The Lawrence Berkeley National Laboratory (LBNL) is presently designing and building the 2.5 MeV injector for the Spallation Neutron Source (SNS). The design includes various beam intercepting devices such as beam stops and slits. The target power densities can be as high as 500 kW/cm{sup 2} with a beam stopping range of 25 to 30 microns, producing stresses well above yield in most materials. In order to analyze the induced temperatures and stresses, a finite element model has been developed. The model has been written parametrically to allow the beam characteristics, target material, dimensions, angle of incidence and mesh densities to be easily adjusted. The heat load is applied to the model through the use of a 3-dimensional table containing the calculated volumetric heat rates. The load is based on a bi-gaussian beam shape which is absorbed by the target according to a Bragg peak distribution. The results of several analyses using the SNS Front End beam are presented

Oshatz, D

Staples, J W

Virostek, S

CERN Document Server

A Thermal Analysis Model for High Power Density Beam Stops

Virostek, S.

Oshatz, D.

Staples, J.

eScholarship - University of California

Lawrence Berkeley National LaboratoryLawrence Berkeley National LaboratoryTitleA thermal analysis model for high power density beam stopsPermalinkhttps://escholarship.org/uc/item/2485t53nAuthorsVirostek, S.Oshatz, D.Staples, J.Publication Date2001-06-08eScholarship.org Powered by the California Digital LibraryUniversity of CaliforniaLBNL-47341A THERMAL ANALYSIS MODEL FOR HIGH POWER DENSITY BEAMSTOPS∗S. Virostek, D. Oshatz, J. Staples,LBNL, Berkeley, CA 94720, USA                                                          ∗ This work is supported by the Director, Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energyunder Contract No. DE-AC03-76SF00098.AbstractThe Lawrence Berkeley National Laboratory (LBNL) ispresently designing and building the 2.5 MeV injector forthe Spallation Neutron Source (SNS).  The designincludes various beam intercepting devices such as beamstops and slits.  The target power densities can be as highas 500 kW/cm2 with a beam stopping range of 25 to 30microns, producing stresses well above yield in mostmaterials.  In order to analyze the induced temperaturesand stresses, a finite element model has been developed.The model has been written parametrically to allow thebeam characteristics, target material, dimensions, angle ofincidence and mesh densities to be easily adjusted.  Theheat load is applied to the model through the use of a 3-dimensional table containing the calculated volumetricheat rates.  The load is based on a bi-gaussian beam shapewhich is absorbed by the target according to a Bragg peakdistribution.  The results of several analyses using theSNS Front End beam are presented.1 INTRODUCTIONThe SNS is an accelerator-based user facility whichwill produce pulsed beams of neutrons for use inscattering experiments.  The project, a collaboration of sixUS national laboratories, is funded by the US Departmentof Energy and is currently under construction at OakRidge National Laboratory (ORNL).  LBNL has designedand is fabricating the SNS Front End [1], which willaccelerate a 38 mA, 6% duty factor H- beam to 2.5 MeVfor injection into a 1 GeV linac.  The Front End consistsof an ion source and low energy beam transport line(LEBT) [2], a radio frequency quadrupole (RFQ) [3,4]and a medium energy beam transport line (MEBT) [5].The Front End design contains several different beamintercepting devices including an emittance scanner slit, abeam chopping target (Figure 1) and a beam stop.  Thepeak power density of the beam is up to 500 kW/cm2during the 1 ms long pulse.  A beam of this intensityimpinging on a target at a normal incidence will producestresses well above the yield stress in virtually allmaterials.  The power density incident on the target can besignificantly decreased by reducing the angle of beaminterception from normal incidence to just a few degrees.This, in combination with the selection of a materialhaving the optimal combination of thermal andmechanical properties, allows the design of a target whichcan survive the short term peak temperatures and stressesas well as long term fatigue effects.This paper will summarize the details of a finiteelement model developed to predict the temperatures andstresses in various beam intercepting devices.   