CLAS12 TORUS Magnet

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CLAS12 - TORUS Magnet
Volker Burkert Jefferson Lab
(Representing the work of the ITEP-Kurchatov-TRINI
TI JLab group)
Conceptual Design and Safety Review of
Superconducting Magnets Jefferson Lab September
26-28, 2006
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Outline

• Brief overview of current CLAS Torus
• Requirements for CLAS12 and TORUS geometry
• TORUS coil shape, s.c. margins, general
  properties
• Magnetic field
• Forces, stresses
• Coil construction and magnet assembly
• Protection
• Heat budget and cooling scheme
• Future development
• Summary

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Hall B Overview

• Hall B currently houses the CLAS detector. CLAS
  will be modified and upgraded to CLAS12 which
  will be the only large acceptance, multi-purpose
  detector for fixed target high energy electron
  scattering experiments.
• CLAS12 will operate with an upgraded luminosity
  of 1035 cm-2s-1, an order of magnitude increase
  over CLAS. This is achieved by replacing the
  existing torus magnet with a smaller toroidal
  magnet at forward angles, and by adding a new
  central detector based on a solenoid magnet.
• With these capabilities, CLAS12 will support a
  broad experimental program in fundamental nuclear
  physics.

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CEBAF Large Acceptance Spectrometer (CLAS)
Torus magnet 6 superconducting coils
Large angle calorimeters Lead/scintillator, 512
PMTs
Gas Cherenkov counters C4F10 Gas, 216 PMTs
Drift chambers argon/CO2 gas, 35,000 cells
Electromagnetic calorimeters Lead/scintillator,
1296 PMTs
Time-of-flight counters plastic scintillators,
684 PMTs
Operating luminosity 1034cm-2s-1
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CLAS Torus Installation in Hall B (1995)
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CLAS Torus Coils
View from the upstream end.
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Experience with CLAS Torus

• CLAS Torus design and performance
• Stable operation of the Torus magnet for 10
  years.
• Heavy-weight design allows use of cryostat to
  support tracking detectors.
• Coil geometry for minimum angle coverage of 8o.
• Six coils in separate cryostats mounted on common
  warm support.
• 5 warm cross bars covering the entire angle range
  resist asymmetric magnetic forces and
  gravitational loads.
• Lessons learned
• Separate cryostats for all coils makes accurate
  alignment difficult.
• Magnetic out-of-plane forces limited maximum
  operational current to 88 of design value.
• Coil shapes prevents reaching much higher
  luminosities.
• Difficulties to access detectors for maintenance
  and repair.
• Forces between solenoid and torus limit use of
  polarized target.
• Wide coil cryostat causes significant reduction
  in acceptances at forward angles and for
  multi-particle events at higher energies.

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Requirements for CLAS12 TORUS

• Provide magnetic field for charged particle
  tracking for CLAS12 in the polar angle range from
  5o to 40o.
• Magnetic field distribution that emphasizes high
  ?Bdl (3-4 Tm) at forward angles, and low to
  moderate ?Bdl (1 Tm to 2 Tm) at large angles,
  approximately matching the particle spectrum
  expected at 11 GeV electron beam energy.
• Maximize acceptance in azimuthal angle, gt 50 _at_
  5o, gt 95 _at_ 40o. This requires cryostat width of
  lt 95 mm.
• Allows re-use of existing detectors downstream
  of the magnet.
• Open geometry for charged particle identification
  and for photon and neutron detection.
• Allow the operation of a polarized target at the
  beam axis, with its own magnetic field, i.e. no
  other external field is permitted.
• Limit magnetic interference with close-by
  magnetic field-sensitive particle detectors, e.g.
  photomultiplier tubes.

