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  • 1. Heat and Mass Transfer in Fusion Energy Applications: from the “Very Cold” to the “Very Hot"
    • Presented by A. René Raffray
    • UCSD
    • Seminar
    • University of California, Los Angeles
    • Los Angeles, CA
    • January 25, 2008
  • 2. Unique Set of Conditions Associated with Fusion • Realization of fusion energy imposes considerable challenges in the areas of engineering, physics and material technology. • This is typified by the heat and mass transfer aspects, which cover a wide range of conditions, from the very cold to the very hot. • This seminar highlights this wide range of challenges associated with the heat and mass transfer aspects of fusion technology using examples in both the Inertial Fusion Energy (IFE) and the Magnetic Fusion Energy (MFE) Approaches. Schematic of IFE Power Plant (HAPL) ARIES-CS Power Core Electricity Generator Target factory Modular Laser Array Chamber
  • 3. Example of IFE Energy Source Based on Lasers, Direct Drive Targets and Solid Wall Chambers (HAPL) Target (fabrication at <20K, survival in chamber during injection) Chamber conditions (~0.1-1 eV depending on pre-shot conditions) Blanket Dry wall chamber (armor must accommodate ion+photon threat at up to ~2000-3000K) Electricity Generator Target factory Modular Laser Array Final optics (+ mirror steering) System (including power cycle)
  • 4. The Very Cold: IFE Target Layering
  • 5. An IFE Target Consists of Multiple Layers with Demanding Fabrication Requirements Uniformity of DT fuel layer thickness Inner and outer surface smoothness (order of 1  m) Example Sector of Spherical Target (HAPL) • Rep rate of 5-10 Hz leads to ~500,000 targets per day • Mass fabrication production • Need reasonably fast layering technique also to maintain acceptable tritium inventory (for target microexplosion physics and thermal radiation shielding)
  • 6. In Beta Layering, DT-Fuel moves until Inner Surface is Isothermal
    • Volumetric heating (tritium decay) causes sublimation until isothermal condition is reached (beta-layering)
    • 3 H --> 3 He + ß - + anti-neutrino
    • (max. ß energy = 18.6 keV)
    • Inner surface temperature increases with increasing layer thickness
    • Thermal gradient causes fuel to redistribute
    • Fuel moves from thicker to thinner region
    • Uniform temperature field leads to uniform layer thickness
    Uniform temperature field on outer target (TP of DT ~19.79K) A. J. Martin, Beta energy driven uniform deuterium-tritium ice layer in reactor- size cryogenic inertial fusion targets, J. Vac. Sci. Technol. A6 (3) (1988)  T,  P Migration of fuel 1- D simplification Temperature Radius Thicker layer (hotter) Thinner layer (colder) Unlayered Target q DT = 0.05 W/cc
  • 7.
    • Gas stream heats up as the heat from the beta decay is removed from the particles
    • Temperature difference causes a non-uniform layer
    • Temperature variations of helium gas are only slightly impressed onto fuel
    • Successful layering will depend on fluidized bed design
      • Minimal temperature gradient
      • Maximum spin and circulation rate of the pellets to provide a time-averaged isothermal environment
    Helium gas: 0.5 atm, <18K> Fuel Capsule Fuel Approximate as semi-infinite slabs Gas T varies 3 mK across capsule 5 Hz target rotation 5 Hz rotational speed lead to acceptable layer uniformity (~1.2%) Apply dynamic ∆T to static model of fuel layer movement 3 mK local ∆T GA Has Proposed a Fluidized Bed for Mass Production of IFE Fuel Pellets LAYERING Fluidized Bed Gas Flow Frit SPIN CIRCULATION Hotter Cooler Temperature at an instant Fuel outer surface varies only 0.008mK
  • 8.
