MEG Cryostat
Fabrizio Raffaelli
INFN sezione di Pisa
The MEG experiment is being built by a collaboration of INFN (Pisa, Pavia, Lecce and
Roma I),: KEK (Japan), ICEPP (Japan), Waseda University (Japan), PSI (Switzerland)
and BINP (Russia).
The aim of the experiment is to reveal the possible decay m+->e+ g with a very high level
of sensitivity (BR<10-13) with respect to the normal muon decay.
The experiment will be performed at Paul Scherred Institut Switzerland. It includes two
main detectors: a liquid Xenon electromagnetic calorimeter for photons detection and a
magnetic spectrometer for positrons detection.
Period 2004-2006
INFN of Pisa had designed and built the liquid Xenon cryostat and the internal
supporting structure of the photomultipliers. The design has been carriedout using the
ASME VIII div 2 and the ANSYS program. The tecnological difficulties are realated to the
limited space avaible to make the cold and the vacuum chambers. The cleaness
requirement are very stringent. Furthermore in some aresas the material must be
minimized. The thermal imput must be limited to 100 Watt.
MEG Cryostat
Experimental area at Paul Scherred Institut (PSI)
Liquid Xenon cryostat
Materials and space limits
Vacuum chamber
Cold chamber
CORBA Magnet
Cryostat must be positioned with the
internal detecting structure with a
precision of a couple of mm respect the
Superconducting magnet (Corba).
The limited space between the magnet
and the cryostat dose not allowed to
make the flanges of the cold chamber
enough stiff. The material used must
have a magnetic relative permeability
less of 1.008 to damage to the
superconductive magnet.
Usually the austenitic stainless steel
have a relative permeability equal or
bigger or than 1.01. For this reason
many parts are made in 25 Cr-20Ni SA240 Plate ASME 310S.
Limted material areas
In the angular regions between +/-630 and
longitudinal +/-250. the material thickness
must be limited as much as possible.
CORBA Supercondacting magnet
Warm magnet
ouround the S.M.
are used to to cancel
the magnetic field i
saome areas.
Materials oround
with magnetic
permeability bigger
than 1.008 can casue
forces on the cold
mass such to break
the magnet
Corba magnet: central part is made by a
superconducive magnet with an alluminum
cryostat such that will be as mauch as possible
transparent to the particles.
Some tecnical requirements
Cold Chamber:
Working temperature 165K; design and test pressure respectively of
0.2 Mpa e 0.4 Mpa. Leak rate below 10-9 atm cm3/sec. This last
requirements implied the use of metallic seals.
Typical pre-setting values are 285 Newton/mm.
Welding procedures must guarantee the cleanness of inner surfaces
Xeno must not enter in front of the detecting system
Vacuum chamber:
External pressure 0.1 Mpa at the working temperature of 20 0C
The front part of the internal structure must be adherent to the cold
windows
Peek
Aluminum 5083-0 AISI 316L
Inner Vessel flanges
Cold chamber
the space avaible has force us to
limited the flanges dimension and
study the positiong of the welding.
Intermediate test during construction.
Cold chamber:
After the warm and cold chambers
have been welded. We test them with
helium. Sealing the volume using
covers with special compound to verify
that no leak are present in the
weddings and materials. The Fea model
in this configuration shown excessive
deformation that does not allowed to
make the test on the cold chamber. This
test was possible only adding an
internal structure to stiffening the
chamber.
Intermediate test during construction
Cold chamber: case open with cover not connected under
an external pressure of 0.1 Mpa. In the first attemped to
make vacuum. We stopped the test after the structure
move of 20 mm loosing the thightness.
The model shows the max deformation of 40mm and high
stress on the inner ring.
Intermediate test during construction
Vacuum chamber: no cover under an external pressure of 0.1 Mpa. In the lower part
reinforced plate were welded in the corrispondence of the supports.
Test in pressione della camera fredda chiusa
Pressure test of the cold vessel max displacement
measured at the pressure test 4 bar
Dial indicator 1 disp.= 1.2 mm
Dial indicator 2 disp.= 1.25 mm
Dial indicator 3 disp.= 0.55 mm
Dial indicator 4 disp.= 0.055mm
1
4
3
2
1.7 mm Dial. N. 2
1.3 mm Dial. N.1
cover
According to ASME the thickness of flanges are undersize considering the allowable stress.
However we reinforced the weldings with a large fillet.
front wall
DESIGN
PRESSURE
ASME
rule
reference
MINIMUM
REQUIRED
THICKNESS
ACTUAL
THICKNESS
Inner wall
(cylindrical
shell)
0.2 MPa
External pressure
UG – 28
5 mm
18 mm
Outer wall
(cylindrical
shell)
0.2 MPa
Internal pressure
UG - 27
1.7 mm
8 mm
Flat head
(plate)
0.2 MPa
UG - 34
17 mm
20 mm
Cover
(plate)
0.2 MPa
UG – 34
24 mm
24 mm
WALL
cover
outer wall
inner wall
front wall
head
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After the qualification test we do the final
machining to prepare it for the windows
weldings.
Welding foil of 0.3mm to garantee the
sealing and cleneass.
