[Jenne, Dan, Sheila, Alexei]
Today we've been moving the ITMY spot position around by changing the L2 P2L gain. It seems that we can repeatably change the coupling of the 9 MHz RIN (as measured by the 70 Hz line injected into the RFAM stabilization box) when we move the spot.
Dan has been watching the ITMY HWS, and noted that the first lock, I was moving away from the point absorber when I was moving farther from the center of the optic. Then, we decided to try to 'jump' over the point absorber, and get closer to the geometric center of the optic. The next 2 locks seemed like we were very nearly on top of the absorber, and the ASC system did not like that. Also, our buildups (arm circulating power and POP18) were much worse when we were on top of the absorber. Now we're trying to move even farther from the center of the optic, to be much farther from the point absorber. So far, this is bringing the buildups back up, however we'll need to move the PRC cavity axis to move the POPX spot. The POPX PZT was starting to rail in pitch during our most recent lock, and will get worse if we continue to move the ITMY spot.
In the attached screenshot, I've got the RIN injection peak, as measured by DARM, at several points in time during these locks. The upper left plot is the first lock where I went up in pitch and things were getting better, but we were getting quite far from the center of the optic.
The lower left plot and upper right plots are attempts to go below the point absorber, but we seem to have been very nearly on top of it. Both of these locks were very short, and they seemed like ASC problems.
The lower right plot is much farther below the center of ITMY. At this location, the buildups were getting better, although not yet as good as they were during the first lock. Also, the RIN coupling was lower than the middle 2 locks, also indicating that we're starting to get farther from the point absorber. This last lock was lost during the Lownoise ESD ETMX transition.
Next steps are to see that the latest position on ITMY is in fact good over a long lock stretch, and move the power recycling cavity axis so that we can try moving even farther on ITMY. I'd like to also see if there is any advantage to moving the spot position in yaw, to be as far as possible from any of the small point absorbers that are also on ITMY. Once we've found a good place on ITMY, we should check to see if we can get better buildups in the arms by moving the spots on the ETMs to match this new ITM position.
The ITMY spot position is defined by the A2L gain. So far today, all I've changed is H1:SUS-ITMY_L2_DRIVEALIGN_P2L_GAIN. It started the day at -1.33 (and is currently at +3.00), so if there are problems relocking, check that and consider setting it back to it's old value.
Here are some HWS images from our spot moving today. Initially we moved up to 17mm, here we can clearly see that the topmost point absorber is getting hot. The other images show the spot position moving down, 9.9mm image is the nominal position. Images for the P2L gains of 1.5 and 3 are also attached. If you flick through them you can see the absorption of the beam gradually shifting down. We were hoping to get over the main point absorber with the P2L gain of 3 however the IFO did not like this spot position. DARM spectrum was particularly bad when we finally locked, there appeared to be a lot of additional frequency noise too. The lock didn't last too long so we put the P2L back to the nominal value.
The longest lock we had was when we moved the beam up to 17mm. During this time we saw a good reduction in the 9MHz coupling - nominally the line height is ~10. The other locks were too short for anything to thermalise completely so hard to say whether much improved or not.