The resultsof studies using numerous target materials, beamintensities and angles of incidence will be presented.Also, a comparison of results obtained using a Bragg peakheat load distribution versus that obtained with all heatapplied at the surface will be given.Figure 1: Beam chopper target made of TZM moly alloy.2 MODEL DESCRIPTION2.1 OverviewIn order to better understand the performance limits ofbeam intercepting devices, an ANSYS finite elementmodel has been developed to predict the transienttemperature and stress distributions within the targetmaterial.  The model simulates the target with 3D solidthermal elements configured as a flat plate with quartersymmetry and uniform cooling on the back side.  Uponobtaining the thermal solution, the elements are convertedto structural elements for stress analysis.  A key feature ofthe model is the method of application of the heat load.  A3D table is generated based on the bi-gaussian distributionof the beam and the deposition of heat into the target for agiven material and beam energy.  The volumetric heatload table is generated and applied completelyindependent of the model mesh.  An algorithm withinANSYS automatically superimposes the 3D table valueson the model and applies heat to individual nodes asappropriate.  This technique allows the load to be appliedto any model geometry without the necessity of trackingeither the numbering or positions of the nodes.  Figure 2shows a typical ANSYS temperature contour plot from ananalysis of the SNS beam chopper target.Figure 2: Typical temperature contour plot.2.2 Parametric Model InputIn order to readily analyze a wide variety of devices andperform case studies, the primary input file whichgenerates the model has been written to allow convenientmodification of all key parameters.  These include thebeam power and spot size, the pulse length, the targetmaterial total thickness and cladding thickness, the angleof incidence and the element mesh densities.  Otherfeatures of the model permit the user to specify solutionfrom a cold start or a steady state condition, application ofheat based on the stopping range profile or surfaceheating, and the use of temperature dependent or constantmaterial properties.2.3 Mesh DensityThe computation of the model mesh densities is codedinto the input file based on the range of the beam in thetarget, the size of the beam and the angle of incidence.  Atthe target surface, the mesh is very fine in order to resolvethe stopping range profile.  The mesh then fans out to alarger size towards the rear surface.  Similarly, the meshdensity is high at the center of the bi-gaussian beam spotand decreases towards the outer edges of the model.2.4 Volumetric Heat LoadingThe calculation of the 3D volumetric heat load table isprogrammed within the model input file using the beampower and rms size in combination with the Bragg peakprofile for the given material and beam energy.  TheBragg peak defines the rate at which the beam energy istransferred as heat to the target atoms.  This energytransfer increases as the beam penetrates the targetsurface, reaches a peak and then drops off to zero soonafter.  The stopping ranges for the materials of interesthere are 20 to 30 µm for the 2.5 MeV beam.  The Braggpeak profile is obtained by inputting the target materialand the beam ion species and energy into the programSRIM (Stopping Range of Ions in Matter) [6].  Theprogram was developed by IBM researchers to producetables of stopping powers and range distributions for anyion at any energy.  The resulting energy deposition datafor the SNS Front End beam for numerous targetmaterials of interest have been stored in a separate inputfile which is accessed by the primary model file.2.5 Target CoolingThe finite element model described here simulates theactual cooling geometry by applying a uniform filmcoefficient over the entire rear surface.  Most beamintercepting devices achieve heat removal by means of aseries of narrow, deep cooling channels located near thesurface.  The channels function like cooling fins toremove heat more efficiently than by the method used inthe model.  To account for the difference, the model filmcoefficient must be increased to reach an equivalent levelof cooling.  The correct value is determined for a givenmaterial and geometry through the use of a 2D thermalsubmodel of a single cooling channel.  The resulting heattransfer rate is used to calculate a correspondingcoefficient for the model.2.6 Material PropertiesThe temperature dependent thermal and mechanicalproperty tables for all materials of interest are stored in aseparate input file.  A material number flag in the primarymodel input file identifies which set of properties is to beused.  