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Windows designs
0.3mm sheet to seal the liquid Xeno
Honeycomb and carbon fiber panel
Cold chamber
0.3 mm sheet window
vacuum chamber
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Welding of the parts. Three days for welding the foil and 3 days to
check and repair leaks test box.
•WARM WINDOW
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Vacuum window test
We repet thress time the measuraments to verify the
stability of the vaccum window. The menbranal stress in
the central part is 250 Mpa. We were more anbicius
thinking to use .15mm sheet, but we had weldings
problems, finally the thickness used was 0.3mm
•H2O
•100
mm
•1E,1
C
•2E,2C
•3E,
3C
Four steps: 0.6 bar, 0.9 bar, 1.2 bar, 1.5 bar
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Test of the vaccum chamber
Dialed indicator positions
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A good
agreement on the
measurements
2mm were measured
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2mm misurato
Lateral plugs Small window 0.1mm SS. sheets
Liquid Xeno
Vacuum
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21
2mm misurato
Cold window first panel carbon fiber aluminum honeycomb
High module pitch carbon fiber 550Gpa
6 plies (0/+45/-45)s Total Thickness .7mm per skins
Honeycomb thickness 19 mm
Density 50Kg/m3.
Glued with film adhesive Redux 312UL.
Material with low outgassing rate resin cyanate ester
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2mm misurato
First Panel
The film adhesive used Redux 312UL (-55 0C) is
not suitable for the nitrogen temperature (-200
0C)
The pitch fibers used are very brittle.
Delaminations around the holes are visible.
The construction method was not adequate for
the precision required. Mechanical calculation
and Fea model were made to verify the stability ,
the strength and the stiffness verification .
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2mm misurato
•Fea analysis stiff99 Ansys
Stacking sequence of the central part.
CF lamina properties
Honeycomb properties
Max disp. 0.65 mm
Fea model ¼ with symmetry boundary condition and hinged edges
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Strength ratio
The coupling factors are related to the theory of
coupling of the transverse properties of the laminate.
To be able to apply the index factor we need to have
nine stress limits and three coupling factors. Used
symbols x fiber direction, y perpendicular to the
fiber, c compressive t tensile z is perpendicular to
the lamina, f means allowed
Layer 13 stress direction of fiber Bottom Mpa Layer 13 stress direction of fiber top Mpa
9
Ts 0  2.193  10 Pa
tensile strenght parallel to the fibers
6
Ts 90  44.057  10 Pa
Tensile strenght perpendicular to the fiber
3
TC0  58.7 10 psi
6
TC0  404.722  10 Pa
6
Fst  882.529  10 Pa
6
SS  89.632  10 Pa
Tensile compressive strenght parallel to the fibers
bending strenght
Shear strenght in plane
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2mm misurato
Linear buckling studies
27
2mm misurato
The shell model does not see the effects on the edges where we change the cross section.
In the first panel the transition was not proper done:
1) A layer of pre-preg fabric should be interplayed each two ply of unidirectional prepreg with a 1 inch of overlap.
2) At least three plies of fabric should be there and since must be placed staggered. The
total reinforcement should be extended for 6 cm.
3) A potting (epoxy loaded with glass fiber was used to assure the continuity of the
honeycomb.
Fabric pre-preg overlapped
28
2mm misurato
Insufficient transition area and internal
reinforcement, cause the initial breaking
of the panel.
Pressure test Breaking at 3.7 bar.
The skin probably break on the side in
contact with the stainless steel and
after the panel is so weak that breaks
on the other side.
The material is weak out of plane and
the limit for inter laminar shear stress
are very low respect the in plane
tensile stress.
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2mm misurato
The sheet after the panel breakage
During the test the displacement were in
agreement with the model
Sheet wrinkles
Strain gage position
4E
14.5
mm
1E,1
C
2C
2E,3C
3E
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30
2mm misurato
New Panel design .
Different material:
1) Fabric plain wave
2) Fiber low intermediate high module T300
3) Resin epoxy space approve
4) Honeycomb in aluminum
5) Small filling parts in Rohacell 51
Different construction method All parts were glued at room temperature
with a post-curing a 50-60 0C. Structural glue qualified at low temperate
(Hysol EA 936 Henkel). Glue data at the nitrogen temperature shear
strength of 27.6Mpa (With sinle lap shear test aluminum on aluminum
(2024T4). The final skin thickness was 1.2mm obtained with 8 plies
+90 0 -90 to take in account the low module of T300. We made in home
the test on the Carbon fiber material at the nitrogen temperature.
Internal Z reinforcement.
31
2mm misurato
Test on the materials
SR= 518-680 Mpa
Alr 1.2-2.9%
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32
2mm misurato
Internal Z reinforcement
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Stampo pelle interna
2mm misurato
Stampo per
Rinforzo interno
Stampo pelle
interna
34
34
2mm misurato
35
2mm misurato
Final test on at the nitrogen temperature
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2mm misurato
The use of ansys allowed us to have information on the behaviors
of the mechanical structures analyzed driving us on the design
solution s like:
1) Positioning of the stiffening
2) Analyzed the shaping of the thin sheet windows
3) Analyzed the Caron fiber honeycomb panel.
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