Den, Kiwamu, Evan

Tonight we again made it to low noise, although the locking procedure is still not robust enough. In the end, it seems that the sensitivity degredation is not due to a new mystery noise, but rather is a consequence of a shift in the interferometer alignment and some left-over temporary equipment.

List of changes/observations for locking is as follows:

• We found that engaging true differential hard loops (instead of differential ETM loops) during the CARM reduction sequence causes the sideband powers to drop too much. So ITM feedback should only be turned on once resonance is achieved.
• We are still coming into resonance with middling recycling gain, sometimes too low to engage the Refl 45 MHz → PR3 loop. The procedure that works most of the time is to gently engage the soft yaw loops, which brings the recycling gain above 36 W/W. Then the soft pitch loops are engaged with error point offsets (0.1 ct for differential and −0.1 ct for common). The offsets in the QPD pitch/yaw filters have not been changed.
• With good recycling gain (>38 W/W) at 2 W, we adjusted the ALS Y camera offset setpoints. The ALS X setpoints didn't seem to need adjustment.
• Kiwamu found that the TMSX yaw OSEM readbacks had moved several days ago, coincident with the amplifier swap. We don't necessarily believe that this represents a real motion of the TMS.
• In the guardian we now engage 5 Hz low-pass filters in the beamsplitter angular loops, and a slight boost in the Michelson length loop. This prevents loop instability during power-up.
• Several times we saw the return of the 0.4 Hz angular instability when powering up with mediocre recycling gain.

Once we reached the low-noise state, we again saw the excess noise that has been bothering us for the past few days. We were able to partially mitigate this as follows:

• The dsub breakout board for OMC measurements was removed. This seems to have fixed the majority of the broadband excess noise from 20 to 100 Hz.
• We retuned the test-mass angle-to-length decoupling. At first we tried using the script, but it didn't do a good job. So we did it by hand. This fixed the excess noise below 20 Hz. Mistuned A2L is consistent with the interferometer being in a different alignment state.
• The SRCL feedforward had to be retuned. Again this is consistent with the interferometer being in a different alignment state. For now, we just retuned the gain. Den suggests using a flat FF filter (which will take care of the radiation pressure coupling) and not worrying about the junk coupling above 80 Hz.
• The jitter coupling into DARM is worse, particularly around 290 Hz. Again this is consistent with the interferometer being in a different alignment state. The 290 Hz peak is also visible in the pitch/yaw IM4 transmission signals, but it seems to have been there last week as well. So it seems the jitter exiting the IMC has not changed.

The sensitivity is now 70–76 Mpc.

Finally, we looked for high-frequency frequency noise downconverting into DARM. We didn't see anything, but here is what we did:

• We used an SR785 to drive a ~20 mVpk sine wave into the CARM error point. We swept the frequency from 30 kHz to 100 kHz while watching the DCPD sum. We didn't see anything.
• Then we used the 4396B to drive a ~20 mVpk sine wave from 100 kHz to 10 MHz. We didn't see anything.