    • - A fluidized bed has been chosen for mass production layering of IFE targets (GA)
    • Room temperature fluidized bed experiments
    • - For initial observation and characterization of bed behavior
    • Numerical fluidized bed model proposed
    • - Must include particle imbalance
    • - Validation through theory and experiments
    • Experimental water-alcohol layering
    • - Validate layering model
    • - Show proof of principle
    • Find optimized parameters for D 2 Layering prior to experiment
    • - Successful D 2 layering in Mass Production Layering Experiment (MPLX) (GA)
    Plan for GA/UCSD Collaboration on Target Layering (UCSD/GA) K. Boehm HiP cell Fluidized bed Target Vacuum Pickup
  • 9. Desktop Experiment Set-Up to Help Understand Fluidized Bed Behavior Under RT Conditions • Use unfilled plastic target shells in a nitrogen flow (  N2 (RT) ~  He (18K)) • Spin rates of unbalanced spheres • One shell was partially filled with whiteout • An unbalanced target needs 2X bed expansion to spin in example case
  • 10. Surface Degradation is Observed During Fluidization
    • A random patch of the target surface is scanned under an SEM for roughening study
    • Measurements on (60-40) Au–Pd sputter-coated targets
    • Unacceptable Au-Pd layer damage for radiation protection and possibly for target physics requirement
    • Need to improve coating process and minimize fluidization time
    One patch of the shell analyzed Au-Pd Surface before Fluidization Surface after 16 hrs of room temperature fluidization
  • 11. Several Parameters Define Window of Operation for Fluidized Bed
    • 3 main objectives
    • - Minimize  T across bed (~3 mK per target)
    • - Optimize spin and circulation rates at operating point
    • - Preserve surface quality
    - Bed expansion ratio: E= h/h’ - Representative of gas speed or mass flow Gas Flow≠0 h Gas Flow=0 h’ Crush target  =1.2  =1.5  =2.0  =4.0  =8.0 1 Q DT Surface Damage Example operating point: 0.5 atm h/h’= 2  T=70mK
  • 12. Unknowns in Bed Behavior Call for Numerical Analysis
    • The behavior of unlayered shells is unknown (unbalanced spheres)
    • Experimental observations are restricted to the particles close to the wall
    • Tests on the cryogenic apparatus are expensive, time consuming and complex
    • Results might be hard to interpret since shells are not easily accessible
    • Optimize operating conditions and define narrow window of operation for successful deuterium layering prior to completion of entire setup
    • - gas pressure, flow speed, bed dimensions, additional heating, frit design, layering time…
  • 13. Proposed Numerical Model Consists of Three Parts
    • Granular model
    • Fluid-Solid interaction
    • Layering Model
      • Thermal/mass transfer
    Fluidized Bed Model
  • 14. Fluidized Bed Model Part I: Granular Model
    • Discrete Particle Method (DPM)
    • Motion of individual particles tracked by computing forces acting on the particles at each time step
    • Apply Newton’s second law of motion
    • Spring – dashpot and/or friction slider model applied for particle collisions
    • Limitations: Not developed for unbalanced spheres
    Forces are computed based on relative velocity at contact point For unbalanced sphere: - Contact forces are a function of relative velocity at contact point - Depends on the orientation of the particle Contact point velocity is not parallel to wall Orientation defined by Euler angles Center of mass Geometrical Center wall displacement wall Normal Force Tangential Force Torque around center of mass
  • 15. Equations to Compute Contact Forces in Model Distance between two sphere centers Distance between two mass centers Convert spin into space fixed coordinates Compute contact point velocity *Single particle experimental observation to determine parameters such as k eff and c eff Apply Forces to: Normal and Tangential Force Component*
  • 16. Fluidized Bed Model Part II: Fluid/Drag Model
    • • Problem with fluid cell sizes for traditional porous flow model:
    • - Minimum of seven pellets per fluid cell for cell average to work in control volume method
    • - Not useful to solve fluid equation for 3x4 grid
    • - Choosing a grid size smaller than the shells leads to complication determining the “average void fraction” around a sphere
    • DNS model to resolve flow around each sphere complex, computationally VERY expensive and not warranted for the purpose of this model
    • • The most important information we are trying to get is the particle spin and circulation rate
    • • Experimental observations show, that the spin of the particles is dominantly induced by collisions, not by fluid interaction
    • • 1-D Lagrangian model to determine void fraction
    • - Compute the void fraction for each slice of the fluidized bed, bounded by one radius in each direction of the center of each sphere.
