Kiwamu, Stefan

We had two long-lock attempts tonight. During both we tracked the the REFL optical gain to frequency noise, as well as the ASC optical gain to an IM4 pitch dither.

The first one we started from a 20W lock that had more CO2 central heating on the X compensation plate (see Kiwamu's log).During this first lock we again saw the sign of the REFL45_I signal to frequency noise flip. This happened about 15min after reaching 50W.

The second lock started with the regular TCS settings. We reached 50W, and reduced the 45 modulation index by 2dB. Guardian now uses the now BS ASC input matrix, only using AS_A_36 with a gain of 1.5 (PIT and YAW). 
In this setup we were able to hold the lock for 1h20min.

Attached is a gain tracking plot for that 2nd lock. Note that there is a number of changes in REFL gains that will affect out ASC system:

- While the power is ramped up there seems to be a Gouy phase shift - with the signal in REFL_A_45 decreasing, and the signal in REFL_B_45 increasing.

- However, after that both 45 ASC signals show almost no change in gain.

- On the other hand, the two 9MHz signals drop their gain by about a factor of 3.

- Also note the episode around -50min - we manually tried to tweak IM4 and PR3, because we were afraid that the input matrix wasn't putting us to a good spot any more.

- Over the 80min the sideband 2-omega kept dropping slightly, but was fine otherwise. Towards the end we notced the AS_90 get rattier. We suspect that this is either due to the BS being slightly misaligned at DC, or due to some ASC loops significantly loosing gain.

Bottom line:

- It still seems possible to get the interferometyer running indefinitely at 50W, but we have to spend some more time understanding how the ASC gains change.

- But this still leaves some pretty uggly higher order modes in the interferometer - I have no idea what that will do to our noise couplings.
 

LEGEND for plot:

H1:LSC-LOCKIN_1_DEMOD_4_I_OUTPUT         LSC: laser frequency to REFL_9_I  (blue)
H1:LSC-LOCKIN_1_DEMOD_3_I_OUTPUT         LSC: laser frequency to REFL_45_I (dark red brown)
H1:ASC-LOCKIN_OSC1_DEMOD1_I_OUTPUT   ASC-REFL_A_RF45_I_PIT gain  (IM4 dither) (orange)
H1:ASC-LOCKIN_OSC1_DEMOD2_I_OUTPUT   ASC-REFL_B_RF45_I_PIT gain  (IM4 dither) (yellow)
H1:ASC-LOCKIN_OSC1_DEMOD3_I_OUTPUT   ASC-REFL_A_RF9_I_PIT gain  (IM4 dither) (light blue)
H1:ASC-LOCKIN_OSC1_DEMOD4_I_OUTPUT   ASC-REFL_B_RF9_I_PIT gain  (IM4 dither) (dark blue)

PS:

Some of the mysteriously unstable loops earlier today were due to the DOWN script now setting back all the gain changes related to the RF modulation depth reduction. This was fixed tonight.