Based on noise budget injections that we have been running since the vent recovery, I put together a low frequency LSC and ASC sub-budget, and made a total budget.
The results show that the noise that limits DARM from 10-20 Hz is very well understood by LSC and ASC noise. Above 20 Hz, ASC noise drops quickly to be more than a factor of 10 below DARM. LSC noise first drops to a factor of 3 below DARM at 20 Hz and then closer to a factor of 5 by 40 Hz.
For the ASC subbudget, I focused on the "usual suspects", choosing to show the HARD loops as well as the MICH ASC. During vent recovery, both MICH ASC loops were put into a higher bandwidth state to counter various instabilities. I'd like to try to put them back in a lower noise control design, which may directly lower DARM noise to at least the level of DHARD Y in the ASC budget. This budget incorporates the slightly lower noise DHARD that I implemented yesterday. Since the vent, CHARD P noise has been the lowest it has been for O4 due to the improved isolation of the HAM1 chamber. Notably missing here is CSOFT P, which I haven't been able to measure yet, but tends to strongly couple to DARM from about 10-15 Hz. The other soft loops couple less, and are not very likely to have a significant effect on this result.
If we can achieve some reduction of the ASC noise, we will reach the LSC noise limit, which is limited by our ability to fit the feedforward well at low frequency.
This noise budget does not include DAC noise or OSEM noise, although we've previously shown those to be smaller than the LSC and ASC noise, especially above 20 Hz.
These budgets were made using excess power projections. The DARM trace and ambient noise levels were chosen from an observing time last night, and the coupling functions were measured recently in June.