    • - This “region of interest” moves with each particle from time step to time step
    • - Once the void fraction is known, the drag force can be computed
  • 17. Richardson-Zaki Drag Model is Applied Based on Known Void Fraction of Region Void Fraction is known based on 1-D Lagrangian Model Richardson-Zaki Drag Force for homogeneous fluidized beds: Terminal Free Fall Velocity is a constant system parameter: Dellavalle Drag Model to Compute Terminal Free Fall Velocity: Archimedes Number: Drag force is added to the total force on the particle at each time step
  • 18. Proposed Fluidized Bed Model Particles need to be spaced apart Initialize position, velocity and quaternion vectors Start time stepping Predictor step Compute forces based on predicted positions Compute the resulting pressure drop Determine bed expansion Correct positions, velocities and accelerations based on the updated forces Create time averaged statistics Write Output every 1000 time steps Particle – wall collisions Compute void fraction Compute drag force Compute effective weight Compute force due to particle – particle collisions Loop over all particles
  • 19. Preliminary Results from Fluidized Bed Model Show Good Qualitative Reproduction of Experimental Results Exact system parameters need to be determined
  • 20. Model Has Been Calibrated through a Series of Runs for Balanced Spheres Total Kinetic Energy in System during Granular Collapse for decreasing time step size (J) 200 particles M = 2E-6 Kg Diameter = 4 mm K_eff = 1000 N/m C_eff = 0.004 N s/m  = 0.0125 N s/m  = 0.4 I = 5E-12 Kg s^2 Example: Stability and convergence in modeling granular collapse Next Step: Validation for cases with unbalanced spheres 6e-05 4e-05 3e-05 2e-05 1e-05 5e-05 0.1 0.2 Time (s)
  • 21. Layering Model
    • Compute the redistribution of fuel based on the fluidized bed behavior
    • Solve 1-D equations simultaneously:
    • This leads to a layering time constant of
    • Time step: ~10 -5 s for fluidized bed v. ~30-60s for layering
    • Based on the time averaged motion and preferential position, we can compute the average temperature/ temperature difference between the thick and the thin side of the shell
    Latent heat Volumetic heating Mass Transfer Equation Heat Transfer Equation
  • 22. Experimental Demonstration of Layering Proposed Using H 2 O – IPA Mixture
    • Water-alcohol filled shells (freezing point ~240K)
    • Fluidized be with cooling stream of N 2 or He
    • Vol. heat source (IR with bandpass filter or microwave)
    • Objectives
    • - Analyze fluidizing behavior for unbalanced shells
    • - Demonstrate target layering process
    • - Investigate the effects of volumetric heating and other bed parameters on the layering process
    • - Validate layering model
    Fluidized Bed IR- Halogen Light TOO MUCH HEAT GETS ABSORBED IN FIRST LAYER TOO TRANSPARENT Small margin Room temperature and pressure T~240 K in vacuum Honed copper tubes as waveguides Bandpass filter IR halogen lamp Glass window Vacuum vessel Fluidized Bed Reflective coating to trap radiation
  • 23. From the Very Cold to the Very Hot: IFE Target Injection
  • 24. The Cryogenic Direct-Drive Target will be Subjected to Challenging Conditions when Injected into an IFE Chamber IFE Chamber (R~6-10 m) Chamber Gas: protective gas (Xe) and/or burn products (D, He) Chamber wall ~ 1000–1500 K, q’’ rad = 0.2 – 1.2 W/cm 2 Target Injection (~100 m/s) Target Implosion Point
  • 25. Modeling Target Injection Using DS2V Example Temperature Field Around a Direct Drive Target - Xe flowing at 400 m/s in the positive x-dir. - 4000 K stream temperature. - 3.22x10 21 m -3 stream density. - Sticking coefficient = 0 - Target surface temperature = 18 K
    • Assumptions
    • Axially symmetric flow.
    • Target is stationary; gas stream velocity to simulate injection.
    • Target surface temperature is constant (~18K).
    • Sticking coefficient = 0 or 1,
    • Accommodation coefficient = 1
    • Target doesn’t rotate.
  • 26. Thermal Modeling Indicates Limited Gas Density in Chamber to Prevent Target DT Reaching its Triple Point
    • For a 16 K target injected at 100 m/s in a 10.5 m chamber, q’’ to reach the TP ~0.5 W/cm 2
    • Radiation q’’ ~ 0.2 W/cm 2 for a 1000K wall and target  =0.96
    • From DS2V, for a target injected at 100 m/s in 1000 K D 2 , q’’ = 0.3 W/cm 2 for D 2 density of ~4 mTorr
    • Not much margin left for increases in radiation due to reflectivity change
    1-10  m Polymer Shell with Au or Pd Reflective Coating 290  m Solid DT/Foam 190  m Solid DT DT Vapor Core ~ 4 mm
  • 27. What About the Effect of 3 He Nucleation Sites?
