John Mandrekas Fusion Energy Sciences Office of Science US Department of Energy

1. High Performance Computing in Fusion Energy Sciences Presented at the Smoky Mountains Computational Sciences and Engineering Conference John Mandrekas Fusion Energy Sciences Office of Science US Department of Energy September 3, 2014

2. Outline • Brief Introduction to Fusion • The Fusion Energy Sciences Program • The Science of Magnetic Confinement • Role of HPC in Fusion Energy Sciences • Challenges and Future Directions

3. Fusion in a Nutshell What is fusion? • When light atoms (such as H isotopes) fuse together, they release energy • High temperatures (several times hotter than the interior of the sun) are needed to overcome the Coulomb barrier • At these temperatures, atoms are ionized, leading to a hot gas consisting of positive nuclei and electrons  plasma • The plasma must be confined for sufficient time so that more energy is produced by the fusion reactions than is used to heat it • Advantages of fusion: • Plentiful fuel: • D can be extracted from water • T can be generated from Li (plentiful) • No carbon emissions • Safe (uncontrolled release of energy impossible) • Any radioactivity is short-lived (decades to decay rather than thousands of years)

4. How to Confine a Plasma? • Magnetic Confinement: • Charged particles gyrate around magnetic field lines • Currently the leading approach for confining a hot plasma • Inertial Confinement: • Fuel is compressed, raising its temperature • Inertia keeps the fuel confined long enough for fusion to happen • High Energy Density Laboratory Plasma (HEDLP) physics is the science behind inertial confinement For any approach to fusion, plasma physics will be central

5. Progress in fusion has been remarkable: comparable to Moore’s Law fusion triple product Confinement time: “Plasma needs to hang on to fusion heat to make more fusion happen” nT Fusion fuel density: “need a lot of collisions per unit time” Fusion fuel temperature: “nuclei need hard, fast collisions for any fusion to happen” 10,000-fold increase in 30 years, another factor of 6 for a power plant

6. U.S. Fusion Energy Sciences Program Supports Fusion and Plasma Science Strategic Goals • Advance the fundamental science of magnetically confined plasmas to develop the predictive capability needed for fusion energy • Support the development of the scientific understanding required to design and deploy fusion materials • Pursue scientific opportunities and grand challenges in high energy density plasma science • Increase the fundamental understanding of basic plasma science, beyond burning plasmas Mission The mission of the U.S. Fusion Energy Sciences (FES) program is to expand the fundamental understanding of matter at very high temperatures and densities and to build the scientific foundations needed to develop a fusion energy source. This is accomplished by the study of the plasma state and its interactions with its surroundings.

7. ITER & the Burning Plasma Era • World’s first MFE experiment to achieve self-heated or burning plasmas • Aims to generate 30 times the level of fusion power achieved to date and to exceed the external power applied to the plasma by at least a factor of 10 • Will demonstrate scientific and technical feasibility of fusion as a future energy source • Under construction in St. Paul-lez-Durance, France http://www.iter.org/

8. Fundamental Description of Magnetic Confinement • The fundamental equations describing the behavior of magnetically confined plasmas are well-known (Boltzmann-Maxwell system): Coupled via ρ and J • fα(r, v, t) is the 6D (plus time) phase space distribution function for all charged species (electrons, ions including fusion-generated alpha particles, multiple impurity species, etc.) • The collision C(fα,fβ) term (usually Fokker-Planck) and the source term S(fα) introduce additional rich physics (and associated computational challenges), such as RF heating, neutral beam injection, pellet injection, interactions with neutrals, atomic and molecular physics, etc.

9. A Grand Challenge Problem Complicated geometries & magnetic topologies W7-X Stellarator Tokamak Intrinsic nonlinearities (especially in burning plasmas) • Extreme anisotropy (||and  to the B field) The “brute force” solution of the 6D nonlinearly coupled system is not possible with current HPC capabilities; even at the exascale level, routine solution of the 6D system for experimentally relevant configurations and time scales will be a challenge

10. Today’s Approach: Scale Separation 10-8 10-10 10-6 104 10-4 102 10-2 100 sec Average over gyromotion (5D)  Gyrokinetic codes (PIC & continuum): plasma turbulence and transport RF codes: wave heating & current drive; Fokker-Planck equation for particle response Velocity moments of kinetic equation (3D); neglect electron inertia  extended MHD codes: macroscopic stability Flux surface averaging  1½D transport codes: discharge time scales A number of critical effects (Neoclassical Tearing Modes, Energetic Particles, Disruptions, etc.) are inherently multiscale Adapted from: D.E. Post, J. Fusion Energy, 2005

11. Multiscale Challenges Developing true predictive capability requires an integrated multiscale approach FSP Workshop Report 2007