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CLAS Torus installation in Hall B (1995)
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CLAS12 - TORUS Design Options

• Option 1 Concept based on CLAS Torus
• reduced coil size using SSC cable with higher
  current density
• cold common hub for coils
• Option 2 Concept based on different winding
  technique
• developed by ITEP-Kurchatov-TRINITI (IKT) group
  within the CLAS12 collaboration
• concept is based on laminar winding technique

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CLAS Torus Coil and possible CLAS12 Torus Coil
Modified Oxford Design
CLAS Torus Oxford Design
Width 144 mm
Width 120 mm
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CLAS12 - TORUS Design Options

• Option 1 Concept based on CLAS Torus
• reduced coil size using SSC cable with higher
  current density
• cold common hub for coils
• Option 2 Concept based on different winding
  technique
• developed by ITEP-Kurchatov-TRINITI (IKT) group
  within the CLAS12 collaboration
• concept is based on laminar winding technique

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TORUS Design with IKT Group

• For the conceptual design we are collaborating
  with research institutions in Russia (Moscow
  Troitsk).
• Institute for Theoretical and Experimental
  Physics (ITEP, Moscow)
• Kurchatov Institute, Russian Research Centre
  (Moscow)
• TRINITI (Troitsk)
• ITEP is a long time collaborator with JLab and
  Hall B with expertise in design and construction
  of large scale detectors for particle physics,
  and construction of magnets.
• Kurchatov Institute has experts on applied
  superconductivity. They developed the laminar
  winding technique for superconducting magnets.
  They built a number of moderate scale magnets
  based on this technique.
• TRINITI has experts on superconducting
  technology, and has test facilities.
• ITEP has been a long time collaborating
  institution with Jefferson Lab and Hall B. As
  collaborators they provide scientific and
  engineering manpower in support of the project,
  and have been involved in CLAS and in CLAS12
  detector developments.

Presented design is based on the IKT approach.
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CLAS12 - TORUS Design Concept

• TORUS magnet with six trapezoidal shaped coils
  covering geometrically the forward 40o,
• limits toroidal field to forward region, where
  needed
• allows to re-use existing forward CLAS detectors
• keeps area in front of coils free
• Individual coils mounted on common cold support
  allows improved access to forward angles and
  better coil alignment to generate more symmetric
  magnetic field distribution.
• Reduce total width of cryostat vacuum shell at
  the front end facing the production target.
• Toroidal symmetry provides B0 at symmetry axis.
• Fast falloff of magnetic field with distance to
  beam line beyond the coil region resulting in
  little interference with detectors and with the
  solenoid field.

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CLAS12
Forward Calorimeter
Preshower Calorimeter
Forward Cerenkov (LTCC)
Forward Time-of-Flight Detectors
Forward Drift Chambers
Superconducting Torus Magnet
Inner Cerenkov (HTCC)
Superconducting Solenoid
Beamline Instrumentation
Inner Calorimeter
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TORUS Magnet
CLAS12
Institutions ITEP, Kurchatov, TRINITI, JLab
40o
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TORUS - Vacuum Shell Geometry
The vacuum shell occupies the shadow zone of its
leading edge. It provides sufficient space for
- stringers re-enforcing its flat walls -
helium flow channels joints - current joints
near the rear edge. A rear stiffener can be
incorporated that is wider than the front one.
Target position
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A stainless steel cone is used to bear the
central forces
TORUS Magnet - Coils
CLAS12
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Super Conductor RD forCLAS 12 Torus and
Solenoid
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TORUS Magnet - Conductor
CLAS12
1.1 mm
Mid-thickness1.156 mm
(De-Keystoned )
Keystone Angle1.01
11.68 mm
11.68 mm

• The SSC cable will be rolled to provide a
  rectangular cross section. The rolling will
  reduce both critical current and n index.
• Make the cable more suitable for intended use in
  the TORUS laminar winding technique.
• Expected to improve the conductor stability.
• Exact values will be determined in a tests
  program planned with the re-rolled SSC cable.

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Torus Peak Field Load Line
CLAS12
Cable Measurements CLAS12 Torus
Approximation used in design
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Temperature Margins
CLAS12
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Margins on the load line
CLAS12
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Torus Magnet - Parameters
CLAS12
2650
2950
140
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TORUS - Effect of Split Coils
CLAS12
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TORUS - Effect of Split Coils
CLAS12
Magnetic field distribution at the inner turns of
a double pancake. Field peaks use to be located
at the corners of the winding. Splitting of the
winding suppresses them at corners disposed near
the horizontal leg of the winding and reduces
maximum magnetic field. .
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TORUS Forces in Windings
CLAS12

• Force component distributions along the coil
  windings.