    • 3 He generated into the DT by tritium decay (half life = 12.3 years) could accumulate in clusters and provide nucleation sites
    • Time between layering and injection = 4-10 hrs
    • • Estimated nucleus radius
    • - 0.4 microns after 4 hrs
    • - 1.6 microns after 18 hrs
    • 2-D model to more accurately simulated bubble nucleation and growth
    Time (delta time = 5 min) Mole fraction of 3 He diffused into the Trap Radial distance from the center of the trap (m) Molar fraction of 3 He
  • 28.
    • Bubble grows as fast as heat for the phase change can be delivered to the interface
    • As the bubble starts growing, the pressure in the bubble drops even further
    • Bubble grows step wise depending on the grid
    • Conservation of energy requires variable time steps
    • In order to determine the length of the time step, the heat flux to the bubble needs to be computed
    • As the bubble grows from one grid to another, the temperature field is adjusted
    2-D Thermal Diffusion Model with Stretched Grid and Bubble Growth Bubble grows radially outward starting somewhere in the domain
  • 29. Temperature Field Around the Bubble • Small thermal boundary layer Time: 4.86 E-6s Radius: 2.94 E-6m Time: 3.52 E-5s Radius:8.66-6m Time: 8.53 E-5s Radius:1.09E-5m Time: 1.47E-4s Radius:1.34E-5m Time: 2.23E-4s Radius:1.64E-5m Time: 3.15E-4s Radius:2.00E-5m
  • 30. Comparison with LANL DT Bubble Growth Experimental Observation • Two stages of bubble growth - Could be explained by the temperature fields • Bubble grows into the solid layer - Low strength of DT around triple point • Small bubbles nucleate later but grow fast for a longer period of time - Higher superheat required - Higher surrounding temperature results in a faster growth
  • 31.
    • Melt layer around the target does not violate symmetry requirements
    • Bubbles do violate symmetry
    • Nucleus of a certain size present in the DT
    • Temporal separation of melt layer occurrence and bubble nucleation
    • If melt layer acceptable for target physics requirement and limiting criterion based on bubble growth, possibility of significant impact on target and chamber design
    • - Increase heat flux by a factor of 3-10
    Applying Model to Characterize Target Design Limitations
  • 32. High Temperature Operation in IFE IFE Chamber
  • 33. What are the Threats on the Chamber Wall?
    • X-ray, ion and neutron fluxes to the chamber wall several times per second.
    • Neutron flux penetrates deeper and not an issue for armor.
    • Need to develop armor that can accommodate X-ray and (more importantly) ion threats.
    Target micro-explosion Chamber wall X-rays Fast & debris ions Neutrons Example energy partitioning for 350 MJ-class direct drive target (HAPL reference from J. Perkins, Oct. 2005)
  • 34. Ion and Photon Threat Spectra Cover Appreciable Energy Ranges • Example spectra for 350 MJ-class direct drive target (HAPL reference, J. Perkins, Oct. 2005).
  • 35. Ion Power Deposition Occurs in a Very Thin Armor Region (~  m’s) over a Few  s • Example case for 10.75 m chamber without protective gas to avoid target survival and placement issues. • Only thin armor region sees large energy deposition and temperature transients (next slide). • This led to the configuration choice of a thin armor layer (~ 1 mm) on a FS substrate. • Blanket at the back sees quasi steady state (similar to MFE). • W chosen as preferred armor material (high-temperature capability, no tritium concern). • Armor lifetime is a key issue and is the focus of the R&D in this area. Coolant FS W q
  • 36. Temperature History and Gradient for W Armor in a 10.75 m Chamber Subject to the 350 MJ-Class Baseline Target Threat Spectra (from HAPL) • 1-mm W on 3.5 mm FS at 580 °C. • No chamber gas. • Time-of-flight spreading of ion energy deposition results in much lower temperature rise than assumption of instantaneous energy deposition. • Peak temperature ~2400°C.
  • 37. Impact of Threat Spectra on W Armor Lifetime
    • • Several possible mechanisms could lead to premature armor failure:
    • - Ablation.
    • - Melting (is it allowable?).
    • - Surface roughening & fatigue (due to cyclic thermal stresses).
