SNS DTL Faraday Cup Engineering Review March 12, 2002

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SNS DTL Faraday CupEngineering ReviewMarch 12,
2002

• Steve Ellis
• SNS-03

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General Requirements

• Six designs required, one for each DTL tank
• Assemblies 1 through 5 mount in beam box after
  DTL tank
• Assembly 6 mount after CCL segment 4
• Large aperture variant of tank 1 design also
  required for D-plate
• 1.25-in aperture, limited axial beam box space
• Energy degrader bias ring
• 26-mA beam, 50-ms pulse, 10Hz

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Faraday Cup Section View
Faraday Cup 3
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Absorbers

• Sized to stop all ions below specified energy
  levels
• Simple, disc shaped, fastened to FC face
• Very accurate thickness tolerance
• Absorb approximately 70 of beam power
• Cooled via conduction through FC body to heat
  exchanger
• Graphite utilized for first three assemblies
• Low energy graphite absorbers are quite thin,
  fragile
• Glidcop AL-60 utilized for subsequent assemblies
  due to axial space limitations

FC 3 graphite absorber
Absorber Material Thickness
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Collectors

• Graphite for all designs
• 0.25-inch thick, 1.75-inch diameter
• Grooved design necessary for three low energy
  designs
• dE/dx significantly higher at lower energies
• Simple disc geometry utilized for three high
  energy designs
• Absorb approximately 30 of total beam power
• Vespel Macor insulators for electrical
  isolation
• Cooled via conduction over entire back surface
  through Macor insulator to heat exchanger
• Kapton insulated signal wire attached with
  threaded fastener to radius

FC 1 - 3 graphite collector
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UCAR ATJ Graphite

• Isostatically molded graphite
• Superior thermal conductivity
• Low density, high specific heat
• Low CTE, low modulus
• Nominal room temperature thermal properties
• Nominal room temperature mechanical properties
• Conventional machining techniques
• Vacuum compatible

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Cooling Scheme

• Heat exchanger fastened to back of each FC
• OFE copper body
• Simple coaxial tube flow scheme
• Nominal flow parameters
• 1/2-gpm
• 6-psi drop
• Adequate thermal performance

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Fabrication

• Common materials
• Low carbon steel, OFE copper, 304L stainless,
  isomolded graphite
• Ordinary concerning machining difficulty
• Conventional fabrication techniques
• Primarily mill lathe work
• One braze operation
• No unusual surface finish requirements
• Reasonable tolerances
• No heat treatment

FC 1 - 6 steel body
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Supporting Engineering Analysis

• Cooling scheme
• Required flow, convective film coefficients,
  pressure drop, etc.
• Beam heating
• Absorber geometry material selection
• Thermal response thermally induced structural
  loading
• Calculation of temperature distribution due to
  beam impingement
• Calculation of corresponding thermally induced
  stress
• Transient (single pulse) as well as steady state
  solutions

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Beam Heating

• Energy deposition due to proton kinetic energy
  loss
• 7.5-MeV to 86.8-MeV H- ions, 26-mA
• 50-ms pulse durations
• 10-Hz repetition rates
• 3-D spatial energy deposition
• Bi-Gaussian transverse beam distribution
• Depth dependant energy loss

Bi-Gaussian beam density plot
Proton energy loss per depth increment
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Thermal Structural Analysis

• Finite element code ABAQUS utilized for numerical
  solution
• Problem symmetry allowed the use of axisymmetric
  mesh ¼ symmetry 3-D mesh
• Temperature dependant material properties
  necessary
• Maximum temperature excursion 1000 K
• Isotropic material behavior utilized with respect
  to thermal mechanical properties
• Accurate spatial body heating due to beam
  impingement applied to mesh with FORTRAN
  subroutine
• Function of x, y, z, sx, sy, penetration depth,
  beam current, energy
• Requires very fine mesh to accurately capture
  behavior near Bragg peak

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7.5-MeV Faraday Cup CollectorTransient Thermal
Response
BEAM

• Collector axisymmetric model
• Small section of collector modeled
• 26-mA beam current
• 50.0-ms pulse
• 1.31-mm RMS beam size
• Spatial body heating
• Calculated temperature rise, Kelvins

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7.5-MeV Faraday Cup CollectorTransient Thermally
Induced Stress
BEAM

• Collector axisymmetric model
• Small section of collector modeled
• 26-mA beam current
• 50.0-ms pulse
• 1.31-mm RMS beam size
• Spatial body heating
• Calculated thermally induced max principal
  stress, psi

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7.5-MeV Faraday Cup CollectorSteady State
Thermal Response

• Collector axisymmetric model
• 26-mA beam current
• 1.31-mm RMS beam size
• 10-Hz operation
• Heating applied as steady surface flux
• 30-Watts
• Calculated temperature rise, Kelvins

BEAM
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7.5-MeV Faraday Cup CollectorSteady State
Thermally induced Structural Response

• Collector axisymmetric model
• 26-mA beam current
• 1.31-mm RMS beam size
• 10-Hz operation
• Heating applied as steady surface flux
• 30-Watts
• Calculated thermally induced max principal
  stress, psi

BEAM
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86.8-MeV Faraday Cup AbsorberTransient Thermal
Response

• Absorber 1/4 symmetry 3-D model
• 26-mA beam current
• 50.0-ms pulse
• 1.44-mm by 0.79-mm RMS beam size
• Spatial body heating
• Calculated temperature rise, Kelvins

BEAM
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86.8-MeV Faraday Cup AbsorberTransient Thermally
Induced Stress Levels

• Absorber 1/4 symmetry 3-D model
• 26-mA beam current
• 50.0-ms pulse
• 1.44-mm by 0.79-mm RMS beam size
• Spatial body heating
• Calculated thermally induced von Mises stress,
  psi

BEAM
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Analysis Summary

• Pulse length ability gt50mS strongly desired
• Want to operate very close to material structural
  limits
• Maximum pulse length limits calculated for each
  device (10-Hz operation)
• Absorbers for FC 4 - 6 at structural material
  limits on aft face
• May generate small surface cracks on aft surface,
  will relieve stress
• Recommend inspection possible absorber change
  out during routine maintenance

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Current Status

• Engineering analysis complete
• Design engineering report near completion
• Drawing package complete
• Need final modifications if any to degrader
  thickness
• Prototype fabrication of FC 1 underway