Minutes of the bKAGRA1 configuration meeting with MIF & MIR on 2015/2/26

2015/2/26 15:00 - 18:00

Main conclusions

• Although negative g-factor has some advantages over positive g-factor, the difference from the viewpoint of ASC is not so drastic. Therefore, the choice of g-factor (~vendor choice) should be mainly based on other factors, like polishing quality, costs, etc.
• We don't yet have enough information to decide whether AR IBF is necessary or not.
• Output optics configuration needs more discussion

Action items

• Perform the FFT simulation with RFSB to see the effect of inhomogeneous ITM substrate. (Hirose, H. Yamamoto)
• Check the IMMT feasibility for positive g-factor (Aso)
• Determine requirements on the ROC error for positive g-factor (Aso)
• Continue the discussion of the output optics configuration via email and remote meetings. (Somiya, Aso, everyone)

Discussion

IFO configuration

• PRFPMI relies on low frequency sensitivity which is dangerous as we all know that technical noises are larger at low frequencies.
• Plot IR vs Cost for many configurations ?

Mirror polishing

• Possibilities of vendor A and B: A has better quality but more expensive.
• Vendor B does not have a necessary reference sphere for 1.9km. Therefore, B may only be able to make 7km (positive g-factor) mirrors.
• Does vendor B has experience in coating on sapphire ? In the case of silica mirrors, coating determines the final figure error.
• ASC noise comparison between positive and negative g-factors: In terms of IR, the difference is very small (1Mpc out of 170Mpc), if we assume the ASC noise is shotnoise limited.
• The safety mergin for the ASC noise is, however, different for positive and negative. 10 times increase in ASC noise will degrade IR by 8Mpc for positive, but less than 1Mpc for negative.
• Error tolerance of ROC for the positive g-factor should be given
• In the case of positive g-factor, we may be able to make the beam size a bit larger
• Necessity of the AR IBF should be judged based on RF SB simulations.

Output optics

• In order to evaluate OMC Gouy phase is acceptable or not, we need to know the HOM waiting factor at the AS port.
• Majority opinion from experienced commissioning experts is that the output faraday is indispensable.
• aLIGO output Faraday is suspended.

• Why OMMT mirrors are so expensive ? -> Assumed the same price as IMMT, which is expensive because of the error tolerance.OMMT mirrors (as well as IMMT) could be cheaper.

• OMMT suspension could be also cheaper ?
• If we ommit OMC and go with RF, the RFPD has to be suspended.