    • - Accumulation of implanted helium.
    • - Fatigue failure of the armor/substrate bond.
    • Because the exact IFE ion and X-ray threat spectra on the armor cannot be duplicated at present, experiments are performed in simulation facilities as part of the HAPL program: - Ions (RHEPP@SNL). - Laser (Dragonfire@UCSD). - Fatigue testing of the W/FS bond in ORNL infrared facility. - He management is addressed by conducting implantation experiments (UNC, UW) along with modeling of He behavior in tungsten (UCLA). Ablation Depth T or  T No net ablation, but surface roughening Threshold for ablation Threshold for roughening Net Ablation
  • 38. Long Term Exposure Experiments Suggest Roughening Threshold and Temperature Dependence for W, for example: • Results from the RHEPP ion beam facility at SNL (0.8-1.6 MeV) indicates roughening threshold for powder metallurgy W (PM W) at ~ 1 J/cm 2 at RT. - some improvement with heated W samples. - single crystal W better. - rhenium (Re) and Re/W alloy much better. • Does roughening matter if it does saturate and does not lead to armor failure? Probably not. • Additional testing and diagnostics needed for confirmation of initial experimental indications on saturation and threshold for W armor. • For HAPL, as an initial armor survival constraint from these early results, a temperature limit of 2400°C was assumed for the W armor (e.g. corresponding to a RHEPP fluence of ~1.2 J/cm 2 ). • Results from Dragonfire laser testing facility at UCSD (YAG laser, 10 Hz) with W (~3000°C) indicate possible roughening saturation after ~10 5 shots. • Also, PM W behavior seems to depend more on T than  T. 11A, 200mJ, 773K, Max: 3,000K (~2,200K  T) 10 3 shots 10 5 shots 10 4 shots
  • 39. Another Major Issue is Ion Implantation Effect on W
    • • He retention and surface blistering characteristics of W
    • • Carbide formation and change of thermophysical properties of surface
    • • R&D activities as part of HAPL program include:
    • • He implantation/anneal cycle experiment (UNC/ORNL)
      • - ~850°C base T, ~1.3 MeV He, pulsed implantation and anneals at 2000°C over ~1000 cycles to fluences of ~10 20 He/m 2
      • • He + D implantation in IEC facility (UW)
      • - ~800°C base T, ~10-100 keV ion, pulsed implantation to fluences of ~10 22 He/m 2
      • • Modeling of implanted ion behavior including bubble formation and behavior (HEROS code at UCLA)
    • He ion irradiation of W at 800 °C shows significant surface damage at modest exposure • Need better understanding (including pulse effect and net mass loss) • Possibility of porous material to enhance release and relieve thermal stress
  • 40. 1 MeV He Implantation and Release in W (UNC) • He retention in W decreases drastically when a given He dose is spread over an increasing number of pulses, each one followed by W annealing to 2000°C • Simple diffusion model suggests values of effective activation energy as a function of dose • Results suggest porous material with 50-100 nm microstructure would keep at% of retained He in W to a few % - Vacuum plasma spray porous W with ~10-100 nm microstructure (PPI/UCSD). IFE Case ~5 x 10 16 atoms/m 2
  • 41. Armor Survival Constraints Impact the Overall IFE Chamber Design and Operation • W temperature limit of 2400°C assumed for illustration purposes (~1.2 J/cm 2 roughening threshold from RHEPP results) • Limit to be revisited as R&D data become available • Example chamber parameters for 0 gas pressure: - Yield = 350 MJ; R=10.5 m; Rep. rate ~ 5 for 1750 MW fusion • Desirable to avoid protective chamber gas based on target survival and injection considerations • Large chamber maintained as baseline for HAPL • Possibility of advanced chambers explored, in particular use of magnetic intervention to stir away the ions and help achieve a more compact chamber  Required P Xe as a Function of Yield to Maintain T W,max <2400°C for 1800 MW Fusion Power and Different R chamber                          0 10 20 30 40 50 0 5 10 15 20 25 30 35 0 50 100 150 200 250 300 350 400 450 Xe Pressure (@ST) (mtorr) Repetition Rate Yield (MJ) 3.5 mm FS T coolant =572°C h=67 kW/m 2 -K chamber 60 40 1 mm W R (m) 5.7 6.5 7 8 10
