Design of NuMI Magnetic Focusing Horns Presented by: Kris Anderson Fermi National Accelerator Lab

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Horn System ReviewAugust 10, 2001

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Design of NuMI Magnetic Focusing Horns

Presented by: Kris Anderson

Fermi National Accelerator Lab

Mechanical Engineer

August 10, 2001

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Page 2Presentation Outline

I. Horn Overview and Experiment Requirements

II. Horn Support Structure (Module and Carriage)

III. Discussion of Design, Loading and Analysis

IV. Design Methodology

V. Summary

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Page 3

Overview: Horn Function in Neutrino Beam Production

120 GeV protons hit target

+ produced at 1 to 100 milli-radian angles

magnetic horn to focus +

+ decay to + in long evacuated pipe

left-over hadrons shower in hadron absorber

rock shield ranges out +

beam travels through earth to experiment

p+

Exp.

Decay Pipe

+

Rock

Hadron

Absorb.

HornsTarget

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Neutrino Beam Requirements Influencing Horn Design

Produce a wide band muon neutrino beam at the MINOS Far Detector with as many muon neutrinos as possible, where the energy spectrum is chosen to maximize the neutrino oscillation signal in the search region (maximize yield)

Effect: Minimize horn inner conductor wall thickness while maintaining conductor integrity, defines horn shape and beam-line location

Facilitate accurate (within 2%) prediction of the spectrum in the MINOS far detector given a measurement of the spectrum in the MINOS near detector.

Effect: Horn construction tolerances (generally within 0.005” or better), field quality

Center the neutrino beam on the MINOS detector.

Effect: Alignment and survey tolerances, field quality, alignment stability

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Neutrino Beam Requirements Influencing Horn Design

Accommodate a primary beam intensity of 4x1013 protons every 1.9 sec, matching the production capability of the MI

Effect: Duty cycle, thermal control issues, radiation hard materials

Assure long-term reliability, alignment and mechanical stability, and reparability.

Effect: Mounting rigidity and thermal stability, hot horn repair/replacement work cell, use of horn positioning modules

Assure personnel safety.

Effect: Radiological considerations (e.g., quick release horn support mechanisms, remote strip-line connections)

Provide flexibile design for possible future conversions to neutrino beams for defined energy search ranges, anti-neutrino beam, narrow band beam, etc.

Effect: Horn positioning modules, shielding design facilitating component relocation to initially accommodate low, medium, and high energy beams

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Page 6Magnetic Horn OverviewGeneral Design Features

• Large toroidal magnetic field

• Requires large current, 200 kAmp

• Thin inner conductor, to minimize absorption

• Water spray cooling on inner conductor

• Most challenging devices in beam design

• Prototype test 1999-2000 to check design

Focus toward detector

Outer Conductor

Inner ConductorStripline

Drain

Insulating Ring

Spray Nozzle

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Page 7

PH2 Horn Configurations andNeutrino Spectra

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Page 8NuMI Target Station LayoutSchematic of Horn Locations

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Subsystem ComponentsHorn Positioning Module

Design incorporates requirements such as motion capability, potential for relative ease of component relocation, radiation aspects of handling and replacing hot

components, and replacement of defective or failed mechanisms

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Page 10Horn Positioning ModuleStripline Remote Clamp

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Subsystem ComponentsModule Support Carriage/Girder

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Structure Modeled Using Pro-E at ANL

- W14x211 Beam Section- Maximum deflection ~2mm- Beam stress safety factor of ~6.5- Bolted Connections and critical weldments have been analyzed- FNAL needs to review design and complete end support hardware

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Subsystem ComponentsHorn Support Structure in Beamline

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Subsystem ComponentsHorn Support Structure in Beamline

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Page 14

Subsystem ComponentsHorn Positioning Hardware in Shield Pile

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Page 15Design Topics

Specific Design Criteria

• NuMI uses 2 horns with parabolic shaped aluminum inner conductors driven by 200kA peak damped half-sine pulse (horn designed for pulse width of 5.2 ms for resonant extraction; pulse changed to 2.6 ms for single turn extraction)

• Horn 1designed for 1E7 pulse lifetime with 5.2 ms pulse (approximately 1 year integrated run time with provisions for accelerator maintenance periods)

