M. Todd, C. Cahillane, S. Dwyer

Yesterday during commissioning, I ran some intensity noise injections. The excitation amplitudes were much too small to start out, so I tuned them as I went along to make sure we had enough coherence in DARM while not driving too hard (trickier at LFs). The injections were 9 bandlimited excitations injected into ISS_SECONDLOOP_EXCITATION, with the injection script located at /ligo/gitcommon/iss_noise_measurements/iss_noise_injection_caller.py. Should these need to be ran again, the individual bands can be uncommented and so the script can run all the bands sequentially by itself now that the excitations are tuned well.

When I inject below 100Hz, there starts to arise some non-linearities in the ISS_SECONDLOOP_RIN channel, which then get injected into DARM. These get more prevelant and higher amplitude as the excitation frequency band goes down. This is shown in Figure 1.

I will analyze the results and comment on this post with the calculated transfer function of intensity noise to darm using this data. The data can be found in the bandlimit-named folders in /ligo/gitcommon/iss_noise_measurements/data/. Most of the injections also have pictures of the coherence and live psds during the injection.

Notes:

The excitations were tuned by looking at the PSD of the DARM error signal while locked, using quick_psd and mapping the shape and amplitude to match it with a slightly higher amplitude. The amplitude will be far too weak as the TF is not known but by tuning the amplitudes by hand while injecting, I saw that the excitation amplitudes need to be about 15 orders of magnitude greater; but the shape of the excitation worked pretty well.