Following alogs https://alog.ligo-wa.caltech.edu/aLOG/index.php?callRep=7609 and https://alog.ligo-wa.caltech.edu/aLOG/index.php?callRep=7604, I have performed measurements to evaluate:
- The origin of the 6.18Hz noise
- The effect of the feedforward
- The transmissibility while driving HEPI (to be above the noise floor of the instruments when the ISI is controlled and excited by the ground motion)

Results from the past:
1- There was already a tiny feature (barely noticeable) around 6Hz when the isolation loops were closed for the first time on June 20th (https://alog.ligo-wa.caltech.edu/aLOG/index.php?callRep=6832). In the presented spectra, feed forward control was not engaged.
2- In https://alog.ligo-wa.caltech.edu/aLOG/index.php?callRep=4874, feedforward stability analysis was performed on ETMY. It was shown that the risk of instability was limited when using the feedforward from stage 0 to stage 1.


New results:
I drove (white noise) simultaneously all HEPI actuators in Cartesian basis such that GS13s are above their noise floor when the ISI is controlled. Then, I measured transfer functions from the HEPI L4C to the GS13 in two configurations (with and without feedforward). Transfer functions are presented in attachment.
1- The coherence below 100mHz is low since the white noise was injected by the HEPI Feedforward path (parallel path to isolation path) and the HEPI is controlled in position at 100mHz
2- The 6.18Hz spike is not visible in Y, RZ and barely visible in Rz. It is more visible in the vertical degrees of freedom.
3- Feedforward is decently tuned to tackle the [2;15]Hz frequency band. The zero created by the feedforward is around 6.18Hz (Is it coincidence?).
4- There is no coherence at 6.18Hz from the input motion to the GS13. (spikes don’t seem to be created by the input motion)
5- Isolation at 1Hz is pretty good (80dB)
6 - Note a pretty high transmissibility on Y and Z around 30Hz created by the zero of HEPI.