Since the material properties are not constant, thesolution to the resulting non-linear FEA model is obtainediteratively within ANSYS.3 PARAMETRIC STUDIESA series of model runs was performed using differenttarget materials, power levels and angles of incidence todetermine their effect on temperature and stress.  Theintent of the analysis was to identify the material with thebest beam intercepting properties and to determine towhat degree a target can absorb additional power bymeans of shallow angles of incidence.Figure 3 summarizes the results of a target materialstudy for a 6 kW, 1 ms long pulse impacting a target at a4:1 angle of incidence (~76° from normal).  The beamsize was σx=3.62 mm and σy =1.62 mm.  This caserepresents the MEBT chopper target which is used tosharpen the leading and trailing edges of the beam pulses.The comparison to yield is based on the strength of agiven material at the peak target temperature as predictedby the model.  The superior target material was found tobe the molybdenum alloy TZM (Mo-0.5Ti-0.08Zr).Based on the results of this study, TZM is being used inall of the SNS Front-End beam intercepting devices.Figure 3: Target material stress comparison.Another series of runs was performed to determine thepeak stress in a TZM target as a function of both angle ofincidence and pulse length.  The curves plotted in Figure4 represent the stresses on an emittance scanner slit heatedby a 100 kW pulse with a spot size of σx=3.27 mm and σy=4.16 mm.  As seen in the figure, the stress isdramatically reduced for shallow angles of incidence, thusallowing the target to absorb much longer pulse lengths.Figure 4: Peak stress vs. pulse length and angle.4 ENERGY DEPOSITION     Many beam target models are simplified by applyingall of the heat directly on the surface. However, in somecases, this method will cause the calculated peaktemperatures and stresses to be significantly higher thanthe actual ones.  A more accurate result is obtained byapplying the heat according to the Bragg peak profile.  Aseries of runs was performed to determine the degree towhich the peak temperature rise is over predicted forsurface heating at various pulse lengths and angles ofincidence (Figure 5).  The error is highest for shorterpulses since the Bragg peak method initially heats a largervolume than the surface heat method.  For longer pulses,the diffusion of heat into the target dominates, and theinitial distribution of the applied heat becomes lessimportant.  For shallow angles of incidence, the heat isdeposited closer to the surface, resulting in less differencebetween the two methods.Figure 5: Bragg peak vs. surface heating.5 CONCLUDING REMARKS  A parametric finite element model has been developedto analyze the induced temperatures and stresses in targetsused to intercept high intensity beams.  The 3-dimensionalvolumetric heat load applied to the model is based on a bi-gaussian beam shape which is deposited into the surfaceof the target according to a Bragg peak distribution.Through the use of the model, the SNS Front End Teamhas designed a beam chopper target, an emittance scannerslit and a beam stop.  The beam stop operates under themost severe conditions, absorbing a 95 kW, 1 ms longpulse every 16.7 ms.  Fabrication of the devices iscurrently under way.  The first tests with beam areexpected to occur later in 2001.6 REFERENCES[1] R. Keller, “Progress with the SNS Front-EndSystems”, PAC ’01, Chicago, June 2001.[2] R. Thomae, P. Bach, R. Gough, J. Greer, R. Kellerand K.N. Leung, “Measurements on the H-ion Sourceand Low Energy Beam Transport Section for the SNSFront-End System”, Linac ’00, Monterey, August2000, 223-225.[3] A. Ratti, R. DiGennaro, R.A. Gough, M. Hoff, R.Keller, K. Kennedy, R. MacGill, J. Staples, S.Virostek, and R. Yourd, “The Design of a HighCurrent, High Duty Factor RFQ for the SNS”, EPAC’00, Vienna, June 2000.[4] A. Ratti, R.A. Gough, M. Hoff, R. Keller, K.Kennedy, R. MacGill, J. Staples, S. Virostek, and R.Yourd, “Fabrication and Testing of the First Moduleof the SNS RFQ”, Linac ’00, Monterey, August2000.[5] D. Oshatz, A. DeMello, L. Doolittle, P. Luft, J.Staples, and A. Zachoszcz, “Mechanical Design ofthe SNS MEBT”, PAC ’01, Chicago, June 2001.[6] J. Ziegler, J. Biersack, and U. Littmark, “TheStopping and Range of Ions in Solids”, PergamonPress, New York, 1985.Nickel 270Nickel Cladded GlidcopTitanium 4TantalumTungstenGlidcop AL-15Moly TZM0% 20% 40% 60% 80% 100% 120%Peak Stress / Yield at Peak Temperature02004006008000 50 100 150 200Pulse Length, µsPeak Stress, MPa2:1NormalIncidence6:15:14:13:11%10%100%1000%1 10 100 1000Pulse Length, µs(Tsurface -TBragg)/TBraggNormalIncidence6:13:1Moly TZM Target

