 4W of annular CO2 on X and 1.7W on Y. From a cavity eigenmode perspective this is somewhat equivalent to applying 1.6W of RH on X and 0.68W on RH Y. However, the ratio of RoC to substrate lensing is different, so we can't just swap to using a constant ITM RH power and expect the same result if we plan to have constant CO2.
These 5+ hour time constants also making picking the CO2 powers a pain (as with everything TCS related), as the fast CO2 changes we have been doing only show us the CP substrate lensing effect on the IFO.
In alog 66700 I show that leaving the ring heaters on didn't have a large effect on the 2 hour transient, though we only had one true lockloss and relock with the CO2s remaining on.