• Minimize bolted connections and material in secondary particle path to minimize the potential for pion absorption- dictates welded construction with 2mm thick inner conductor for horn 1

Tolerances Extremely Important:

• General alignment budget of ±0.020”, apportion 0.010” fabrication, 0.010” alignment accuracy

• Azimuthal wall thickness tolerance variation of finished inner conductor of ± 0.005” called out on drawings (IHEP 1999 Task A Report call out ± 0.0014” )

• Straightness tolerance of finished conductors over 3 meter effective length specified as ± 0.010”

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Page 16Design Topics

Specific Design Criteria

Design issues to address for implementing a robust horn system:

– Adequate water cooling to control thermal stress, particularly in center conductor region; achieved using appropriate water nozzles

– Conductor corrosion control measures / fatigue life enhancement

– Conductor erosion control and dielectric barrier layer coating

– Use of radiation hard materials

– Fabrication techniques to meet design criteria (e.g., geometric tolerances)

Above Concerns Lead to Prototype Cycle:

– Validate design through fabrication and electrical pulse test of a prototype horn 1 (i.e., horn with highest mechanical loading) to identify and address potential fabrication concerns and investigate effectiveness of cooling, corrosion measures, and conductor design integrity

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Prototype Horn 1 Isometric Cross-Section View

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Page 18Horn 2 Cross-Section

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General 2 Horn System Parameters

Parameter Horn 1 Horn 2

Neck Radius (cm) 0.9 3.9

Wall Thickness, Neck (mm) 4.5 5.0

Outer Conductor Radius to i.d. (cm) 14.9 32.3

Inductance (nH) 685-690 ~457

Resistance (µΩ) 208 (meas.) <112

Average Power from Current Pulse (kW) 17.0 <7.5

Power Flux at Neck (W/cm2) 14.5 <4.7

Temperature Rise at Neck (oC) 22.8 <7.1

* Note: Above heat load numbers are from original design pulse width of 5.2 msec

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Page 20Mechanical Loading and Analysis

Mechanical Loading of Horn is the Result of :

- Current pulse thermal expansion from resistive heating (peak at the

end of the pulse)

- Magnetic forces (peak at the mid-pulse)

- Beam heating from particle interaction in material

Horn 1 Horn 2Inner conductor resistive heating 17 kW <7.5 kW

Inner conductor beam energy deposition 1045 W 371 W

Outer conductor beam energy deposition 14.5 kW 5.4 kW

(1” thick) (1” thick)

* Note: Above numbers from original design pulse width of 5.2 msec

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Mechanical Loading • During the current pulse length of 5.2 ms, mechanical load

disturbances travel the following lengths:

Structural: l1 = (E/)1/2 = 25.2m

Thermal: l2 = (k/cp)1/2 = 0.6 mm

Where:

E = Young’s modulus = 69 GPa

= pulse width = 5.2 ms

= density = 2713 kg/m3

k = thermal conductivity = 180 W/m•K

cp = specific heat = 963 J/kg•K

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Loading and Analysis Summary

• Current pulse is mechanically a very slow load and thermally a very rapid load

• Analyze thermal stresses at beginning, middle, and end of pulse (quasi steady-state) to determine magnitude of cyclic loading for fatigue analysis– ANSYS shell element FEM modeling conducted by Z. Tang

• Magnetic loading (vector cross-product J x B) is greatest during mid-pulse; this force is superimposed with thermal loading and results in axial and hoop stress components, as well as significant end wall loading

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Areas of Highest Mechanical Loading

Neck of Horn

Upstream EndcapHigh Stress AreasIdentified by ANSYS

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Factors Affecting Fatigue Life

• Fatigue strength is dependent upon stress ratio• To compute stress ratio R, whole stress cycle must

be known.– Stress Ratio, R, is defined as the ratio of the minimum

to maximum stress.• Tension is positive, compression is negative

– R=Smin/Smax varies from -1R1 For 6061-T6 Aluminum• R = -1 alternating stress) max=16 ksi• R = 0 Smin=0) max=24 ksi, (1.5X at R=-

1)• R = .5 max=37 ksi, (2.3X at R=-1)

– These values are for N=107 cycles, 50% confidence

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Mechanical Loading and AnalysisAreas of Highest Mechanical Loading