English

A thermal analysis model for high power density beam stops

UNT Digital Library

LBNL-47341A THERMAL ANALYSIS MODEL FOR HIGH POWER DENSITY BEAMSTOPS∗S. Virostek, D. Oshatz, J. Staples,LBNL, Berkeley, CA 94720, USA                                                          ∗ This work is supported by the Director, Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energyunder Contract No. DE-AC03-76SF00098.AbstractThe Lawrence Berkeley National Laboratory (LBNL) ispresently designing and building the 2.5 MeV injector forthe Spallation Neutron Source (SNS).  The designincludes various beam intercepting devices such as beamstops and slits.  The target power densities can be as highas 500 kW/cm2 with a beam stopping range of 25 to 30microns, producing stresses well above yield in mostmaterials.  In order to analyze the induced temperaturesand stresses, a finite element model has been developed.The model has been written parametrically to allow thebeam characteristics, target material, dimensions, angle ofincidence and mesh densities to be easily adjusted.  Theheat load is applied to the model through the use of a 3-dimensional table containing the calculated volumetricheat rates.  The load is based on a bi-gaussian beam shapewhich is absorbed by the target according to a Bragg peakdistribution.  The results of several analyses using theSNS Front End beam are presented.1 INTRODUCTIONThe SNS is an accelerator-based user facility whichwill produce pulsed beams of neutrons for use inscattering experiments.  The project, a collaboration of sixUS national laboratories, is funded by the US Departmentof Energy and is currently under construction at OakRidge National Laboratory (ORNL).  LBNL has designedand is fabricating the SNS Front End [1], which willaccelerate a 38 mA, 6% duty factor H- beam to 2.5 MeVfor injection into a 1 GeV linac.  The Front End consistsof an ion source and low energy beam transport line(LEBT) [2], a radio frequency quadrupole (RFQ) [3,4]and a medium energy beam transport line (MEBT) [5].The Front End design contains several different beamintercepting devices including an emittance scanner slit, abeam chopping target (Figure 1) and a beam stop.  Thepeak power density of the beam is up to 500 kW/cm2during the 1 ms long pulse.  A beam of this intensityimpinging on a target at a normal incidence will producestresses well above the yield stress in virtually allmaterials.  The power density incident on the target can besignificantly decreased by reducing the angle of beaminterception from normal incidence to just a few degrees.This, in combination with the selection of a materialhaving the optimal combination of thermal andmechanical properties, allows the design of a target whichcan survive the short term peak temperatures and stressesas well as long term fatigue effects.This paper will summarize the details of a finiteelement model developed to predict the temperatures andstresses in various beam intercepting devices.   The resultsof studies using numerous target materials, beamintensities and angles of incidence will be presented.Also, a comparison of results obtained using a Bragg peakheat load distribution versus that obtained with all heatapplied at the surface will be given.Figure 1: Beam chopper target made of TZM moly alloy.2 MODEL DESCRIPTION2.1 OverviewIn order to better understand the performance limits ofbeam intercepting devices, an ANSYS finite elementmodel has been developed to predict the transienttemperature and stress distributions within the targetmaterial.  The model simulates the target with 3D solidthermal elements configured as a flat plate with quartersymmetry and uniform cooling on the back side.  Uponobtaining the thermal solution, the elements are convertedto structural elements for stress analysis.  A key feature ofthe model is the method of application of the heat load.  A3D table is generated based on the bi-gaussian distributionof the beam and the deposition of heat into the target for agiven material and beam energy.  The volumetric heatload table is generated and applied completelyindependent of the model mesh.  