Parametric and Alignment (In)stability

This page collects work on tasks related to Parametric Instabilities and Arm Cavity Alignment Instabilities, and is more broadly to the optical design of the arm cavities.

General considerations:

• starting with ET HF but consider a Voyager-like (no Xylophone, 120k) alternative for comaprison

Model preparation (work in progress):

• Finesse models with beam shape feature enabled
• Comsol/Ansys model of mirror surface motion

• We have a tuned, mode-matched Finesse model of ET-HF. This will be used by PIs, LG33, Arm Length and Alignment stability groups
• Ansys models are in development
• LIGO PI tools are being adapted for ET/LIGO comparisons
• Started study of alignment stability in a single ET-HF cavity

• Single arm model for ET-HF
• Compute aligment sensing matrix, alignment to longitudinal coupling
• Compare different scenarios (arm length, laser power, mirror size) depending on suggestions from other groups
• Conclusion should be a first intuitive understanding how alignment and radiation pressure instabilities change when moving into the ET parameter space

We have now fast and tested Finesse models for PI resonances in Advanced LIGO and will adapt this to an ET design. The aim is to a) answer the question if PIs are significantly worse or better in the current (HF) design and b) to prepare the tools to answer this question for designs options emerging this week.

FEM:

• Produce Finesse-compatible maps from Ansys model. Compare these to Comsol maps
• Try 3 geometries of ET-HF mirrors
• (?) Try different materials, ET-LF mirrors, …

PI modelling:

• ET current design vs aLIGO - do we expect more PIs? Single arm vs full interferometer “worst case scenarios”
• Arm Cavity RoCs vs number (and gain) of PIs
• Other parameters/configurations: different mirrors, LG33 case, recycling cavity influence,…

Progress since the workshop:

• Single arm Finesse model for ET-HF made
• COMSOL model of the ET-HF mirrors in progress:
  • We have first maps of modes <10kHz, full range will be up to 60kHz to match out LIGO modelling.
  • eg:
  • First indications: higher mode density in ET mirrors that LIGO (expect a factor 2-3 more mechanical modes over the same frequency range)
• ANSYS model in progress: focus on material choices.
• Finesse model to compare 'worst case scenario' ET and LIGO arms ready to go once ET maps are prepared
  • Counts modes as 'unstable' if they have a parametric gain R>1 within 2kHz of their computed mechanical frequency
  • eg: forest_plot_multidetector.pdf

Coupling of alignment fluctuations (non-longitudinal suspension modes) increases with the beam size on the mirrors roughly as w^6. However, the details are complicated due to radiation pressure splitting the degrees of freedom into hard and soft modes. We want to answer the question whether the g factor of the cavities can be taken simply from arm length and clippong losses or whether a trade-off might be required. This again will be directly lined to arm cavity design considerations this week.

• Compute aligment sensing matrix, alignment to longitudinal coupling

home