Values for 5.2 msec Pulse Width

• US end cap: minimum stress before pulse is -1030 psi; maximum stress at mid-pulse is -9020 psi; mean stress is -5025 psi with an alternating stress of 3995 psi; Stress ratio R=0.11

• Under the above calculated stress levels, allowable maximum stress for 107 cycles at endcap is 26.5 ksi resulting in fatigue safety factor of 2.9

• Neck of horn: stress at mid-pulse is +4351 psi; stress at end of pulse is

-3742 psi; mean stress is 304 psi with alternating stress 4047 psi; Stress ratio R = -0.86 (Note: Negative value of R results in lower value of fatigue stress limit)

• Under the above calculated stress levels, allowable maximum stress for 107 cycles at neck is 15.3 ksi resulting in fatigue safety factor of 3.5

• Stress in conductor weldment regions is very low (<<4 kpsi)

Fatigue data from Aerospace Structural Metals Handbook

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Page 26Inner Conductor Support Modal

Analysis Summary

• Conducted modal analysis of prototype horn 1 using IDEAS Master Series v6.0

• Objective is to determine the appropriate number of spider supports such that the first mode is above excitation pulse frequency

• Finite Element Model generated using 8 node brick elements

• Excitation pulse frequency = 100 Hz– Unsupported inner conductor first mode (bending) natural frequency = 65

Hz

– Single support DS of neck - 165 Hz

– Two spiders either side of neck - 175 Hz

– Three spider support system - 358 Hz

– Additional supports contributed little to increasing first mode frequency - diminishing returns

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Page 27

WBS 1.1.2 Technical ProgressInner Conductor Supports

6061-T6 Al Electroless Nickel Coated Support Struts

Zirconia Ceramic

Belleville Spring Washers

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Page 28Horn Test Stand MeasurementsInitial Vibration Measurements

• Conducted Inner and Outer Conductor Modal Measurements

– Used miniature accelerometers and shaker (white noise) to excite and measure first 15 modes in inner and outer conductor

• General Summary of Results– First mode, global system mode

• Inner and outer conductor simple bending, 120 Hz, highly damped

– Higher order modes (from 198 Hz to 678 Hz) are associated with the inner conductor, are generally local, and are reasonably well damped from conductor supports

• Areas exhibiting maximum modal deflections do not occur in high stress regions identified by ANSYS (i.e., upstream end cap and neck of horn)

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Page 29Discussion of Design Methodology

Cooling

• Initial ANSYS analysis was conducted assuming a heat transfer coefficient h= 1700 W/m2•K

• Note that equilibrium conductor temperature and resultant thermal stress is a function of cooling effectiveness (i.e. heat transfer coefficient)

• We conducted cooling nozzle tests simulating horn neck geometry to ensure that measured values for the heat transfer coefficient exceeded the above value

• Actual measured values for h at 20 psig with 110-005 (.5 gpm) nozzles result in lower bound h values of 4300-4800 W/m2•K

• Actual measured values for h at 20 psig with 110-010 (1 gpm) nozzles result in lower bound h values of 5800-6500 W/m2•K

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Page 30Discussion of Design Methodology

Cooling

Nozzle Heat Transfer Coefficient Measurement

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Page 31

Corrosion ConsiderationsFactors Affecting Fatigue Life

• Moisture reduces fatigue strength– For R = -1, smooth specimens, ambient

temperature:• N=108 cycles in river water, max= 6 ksi

• N=107 cycles in sea water, max~ 6 ksi– Hard to interpret this data point

• N=5*107 cycles in air, max= 17 ksi

– The above data is motivation for utilizing corrosion/encapsulating barrier layer over aluminum substrate

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Page 32Design Methodology

Candidate Corrosion Barrier Layer

Two Possible Candidates:

• Electroless nickel: reasonable corrosion barrier properties, non-dielectric, more expensive, limited vendor base with large tank capacity– Conducted fatigue test of nickel coated aluminum samples at the 107 fatigue

limit and compared results with equivalent non-coated aluminum specimens: coated samples survived 1.7x 107 cycles, non-coated samples failed at 0.6x 107 cycles

– Use high phosphorus electroless nickel (0.0005” - 0.0007” thick) on inner conductor and conductor supports