An algorithm withinANSYS automatically superimposes the 3D table valueson the model and applies heat to individual nodes asappropriate.  This technique allows the load to be appliedto any model geometry without the necessity of trackingeither the numbering or positions of the nodes.  Figure 2shows a typical ANSYS temperature contour plot from ananalysis of the SNS beam chopper target.Figure 2: Typical temperature contour plot.2.2 Parametric Model InputIn order to readily analyze a wide variety of devices andperform case studies, the primary input file whichgenerates the model has been written to allow convenientmodification of all key parameters.  These include thebeam power and spot size, the pulse length, the targetmaterial total thickness and cladding thickness, the angleof incidence and the element mesh densities.  Otherfeatures of the model permit the user to specify solutionfrom a cold start or a steady state condition, application ofheat based on the stopping range profile or surfaceheating, and the use of temperature dependent or constantmaterial properties.2.3 Mesh DensityThe computation of the model mesh densities is codedinto the input file based on the range of the beam in thetarget, the size of the beam and the angle of incidence.  Atthe target surface, the mesh is very fine in order to resolvethe stopping range profile.  The mesh then fans out to alarger size towards the rear surface.  Similarly, the meshdensity is high at the center of the bi-gaussian beam spotand decreases towards the outer edges of the model.2.4 Volumetric Heat LoadingThe calculation of the 3D volumetric heat load table isprogrammed within the model input file using the beampower and rms size in combination with the Bragg peakprofile for the given material and beam energy.  TheBragg peak defines the rate at which the beam energy istransferred as heat to the target atoms.  This energytransfer increases as the beam penetrates the targetsurface, reaches a peak and then drops off to zero soonafter.  The stopping ranges for the materials of interesthere are 20 to 30 µm for the 2.5 MeV beam.  The Braggpeak profile is obtained by inputting the target materialand the beam ion species and energy into the programSRIM (Stopping Range of Ions in Matter) [6].  Theprogram was developed by IBM researchers to producetables of stopping powers and range distributions for anyion at any energy.  The resulting energy deposition datafor the SNS Front End beam for numerous targetmaterials of interest have been stored in a separate inputfile which is accessed by the primary model file.2.5 Target CoolingThe finite element model described here simulates theactual cooling geometry by applying a uniform filmcoefficient over the entire rear surface.  Most beamintercepting devices achieve heat removal by means of aseries of narrow, deep cooling channels located near thesurface.  The channels function like cooling fins toremove heat more efficiently than by the method used inthe model.  To account for the difference, the model filmcoefficient must be increased to reach an equivalent levelof cooling.  The correct value is determined for a givenmaterial and geometry through the use of a 2D thermalsubmodel of a single cooling channel.  The resulting heattransfer rate is used to calculate a correspondingcoefficient for the model.2.6 Material PropertiesThe temperature dependent thermal and mechanicalproperty tables for all materials of interest are stored in aseparate input file.  A material number flag in the primarymodel input file identifies which set of properties is to beused.  Since the material properties are not constant, thesolution to the resulting non-linear FEA model is obtainediteratively within ANSYS.3 PARAMETRIC STUDIESA series of model runs was performed using differenttarget materials, power levels and angles of incidence todetermine their effect on temperature and stress.  Theintent of the analysis was to identify the material with thebest beam intercepting properties and to determine towhat degree a target can absorb additional power bymeans of shallow angles of incidence.Figure 3 summarizes the results of a target materialstudy for a 6 kW, 1 ms long pulse impacting a target at a4:1 angle of incidence (~76° from normal).  