Eddy currents simulation and Force test in the reference mass

1
Eddy currents simulation and Force test in the
reference mass
Virgo Detector Meeting 04/10/2006

• Francesco Cottone
• Virgo Thermal Noise Group - INFN Perugia

2
Outlines

• Motivations
• Possible solutions for Eddy currents and force
  reduction in reference mass
• 3D Finite Elements Analysis of Eddy currents and
  magnetic force reduction by means of hats
• Experimental results of hats shielding properties
• Conclusions

3
Motivations

• the eddy current dissipation enters as a limiting
  quality factor Q_eddy in the fluctuation spectrum
• (G.Cagnoli et al, Rev. of Sci. Instr. 69 (1999),
  2777-2780)

where Hpend(?) is the force-to-displacement
transfer function
and Q_eddy
4
Motivations
its hard to evaluate analitically geometrical
factor related to reciprocal distance between
the RM and magnet for a real complex design
Curtesy of P. Puppo
5
Motivations
M.Punturo 17/10/2005 STAC meeting

• the eddy current dissipation must be reduced
• thermal noise requirements are reached if the
  magnetic moment of the magnets is reduced to
  ?510-3 Am2 but with a factor 10 lees in force

6
Possible solutions

• Complete solution
• Reference Mass made of a dielectric material for
  Virgo
• Provisional solution
• reduce the magnets intensity (magnetic moment
  ?510-3 Am2)
• stainless steel reference mass
• Qeddy 1.8108 instead of
• Qeddy 9.4105 for Aluminium
• Problems of magnet replacement
• it is necessary to minimize the shut-down period
  of the ITF
• it is possible to damage the post or the mirror
  when detaching the magnet from the post ,
  probably causing a large dust pollution on the
  mirror face.

M.Punturo presentation of 7/12/05 detector
meeting
7
The magnetic shielding hat
M.Punturo - 7/12/05 detector meeting
high permeability material bend the magnetic
force lines inside itself due to magnetic
refraction µµ0µrB/H (Ht1/Hn1)/(Ht2/Hn2)µ1/µ2t
g?1/tg?2
H1
?1
?2
H2
8
3D FEM of RM and Magnet with Hat
mag
Finite Element Modelling of 1/4 slice of
Reference Mass ( COMSOL multyphysics )
Hat
9
3D FEA of Eddy Currents
Resistive Heating is defined as the density of
power lost by the Eddy current in a conductive
medium dPJE?J2J2/? W/m3
Finite Element Modelling of 1/4 slice of
Reference Mass ( COMSOL multyphysics ) Magnet
Dipole moment M0.05 Am2
10
2D FEA of Eddy Currents moving mesh
2D Finite Element Modelling of Reference Mass
section ( with COMSOL multyphysics ) Its has
been used the moving mesh analysis with relative
displacement along z axis of 10 nm max
11
3D FEM of Magnetic Force on coil
Finite Element Modelling of coil and magnet with
different hats ( COMSOL multyphysics ) Current
in the coil I10 A Relative distance along
symmetry axis d10mm
12
Experimental setup for Magnetic Force shielding
measurements
balance resolution 0.1mg estimated error lt1
13
Results comparison between force measurements
and simulated data
Magnetic field in the center of coil versus
various hat geometry
magnetic force perpendicular to coil plane
(component Fz) vs different geometry of hats
14
Conclusions

• It is possible to reduce inducted currents
  through ferromagnetic hats as temporary solution
• The shielding power depends on magnetic
  permittivity of material and geometry of hats and
  increase with an increasing of the surface magnet
  covering and thickness (up to 1 order of
  magnitude)
• Simulated data confirm a decreasing of Eddy
  currents and magnetic force for different
  covering configurations down to one order of
  magnitude
• Experimental measurements shows a 4-5 lower
  shielding power then simulated data maybe due to
  not pure steel hats (80 iron and 20 lead)