Minutes of the Special VIS+ISC Meeting on BS Magnets (Nov 4 2015)

15:00-16:30 @ SeeVogh

Participants: Aso, Flaminio, R. Takahashi, Barton. K. Yamamoto, Hirose, Shimoda, Miyamoto, Michimura

Conclusions

Use 2mm dia, 3mm long SmCo magnets (Ni coated) for BS TM.
Use 2mm dia, 5mm long SmCo magnets (Ni coated) for SRM/SR2/SR3 TMs.
No change for magnets on PRM/PR2/PR3 TMs.
Put a hidden magnet with an opposite polarization inside the flag base on IM to cancel the magnetic moment
The size of the magnets for IM should be decided soon (according to the magnetic noise calculation).
Wait until Friday for comments from VIS and ISC people for final decision.

Shimoda-kun's calculation show that the magnetic noise is below requirement by an order of magnitude, when the magnetic field fluctuation of the Earth is used.
Virgo experience show that the magnetic field fluctuation could be 50 times higher.
Volume of the current magnet for BS in KAGRA is 85mm³, whereas that for AdVirgo is 2.6mm³. Also, actuation efficiency of BS TM for KAGRA is 200 times larger than that of AdVirgo.
From lock acquisition point of view, magnetic moment of magnets on BS TM can be reduced by upto 1/900. For SRM TM, upto 1/40.
We decided to reduce magnetic moment for BS TM by roughly 1/40, SRM TM by roughly 1/30.
SmCo is a bit brittler than NdFeB, but we can be careful enough since aLIGO and AdVirgo people are currently using them. Also, Ni coated SmCo is available.
LIGO-T0900061 summarizes the reports on Barkhausen noise.
LIGO-M0900034 summarizes the difference between SmCo and NdFeB.