Looking at the result 32017, the next steps would be:
- Measure the transfer function from the PEM injection to the IMMT2 QPDs (or, is it possible to form such transfer fuction from the exsiting data??): if some peaks can be found around 120 Hz-ish in the measured transfer function, this would suggest the structure which has the peak resonances might be due to the stack itself, not STM1/2 supports, though I am well not sure which frequencies the mechanical resonance peaks of the stack system come in desgin.
- If such peak structures in the measured transfer function is confirmed, I think it would be nice to open the IFI chamber to check if there would not be any stackings in the stack legs as soon as possible, hopefully. If something is touching, just removing them would improve the sensitivity drastically; according to R. Takahashi-san, he could not confirm ALL the pedestals of the stack in the past. Anyway, the sooner done, the longer we can have time to consider the countermeasure.
- If nothing touched, maybe replacing the IFI stack with a certain better vibration-isolation table (not necessarily with the TMS-VIS-like) might become a serious option...