• x-component moment relative to point (0, 0.1631,
  2.8247) along the coil winding.
• The total moment is -2.735 106 Nm.

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TORUS - Magnetic Field
CLAS12
3 m
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Solenoid-Torus Magnetic Field
CLAS12
Field in TORUS sector mid-plane
T 5o
15o
10o
B(Gauss)
B(Gauss)
Solenoid
Torus
40o
20o
30o
B(Gauss)
B(Gauss)
B(Gauss)
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TORUS Construction
CLAS12
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TORUS - Design principle
CLAS12

• The laminar winding was chosen as the main
  design principle. It consists of Rutherford
  cable pancakes adhesively bonded to metal sheets.
  The adhesives in use were developed in aircraft
  industry. They are very strong and reliable.
• An important feature is that the conductor is not
  overstressed in this type windings. It is a
  structure that reacts to the load. For this
  reason there are no problematic mechanical
  excitations in laminar windings.
• Strong thermo-magnetic excitations are eliminated
  due to using a conductor with VAC index n lt 20
  ).
• There is no scale dependence of the quench
  current. Conductor current density can be the
  same in a large SC magnet as in a small one if
  protective mode keeps stored energy in the
  winding.
• High current density in detector magnets provides
  a lot of benefits.
• ) approximate formula E
  E(0.1mV/cm)In.

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The Basis of the Design

• The proposed design is based on successful
  experience of the RRC Kurchatov Institute team.

• The photos illustrates the laminar winding
  techniques and adhesive bonding used here for the
  inductor of a linear motor. Cryogenics (1992)
  32S, (Proc.ICEC 14) 328-331.

Details and experience with this technique are
described in a technical paper E. Yu. Klimenko,
E. P. Polulyakh, Laminar Windings, presented
at ASC/04 (2004)
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Reliability of Technique
This planar separator was exploited actively for
more than 5 years. It is still operational.
IEEE Trans. on Magnetics (1988) 24, 882-885.
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TORUS Main parts of cold unit
CLAS12
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TORUS Coil Assembly
1

• Double pancake windings are attached to
  structural sheets (stainless steel, 2 mm)

• The structural sheets provide strength and
  stiffness of the coils. The margin of strength of
  2mm thick sheets is very high.

• The sheer strength of the adhesive (10MPa)
  provides a 5-fold margin for the most heavily
  loaded part of the conductor.

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TORUS Coil Assembly
2

• Arrangement of cooling tubes inside pancake. They
  are adhesively bonded to the middle and outer
  structural sheets.
• Coils are cooled indirectly by thermal
  conductivity of the structural sheets.
• Positioning of the honey comb structure.

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TORUS Coil Assembly
3

• Mounting of lateral sheets.
• Adhesive bonding of the entire coil assembly.
• Both pairs of inlet and outlet tubes are led out
  at one side of the pancake alternating for even
  and odd coils for the assembly of the entire
  cooling system.
• The bonding process occurs inside evacuated
  rubber bag at 170?C.

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Structural Stress Strain Analysis
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?max2.1 MPa
CLAS12
TORUS - Structural Stresses

• Structural sheet stress at nominal current
• The holes allow placing warm supports between the
  flat walls of the vacuum shell.
• Maximum stress is 2.1 MPa
• The margin is very large.

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CLAS12
TORUS - Structural Strain
Smax8.7 ?m

• Structural sheet strain at nominal current.
• The maximum strain is extremely small 8.7 ?m.

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• TORUS
• Assembly procedure

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TORUS Assembly procedure
4
Stiffener

• Mounting of peripheral stiffener and adhesive
  bonding.
• The shape of the bottom stiffener provides a
  dovetail junction with the structural cone.
• The adhesive bonding strength of the stiffener
  resists the moment Mx

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TORUS Assembly procedure
5

• Joining the coils with the structural cone by
  means of dovetail junction and adhesive bonding.

Structural cone
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TORUS Assembly procedure
5
(continue)

• Assembly of the cold unit

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TORUS Assembly procedure
6

• Fixing the six coils positions.
• Mounting the current joints and cryogenics
  connections.