12. First Steps Toward Integration RF (ECCD) stabilization of Tearing Modes (SWIM – T. Jenkins) Edge-core coupling of plasma turbulence (EPSI – CS Chang) New SciDAC-3 Partnership: Advanced Tokamak Modeling (AToM) Integrated Plasma Simulator (IPS) Framework D. Batchelor / D. Bernholdt- SWIM More integration is needed, leading to Whole Device Modeling (WDM) capability

13. Materials Science Challenges PSI-SciDAC – B. Wirth • The burning plasma environment, characterized by high heat and particle fluxes, high neutron fluxes and fluences, and high temperatures and mechanical stresses, presents extreme challenges to materials. • Deepening the fundamental understanding of the various mechanisms limiting the performance and lifetimes of existing and proposed materials, are among the most significant challenges facing the fusion program. • Plasma facing components (PFCs) must remove plasma exhaust, which involves unprecedented power and particle fluxes & fluences, while limiting release of impurities to core plasma. • Key Issues: • Erosion lifetime and plasma compatibility • Tritium inventory • Thermal Transients • H / He blistering • Heat Removal • Fabrication Technology • Neutron Damage • Structural materials response to intense, 14 MeV-peaked neutron spectrum Multiscale challenges in PSI science Wirth, Nordlund, Whyte, and Xu, Materials Research Society Bulletin 36 (2011) 216-222

14. Role of HPC in Fusion Energy Sciences From the early days of the MFECC (predecessor to NERSC) to today’s Leadership Computing Facilities, HPC has played a significant role in fusion research -- essential component of the strategy to develop predictive capability Initial un-optimized performance • Partnerships between fusion scientists and computational scientists, under SciDAC, have accelerated the rate of scientific discovery in fusion plasma science by improving the performance of fusion codes on leadership computing facilities and by addressing challenging data management and visualization issues associated with high-performance computing. Performance after SUPER optimizations (CPU only) (4x improvement using SUPER & GPUs) Maximum # of Titan cores

15. HPC Advances Enable Higher Resolution ITER-relevant RF Simulations ICRF antenna simulations with sheath BC’s LH Wave propagation at   (ceci)1/2 Surface wave excitation and penetration at  >> ci ICRF Heating at  ~ ci NSTX C-Mod C-Mod ITER GigaflopTeraflopPetaflop Validation of ICRF-generated distributions against experiment ICRF-generated ion distributions CSWPI (P. Bonoli, MIT)

16. HPC Advances Enable Higher Fidelity Plasma Turbulence Simulations Exaflops Multiscale, multiphysics 5D / 6D electromagnetic kinetic simulation of ITER plasma, including effects of fusion reactions Petaflops Non-perturbative multiscale 5D electrostatic / electromagnetic gyrokinetic simulations with multiple kinetic species, integrating the edge and core regions Teraflops Non-perturbative (full f) 5D electrostatic gyrokinetic single-species (ion) simulation with adiabatic electrons of the tokamak edge region in realistic geometry, including background plasma effects Gigaflops Perturbative (δf) 5D electrostatic gyrokinetic single-species (ion) simulation with adiabatic electrons in model geometry CPES / EPSI (CS Chang, PPPL)

17. Impact of HPC Advances on Macroscopic Stability: from CDX-U to ITER CEMM (S. Jardin, PPPL)

18. HPC Advances Required to Assess Plasma Surface Interactions in Fusion Tokamaks Challenge: No viable materials to bridge from today’s tokamaks to future fusion power reactors – predictive capability through HPC needed to assess Plasma Facing Component Performance & design advanced PFCs Exascale: Continuum simulations of Plasma Surface Interactions of reactor scale divertor surface (100 m2) with O(1012) unknowns predict plasma facing component performance & tritium retention 100’s of Petaflops: MD simulations of 105 nm2 spatial domains with gas implantation fluxes of 1025 Hem-2s-1 to 1020He m-2s-1 Mesoscale& continuum simulations for prototypic laboratory-scale experiments 10’s of Petaflops: MD simulations of 104 nm2 spatial domains with gas implantation fluxes of 1026 He m-2s-1 to 1020 He m-2 s-1 SciDAC-PSI, Wirth et al.

19. Challenges & Future Directions • HPC computing will continue to be an important instrument of science for fusion, as we enter the burning plasma era • Increasing the level of integration in fusion simulations is critical for developing predictive capability • What can we do by ITER first plasma? • Should be accompanied by comprehensive V&V and UQ • Fusion science has to overcome a number challenges in the coming years to capture the full potential of extreme scale computing: • Maintain excellent performance of simulation codes and frameworks in a rapidly changing hardware environment (hybrid computing, multi- and many-core, etc.) • Avoid reduced productivity and disruptions during hardware transitions • Code coupling issues • Multiscale and multiphysics challenges • “Big Data” challenges (analysis, visualization, etc.) • Active partnerships between fusion scientists and computational scientists are essential for success

20. Thank you!