The beamsize was σx=3.62 mm and σy =1.62 mm.  This caserepresents the MEBT chopper target which is used tosharpen the leading and trailing edges of the beam pulses.The comparison to yield is based on the strength of agiven material at the peak target temperature as predictedby the model.  The superior target material was found tobe the molybdenum alloy TZM (Mo-0.5Ti-0.08Zr).Based on the results of this study, TZM is being used inall of the SNS Front-End beam intercepting devices.Figure 3: Target material stress comparison.Another series of runs was performed to determine thepeak stress in a TZM target as a function of both angle ofincidence and pulse length.  The curves plotted in Figure4 represent the stresses on an emittance scanner slit heatedby a 100 kW pulse with a spot size of σx=3.27 mm and σy=4.16 mm.  As seen in the figure, the stress isdramatically reduced for shallow angles of incidence, thusallowing the target to absorb much longer pulse lengths.Figure 4: Peak stress vs. pulse length and angle.4 ENERGY DEPOSITION     Many beam target models are simplified by applyingall of the heat directly on the surface. However, in somecases, this method will cause the calculated peaktemperatures and stresses to be significantly higher thanthe actual ones.  A more accurate result is obtained byapplying the heat according to the Bragg peak profile.  Aseries of runs was performed to determine the degree towhich the peak temperature rise is over predicted forsurface heating at various pulse lengths and angles ofincidence (Figure 5).  The error is highest for shorterpulses since the Bragg peak method initially heats a largervolume than the surface heat method.  For longer pulses,the diffusion of heat into the target dominates, and theinitial distribution of the applied heat becomes lessimportant.  For shallow angles of incidence, the heat isdeposited closer to the surface, resulting in less differencebetween the two methods.Figure 5: Bragg peak vs. surface heating.5 CONCLUDING REMARKS  A parametric finite element model has been developedto analyze the induced temperatures and stresses in targetsused to intercept high intensity beams.  The 3-dimensionalvolumetric heat load applied to the model is based on a bi-gaussian beam shape which is deposited into the surfaceof the target according to a Bragg peak distribution.Through the use of the model, the SNS Front End Teamhas designed a beam chopper target, an emittance scannerslit and a beam stop.  The beam stop operates under themost severe conditions, absorbing a 95 kW, 1 ms longpulse every 16.7 ms.  Fabrication of the devices iscurrently under way.  The first tests with beam areexpected to occur later in 2001.6 REFERENCES[1] R. Keller, “Progress with the SNS Front-EndSystems”, PAC ’01, Chicago, June 2001.[2] R. Thomae, P. Bach, R. Gough, J. Greer, R. Kellerand K.N. Leung, “Measurements on the H-ion Sourceand Low Energy Beam Transport Section for the SNSFront-End System”, Linac ’00, Monterey, August2000, 223-225.[3] A. Ratti, R. DiGennaro, R.A. Gough, M. Hoff, R.Keller, K. Kennedy, R. MacGill, J. Staples, S.Virostek, and R. Yourd, “The Design of a HighCurrent, High Duty Factor RFQ for the SNS”, EPAC’00, Vienna, June 2000.[4] A. Ratti, R.A. Gough, M. Hoff, R. Keller, K.Kennedy, R. MacGill, J. Staples, S. Virostek, and R.Yourd, “Fabrication and Testing of the First Moduleof the SNS RFQ”, Linac ’00, Monterey, August2000.[5] D. Oshatz, A. DeMello, L. Doolittle, P. Luft, J.Staples, and A. Zachoszcz, “Mechanical Design ofthe SNS MEBT”, PAC ’01, Chicago, June 2001.[6] J. Ziegler, J. Biersack, and U. Littmark, “TheStopping and Range of Ions in Solids”, PergamonPress, New York, 1985.Nickel 270Nickel Cladded GlidcopTitanium 4TantalumTungstenGlidcop AL-15Moly TZM0% 20% 40% 60% 80% 100% 120%Peak Stress / Yield at Peak Temperature02004006008000 50 100 150 200Pulse Length, µsPeak Stress, MPa2:1NormalIncidence6:15:14:13:11%10%100%1000%1 10 100 1000Pulse Length, µs(Tsurface -TBragg)/TBraggNormalIncidence6:13:1Moly TZM Target

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A Thermal Analysis Model for High Power Density Beam Stops

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