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TORUS Assembly procedure
7

• Mounting of shields and cooling lines. Individual
  sectors are fixed with temporary supports.
• Every sector consists of several insulated strips
  to prevent the shields from collapsing in case of
  a quench.

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• If the shield is solid, eddy currents can press
  it to the coil in the case of a quench. It is
  necessary to use a shield dispersed into several
  strips to limit eddy currents.

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TORUS Assembly procedure
8

• Mounting the vacuum shell.
• Sectors are fixed initially with temporary
  supports.
• Vacuum shell is laser welded.
• The temporary supports are replaced with
  permanent supports.

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TORUS Assembly procedure

• The flat walls of the vacuum shell are reinforced
  with sets of warm supports and inner stringers.

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TORUS Vacuum shell stress analysis
?max196MPa

• Stress distribution in the flat wall under
  atmospheric pressure. The maximum stress is 196
  MPa.

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TORUS Vacuum shell strain analysis
Smax2.1 mm

• Strain distribution in a flat wall under
  atmospheric pressure. The maximum strain is 2.1
  mm.

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TORUS - Protection

• The winding will be self-protected by means of
  quench back method ).
• A preliminary estimation is presented for the
  case of active protection by means of partial
  discharge of the TORUS through an external
  resistor, and winding overheating with rising
  eddy currents.
• The complete computation, including the early
  transition into the normal conducting state, will
  be made after RD on the conductor has been
  completed..

) P.H. Eberhard, M.A. Green, W.B. Michael (IEEE
Trans. on Magnetics, 1977, MAG-13,
78-81.) developed method for medium size
mono-layer windings, which is suitable for
laminar winding of any size.
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TORUS Quench Protection
Time dependence of current and temperature in
the TORUS during a protecting discharge. The
temperature rises to a maximum of 90 K.
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TORUS - Cryogenics
1- structural sheet 2- point of cross bars
fastening 3- holes for warm inter-wall support
4- cooling channels 5- cross bar 6- unit of
peripheral stretching support 7- structural cone
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Parameters for Heat Leak Evaluation
Dependence of thermal conductivity on
temperature ? (0.00138? - 0.00174)0.5
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TORUS Preliminary Heat Load
Detail of the TORUS design at the place of
cross-bar contact with the coil.
1. Structural sheet with cooling channels
attached, 2. N2 shield, 3.Vacuum shell, 4.SC
winding, 5.Support, 6. He-tubes, 7. Inter-wall
support of vacuum shell.
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TORUS - Thermo-siphon schematic
CLAS12
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TORUS - Force-cooling schematic
CLAS12
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Near term plans

• TORUS coil with dispersed windings to optimize
  field distribution.
• Modify vacuum shell for reduced heat load and
  deformations.
• Forced flow cooling scheme and location of super
  cooler unit.
• Warm and bent crossbar supports.

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TOROID - Modified Coil
CLAS12
Opening in coil
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TORUS Alternate Coil Cryostat

• Current study to assess viability of a more
  light-weight structure.

Strain analysis
Stress analysis
Structural sheet stresses at nominal current. The
maximum stress is 14.2 MPa.
Structural sheet strains at nominal current. The
maximum strain is 37 ?m.
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CLAS12 TOROID Coil Detail
A He flow super cooler is placed in the shadow
zone at the rear side where a wider vacuum shell
provides sufficient space for joints
Super cooler cross section
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Warm crossbars to fix coil positions

• Two sets of warm adjustable cross-bars provide
  mechanical stability of the vacuum shell. The
  sets support the outward corners of the fans.
• The curved cross-bars prevent interference with
  particles.

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Modified Coil Preliminary Heat Load
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TORUS - SUMMARY
CLAS12

• We have two options for the CLAS12 TORUS magnet,
  one using the technology of the existing CLAS
  Torus magnet, the other one using the technology
  developed by collaborating ITK institutions. IKT
  also provides scientific and engineering manpower
  to the project.
• The presented TORUS magnet represents an advanced
  conceptual design with the correct magnetic field
  characteristics. Detailed calculations show that
  the design meets, and partly exceeds the design
  specifications in all aspects that have been
  modeled.
• Heat leaks and quench protection have been
  modeled.
• The structural analysis shows wide margins that
  allow further adjustments of the coil shape and
  simplifications of the support structure.