Beryllium window experiment at HiRadMat

1. Beryllium window experiment at HiRadMat 1Chris Densham, 1Tristan Davenne, 1Andrew Atherton, 1Otto Caretta, 1Peter Loveridge, 2Patrick Hurh, 2Brian Hartsell, 2Kavin Ammigan, 3Steve Roberts, 3Viacheslav Kuksenko, 1Michael Fitton, 1Joseph O’Dell, 2Robert Zwaska 1 STFC Rutherford Appleton Laboratory, UK 2Fermilab, US 3 Oxford University (Materials for Fission and Fusion Power), UK

2. Objectives of experiment Identify design limits for beam windows for the next generation of proton accelerator driven facilities by: • Exploring the onset of failure modes (flow behaviour, crack initiation, or fracture, and other degradation) of various beryllium grades/forms under controlled conditions at simultaneous high localized strain rates and temperature rises. • Identifying and quantifying any potential thermal stress wave limits for beryllium windows under intense pulsed beam conditions and how they may differ between grades/forms • Comparing measurements to non-linear failure simulations for validation/modification of material models through the use of state-of-the art material analysis techniques • Investigating the potential effects of resonance, with constructive superposition of stress waves, in windows of particular thicknesses/geometries.

3. Model Inputs Model inputs HiRadMat Proton Beam Parameters Beam kinetic energy - 440GeV Beam Sigma – 0.3 - 0.5mm Bunch spacing - 25ns Number of protons/bunch = 1.7e11 Number of bunches – 288 Spill duration - 7.2μs Fluka and MARS Energy Deposition calcs Max energy density = 0.2 GeV/cc/primary Temperature jump = 1.7K/bunch or 493K/spill Stress simulations (Static and inertial) LS-Dyna, Autodyn and ANSYS Beryllium window – temperature dependent strength properties Bilinear and elastic-viscoplastic hardening models Window dimensions: Radius range = 5-25 mm Thickness range = 0.15-1 mm (0.15mm chosen such that bunch spacing=2*t/cL)

4. Beryllium Material Data Literature data on mechanical properties of beryllium at high strain rates Stress Strain curves for Beryllium S-65B • Combined ITER and US Air Force data used to implement Bilinear Kinematic Hardening material model in ANSYS Classic Yield Strength of Beryllium S-65B Approximated using elastic viscoplastic model in LS-Dyna simulations [ITER MATERIAL PROPERTIES HANDBOOK 1997] [Mechanical Properties of Structural Grades of Beryllium at High Strain Rates, US Air Force Materials Laboratory, 1975]

5. Beryllium Material Data Bi-linear kinematic model used in ANSYS Tangential Modulus = 4.62 GPa

6. Beam Induced Stress Beam induced stress & strain • Static structural analysis of thermal stresses induced by beam pulse • Temperature dependent material properties • Window properties: • 25mm radius, 1mm thickness • Temperature jump of 360°C Bi-linear Model

7. Dynamic simulations Edge strain simulation results • Be slugs: R = 20 mm, L = 30 mm • Beam centered at r/R = 0.9 • Beam sigma: 0.3 mm • Elastic viscoplastic material model (LS-DYNA) • Temperature and strain rate dependent [1] LS-DYNA model showing beam location and temperature after 288 bunches.

8. Dynamic strain response E: elastic material model EVP: elastic viscoplastic model 36 bunches • ΔT = 120 °C • Max. εtot,equiv = 0.08 % 288 bunches • ΔT = 980 °C • Max. εtot,equiv = 2.0 %

9. Plastic strain: generation of permanent surface displacement y-displacements, σ = 0.3 mm • y-displacements (bump height) range from 2 – 10 µm (σ= 0.3 mm, t = 0.25 – 3 mm) – well within resolution of modern profilometers • Damage model being developed to better predict onset of fracture and fracture morphology after cool-down (fracture of centre spot expected)

10. Surface deformation versus beam sigma / intensity At maximum intensity: (288 bunches/pulse) Results are for 0.25 mm window, elastic viscoplastic material model

11. Beam and Applied Pressure Realistic load case: beam pulse + applied pressure • A pressure is applied to one side of window as is the case in an actual beam window • Investigate whether addition of beam pulse could produce significant stress peak • Window is constrained at periphery edge. • Investigated the influence of altering the window radius and thickness and magnitude of pressure load. Stress response of window under beam and pressure load of 4 bar

12. Realistic load case: beam pulse + applied pressure • Interim conclusion • Applying a pressure to the window in conjunction with beam loading does not appear to induce a higher stress peak in the window (good result for actual beam windows!) • Nevertheless, it may still be a valid method of detecting window failure e.g. by using an on-line leak detector