  • 42. High Temperature Operation in MFE MFE Divertor
  • 43. The Divertor Challenge in MFE: How to Accommodate the Heat and Particle Fluxes ARIES-CS Compact Stellarator Concept as example of MFE Device • Provide region for ion neutralization and fusion ash pumping • W armor considered for power plant application - high temperature capability - relatively high sputtering threshold Field line tracing model to estimate divertor heat flux for ARIES-CS
  • 44. ARIES-CS Divertor Design • UCSD/FZK collaboration • Design for a max. q’’ of at least 10 MW/m 2 • He coolant and tungsten-alloys as structural material and for armor • Previous concepts include: • Build on the W cap design and explore possibility of a new mid-size configuration with good q’’ accommodation potential, reasonably simple (and credible) manufacturing Cooling finger with W caps (~1-2 cm) (FZK): - minimize use of W as structural material - accommodate higher q’’ - many small units (~10 5 -10 6 )required, raising reliability and maintenance concerns Plate configuration (~ 1m) (FZK): -fewer units ~100-1000 -not well suited for thermal gradient accommodation
  • 45. T-Tube Divertor Concept (also applicable to Tokamaks) • W allow cartridge inside outer tube • Cooling with discrete or continuous jets • W armor layer • Graded transition connection from W alloy to FS manifold to reduce differential thermal expansion and stress W alloy outer tube W alloy inner cartridge W armor
  • 46. Set Tube Length for Acceptable Deflection • Deflection <0.2 mm for L=10 cm and D = 1.5 cm
  • 47. Excellent Thermal-Hydraulic Performance • Results from and FLUENT (CFD) and CFX indicates that for q’’ = 10 MW/m 2 : - High jet heat transfer coefficient - W alloy temperature within ~600-1300°C (assumed ductility and recrystallization limits, but requires further material development) • Results confirmed through dedicated experiments at Georgia Tech. Example Case: • Jet slot width = 0.4 mm • Jet-wall-spacing = 1.2-1.6 mm • Specific mass flow = 2.12 g/cm 2 • Mass flow per tube = 48 g • P = 10 MPa,  P ~ 0.1 MPa •  T ~ 90 K for q’’ = 10 MW/m 2 • T He ~ 605 - 695 ° C
  • 48. Design Optimized for Reasonable Stresses • Results from ANSYS for q’’ = 10 MW/m 2 : - Maximum thermal stress ~ 370 MPa • Graded transition connection from W alloy to FS manifold results in acceptable stresses  th,max ~ 370 MPa
  • 49. Divertor Unit Cell Design Allows for Assembly in Manifolding Units and Integration in Plates of Required Sizes • T-tubes assembled in a manifold unit. • Typical divertor target plate (~1m x ~1 m) consists of a number of manifold units. Details of T-tube manifolding to keep FS manifold structure within its temperature limit
  • 50. Fabrication Method Being Investigated at Plasma Processes Inc. (PPI, Huntsville, AL) • Vacuum Plasma Spray (VPS) posible • New EL-Form process method preferred - High temperature electroforming based on electrodeposition of compact layers of metals onto a mandrel of the desired shape. - Deposits >99% of theoretical density as deposited - Avoid possible shrinkage linked with long term post-processing sintering or HIPing • Co-axial tube successfully manufactured as part of SBIR I effort • Prototypical geometry + heat flux testing as part of Phase II
  • 51. Only Very Few Off-Normal Events Could be Allowed in a Fusion Power Plant • Disruptions can result in heat loads on the divertor (ITER, 6-25 MJ/m 2 over ~1.5-3 ms) • Melting and evaporation would occur • Very few such events allowed (cannot shut down the power plant + armor lifetime) 2-mm W alloy 3-mm W armor He h = 20 kW/m 2 -K T = 700°C Energy Deposition
  • 52. Summary
    • Realization of fusion energy imposes considerable demands in the areas of engineering, physics and material technology, as typified by the heat and mass transfer aspects, which cover a wide range of conditions, from the very cold to the very hot, e.g.:
    • - IFE target: < 20K
    • - IFE pre-shot chamber environment: ~1000-10000 K
    • - IFE chamber wall: ~1500-3000 K
    • - MFE divertor normal/off-normal conditions: ~1500/4000 K
    • Exciting challenges
    • Much progress toward credible solutions but more work needs to be done.