• Anodizing: best solution for lower stress thick cross-section areas Type III (hard coat sulfuric acid, 0.0023”), Rc 60-65, dielectric strength of ~800 V/mil– Type III hardcoat anodize is selected for outer conductor and thick lead in

portion of inner conductor; not suitable for thinner/higher stress areas of inner conductor due to approximate 60% reduction of fatigue strength

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Prototype Horn 1Hardcoat Anodize

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•Single pass, full penetration CNC weld is required to minimizing conductor distortion, assure repeatability, and control internal weld porosity• Proper cleaning, handling, fixtures, and weld parameters are crucial to minimize conductor distortion and internal weld porosity

NuMI approached welding solution via parallel paths1) Identify vendor base to subcontract critical horn conductor welding

- Vendor base for CNC TIG welding extremely limited and expensive; less flexible fabrication path than in-house- Prototype horn 1 fabricated in this manner using Sciaky as prime contractor, ANL as subcontractor

2) Investigate the development of welding capability in-house- Have specified, benchmarked, purchased, and commissioned a Jetline fully automated TIG welding system for producing controlled conductor weldments- System installed at MI-8 horn facility- Long term solution for welding 4 initial horns (production and spare horn 1 and horn 2)

Horn FabricationPrecision Welding

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Conductor Fabrication

•Inner conductor fabricated from 6061-T6 billet per QQA 200/8•Relatively good strength (UTS ~ 45 ksi, YS ~ 40 ksi, R=-1 FS ~ 16 ksi)•Available in variety of sizes and shapes•Welds readily•Relatively good corrosion resistance

•All prototype horn inner conductor parts CNC machined by Medco to tolerances better than ~ 0.002”•Inner conductor welding complete - CNC TIG - Overall tolerances held to ±0.010” over 133.375” length (straightness and radial deviation from ideal)•Outer conductor overall tolerances better than ±0.010”•Outer conductor anodized, inner conductor uses electroless Ni coating•Stripline contact surfaces use 0.0005” silver brush plating

Technical ProgressPrototype Horn 1 Design Summary

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•Water Seals- Total of 64 water seals in horn- Utilize EVAC aluminum delta seals on KF style flange

•Bolted Connections- Utilize TimeSert threaded inserts, pullout exceeds 9600 lb. on 3/8” insert- As a reference, maximum end wall reaction is approximately 4270 lb.

•Current Contact Surfaces- Current surfaces have 32 µin finish, 0.0003”-0.0005” silver plate finish- Interface clamping pressure exceeds 1400 psi- As reference, lithium lens secondary contact lead is 5.01 in2 for 6285 Arms; Prototype horn 1 contact area is 9.2 in2 for 7250 Arms.

•Corrosion/Erosion Control- Outer conductor and thick lead in section of inner conductor employs 0.0023” thick Type III hard coat anodize followed by mid-temp nickel seal- Inner conductor utilizes 0.0007” thick high phosphorus electroless nickel

•Inner Conductor Spider Support Columns- Design has been experimentally tested to 36 million cycles at defections of ±0.031” with 80 lbs. axial preload with no failures

WBS 1.1.2 Technical ProgressPrototype Horn 1 Design Summary

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Prototype Horn 1 at MI-8

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Prototype Horn Test Summary

- Have successfully operated prototype horn for 2,152,352 pulses at 200 kA peak, 850µsec pulse width

- Magnetic field mapping, powered vibration, and cooling measurements are complete with excellent results

- Experienced two small NW16 water nozzle leaks (one stopped after tightening EVAC clamp, the other resulted in a cracked flange after several polishing/tightening attempts)

- Pulse testing resulted in mild redesign of water lines (samples)

- Recently operated for 1000 pulses at full production pulse of 200 kA peak, 2.6ms pulse width

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Page 41Summary

Production Horn Status

Design of production horn 1 and horn 2 complete except:

- Mild design iteration of horn 1 based on MI-8 pulse test results

- Integration of outer conductor cooling for both horn 1 and horn 2

Production Horn Fabrication

- Outer conductor rough forgings in fabrication for production and spare horns 1 and 2 (Vendor: Scot Forge)

- Inner conductor rough forgings (including weld samples) in fabrication for horn 2 (Vendor: Lenape Forge)

- Horn 2 parts to final vendor fabrication this fall

Documents

Design of NuMI Magnetic Focusing Horns Presented by: Kris Anderson Fermi National Accelerator Lab