13. Outline conceptual design of experiment • Multiple samples exploiting long interaction length in beryllium. • Samples include: • Different commercial grades of Be • Thick & thin windows • Unstressed and pre-stressed

14. Online instrumentation • Strain measurements: strain gages positioned on surface of beryllium slugs to measure • axial strain • circumferential strain • Laser Doppler Vibrometer to compare surface vibrations with simulations and provide independent check on rms beam spot size • Optical pyrometer to measure peak temperature rise (another check on beam size) HRMT14 experiment: Equipped Inermet specimen for strain measurements [2]

15. Off-line materials analysis • Profilometer/AFM to analyse window surface profile and measure out-of-plane plastic deformations. • Advanced microscopy systems for micro-structural and crystallography evaluation (SEM, EBSD, EDS) and potential crack/failure analysis.

16. Proposed experimental methodology • Polish samples before irradiation and characterise using AFM, SEM, EBSD, EDS, nanoindentationand, possibly, micromechanical methods • Carry out experiments: • Scan beam across samples with increasing number of bunches per spill • Carry out multiple shots on single locations to investigate whether beam effects saturate or accumulate • Repeat measurements in step 1 to identify effects of pulse beam interation

17. Material analysis techniques • Used before and after in-beam experiment to quantify effects of pulsed beam interaction with material

18. Atomic Force Microscopy Used to measure surface bump dimensions

19. Electron backscatter diffraction (EBSD) • Electron backscatter diffraction is a technique for the scanning electron microscope which allows crystal orientations in a polycrystalline material to be measured. • Maps of crystal orientation can be collected using EBSD. They remove any ambiguity regarding the recognition of grains and grain boundaries in the sample. • We intend to use EBSD to see how the material flows during plastic deformation and, if a crack develops, how the flow results in fracture

20. Nanoindentation Used to measure changes in hardness across sample after irradiation

21. Focussed Ion Beam (FIB) Methods ZeissNvision dual beam FIB-SEM • FIB technique advantageous for: • Site specific regions • Small volumes – reduction in hazards e.g. activity, toxicity, etc. Sample

22. Micromechanical Testing 2mm Steve Roberts Oxford University Materials

23. 1um 3um 2um 3um 4um Why microcantilevers? • Need for a sample design that can be machined in surface of bulk samples. • Bend testing allows fracture as well as elastic and plastic properties to be investigated. • Suitable for measuring individual microstructural features. • Testing of samples only available in small volumes. • Geometry that can be manufactured quickly and reproducibly.

24. Micromechanical testing Fe-6%Cr – yield stress Yield Stress (GPa) 0.1mm 6.0 Ion-irradiated 4.0 Neutron-irradiated 2.0 Unirradiated 0.0 0.0 2.0 4.0 6.0 8.0 Beam depth (mm)

25. Energy-dispersive X-ray Spectroscopy (EDS) • Used to measure migration of impurities e.g. to grain boundaries

26. Summary of measurements 1) Plastic deformation out-of-plane profile. 2) Vibration (strain gauges) response (onset of yielding, fracture timing (in cool-down cycle?)) 3) Crack/fracture detection through microscopy 4) Fracture surface morphology through microscopy (inter-granular?) 5) Grain orientation and residual strain through microscopy (EBSD) 6) Visual (High Speed or High Resolution Camera) to capture any unforeseen events

27. Interpretations of measurements • Do measurements match the macro-scale simulations and/or material/damage models? (Validation, Benchmarking) • Are results consistent across the various Be grades and conditions tested? Can materials characterisation explain any differences noted? • Do results indicate that certain grades/conditions/orientations exhibit better resistance to thermal stress waves? • Does resonance between bunches have a measureable effect? • Can one primary failure mode be identified for all material grades/conditions or does the failure mode differ depending upon material/grade/condition? • Was anything observed that was not expected?

28. Extra Material

29. Influence of Cp Influence of Cp Constant Cp Temp dependent Cp

30. Inertial stresses from single pulse Stress at window centre following a single bunch. Note ‘small’ magnitude of stress waves and significant reduction in stress wave magnitude within several bunches Axial stress at window centre during first six bunches Axial wave magnitude increases for first three bunches no significant constructive interference of axial waves observed

31. Inertial Stress – complete pulse Inertial stress from complete spill Stress resulting from entire pulse (288 bunches) Plastic deformation starts at about 2μs Peak stress is 260MPa Inertial stress waves don’t appear to significantly add to stress Axial strain rate < 25000 s-1 Radial strain < 900 s-1 Strain rate reduces once plastic deformation occurs

32. Inertial Stress – complete pulse Inertial stress from complete spill Plastic work occurs on beam axis Strain growth rate changing at yield point Axial Deformation of 0.6microns with 0.15mm thick window