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

15:00-16:30 @ See``Vogh

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

[[http://gwdoc.icrr.u-tokyo.ac.jp/cgi-bin/private/DocDB/ShowDocument?docid=4450|Slides by Michimura]]

== Conclusions ==
 * Use 2mm dia, 3mm long Sm``Co magnets (Ni coated) for BS TM.
 * Use 2mm dia, 5mm long Sm``Co 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.

== Details ==
 * 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 85mm^3^, whereas that for Ad``Virgo is 2.6mm^3^. Also, actuation efficiency of BS TM for KAGRA is 200 times larger than that of Ad``Virgo.
 * 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.
 * Sm``Co is a bit brittler than NdFeB, but we can be careful enough since aLIGO and Ad``Virgo people are currently using them. Also, Ni coated Sm``Co is available.
 * [[https://dcc.ligo.org/LIGO-T0900061/public|LIGO-T0900061]] summarizes the reports on Barkhausen noise.
 * [[https://dcc.ligo.org/LIGO-M0900034/public|LIGO-M0900034]] summarizes the difference between Sm``Co and NdFeB.