ADVANCED COMPUTATION IN PLASMA PHYSICS

Forty-Third American Physical Society

Division of Plasma Physics Annual Meeting

Long Beach, California

W. M. TANG

Princeton University, Plasma Physics Laboratory,

2 November 2001

PERSPECTIVE

• GOAL: Reliable predictions of complex properties of high temperature plasmas– Acquire scientific understanding needed for predictive models

superior to empirical scaling

• Plasma Science is both utilizing and contributing to the exciting advances in Information Technology and Scientific Computing.

• Advanced computation in tandem with theory and experiment is powerful new tool for scientific understanding and innovation in research

• Focus of present talk: Magnetically-Confined Plasmas (Fusion Energy Sciences )

Fusion Plasma Science is in Pasteur’s QuadrantProf. Donald Stokes, Dean, Princeton Woodrow Wilson School

Considerations of Use?

No – Yes

Qu

est

for

Bas

icU

nd

erst

and

ing

?

No

–

Yes

Bohr

Edison

Pasteur

Tight coupling of understanding and innovation.

Strong commitment to both!

Plasma Science ChallengesNRC Plasma Science Committee

• Macroscopic Stability– What limits the pressure in plasmas?

• Astrophysical accretion disks

• Wave-particle Interactions– How do particles and plasma waves

interact?• Solar coronal heating

• Microturbulence & Transport– What causes plasma transport?

• Accelerator collective dynamics

• Plasma-material Interactions– How can high-temperature plasma

and material surfaces co-exist?• Materials processing

Challenge to Theory & Simulations

• Huge range of spatial and temporal scales

• Overlap in scales often means strong (simplified) ordering not possible

10-6 10-4 10-2 100 102

Spatial Scales (m)electron gyroradius

debye length

ion gyroradius

tearing length

skin depth system size

atomic mfp electron-ion mfp

10-10 10-5 100 105

Temporal Scales (s)

electron gyroperiod electron collision

ion gyroperiod Ion collision

inverse electron plasma frequency confinement

Inverse ion plasma frequency current diffusion

pulse length

Scientific ComputingCritical to Discovery in Many Scientific Disciplines

Subsurface Transport

GlobalSystems

DOE Science ProgramsNeed Dramatic Advances

in Simulation Capabilities

To Meet TheirMission Goals

Health Effects, Bioremediation

Fusion Energy

CombustionMaterials

Plasma Physics in DOE Advanced Scientific Computing Programs

• New DOE Office of Science Program: “Scientific Discovery through Advanced Computing” ---- FES is an active member of this broader scientific portfolio with access to new resources

• Plasma Science Advanced Computing Institute (PSACI)– Lead role for coordinating Plasma Science component of

DOE’s new SciDAC Program

– Peer-reviewed projects include FES Collaboratory, Magnetic Reconnection, Wave Heating, Atomic Physics, Turbulent Transport, and MHD Simulations

– Program Advisory Committee (with distinguished members from outside & within FES) provides excellent advice/guidance

COMPUTATIONAL CHALLENGES IN FUSION ENERGY SCIENCES IMPORTANT FOR MOST AREAS

(Cross-Disciplinary Opportunities)______________________________________________________________

• Enhance physics models & develop more efficient algorithms to better address scientific issues

Multi-scale physics e.g. Kinetic (electromagnetic) dynamics

Improved algorithms e.g. Adaptive mesh refinement for higher dimensionality

phase-spaceScalability of codes

e.g. Efficient implementation of codes on most powerful MPP supercomputers

• Improve analysis/interpretation of greatly increased volume of simulation data New diagnostic & visualization tools, improved data

management/analysis

Advanced Scientific Codes --- a measure of the state of

understanding of natural and engineered systems

Theory(Mathematical Model)

AppliedMathematics(Basic Algorithms)

ComputationalScience

(Scientific Codes)

ComputerScience

(System Software)

Problem with Mathematical Model?

Pro

ble

m w

i th

Com

put

ati o

n al

Me t

h od ?

Computational Predictions

Agree* w/ Experiments?

No Yes Speed/Efficiency?

Inadequate

AdequateUse the New Tool for Scientific Discovery

(Repeat cycle as new phenomena encountered )

*Comparisons: empirical trends; sensitivity studies; integrated measurements (spectra, correlation functions, heating rates …)

Single Fluid

Resistive MHD

Two Fluid MHD

(electrons and ions)

Two Fluid MHD plus energetic

gyro-particles

Gyro-particle ions and

fluid electrons

Full orbit particle ions and

fluid electrons

Less complex model, valid for high-collisionality, strong fields, long times

More computationally demanding. Required to describe many important

but subtle phenomena.

External kink modes

Neoclassical tearing mode (including rotation)

Collisionless reconnection

MHD modes destabilized by wave-particle resonance with energetic species

Kinetic stabilizationof internal MHD modes by ions

Tilting and interchange modes in FRC

MACROSCOPIC (MHD) SIMULATIONS: DIFFERENT LEVELS OF ANALYSIS CAPABILITY

PPPL, SAIC, MIT, LANL, NYU, GA, U.Wisc., U. Texas, U. Colorado

Neoclassical Tearing Mode (NTM) Analysis Capability

• Self-consistent closure for Neo-classical Fluid Eq.’s being developed & applied [e.g., NIMROD]

• Results to be cross-benchmarked & validated against experimental results

• Enable assessment of NTM impact on beta limit for long-pulse, high-performance tokamaks

•

• UNSTABLE INTERNAL KINK (LEFT) EVOLVES (RIGHT)

MHD SIMULATION OF INTERNAL RECONNECTION EVENT

Hot Inner Region Interchanges with Colder Outer Region via Magnetic Reconnection

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MPP Supercomputers Provide Access to New Plasma Wave PhysicsORNL, PPPL, MIT

Mission Research, Lodestar, CompX

Improved Physics and All Orders Spectral Algorithm (AORSA-2D)

• Field solutions for conversion of fast ion cyclotron waves to ion Bernstein waves in 2D for a tokamak – collaboration with Computer Science and Math division at ORNL

• Contours of wave electric field strength for mode conversion using DIII-D tokamak parameters

• Patterns shown here are not revealed in a 1D treatment

• Extension to shorter wavelength and to 3D will be possible with new generation computers

UNDERSTANDING TURBULENT PLASMA TRANSPORT

An important problem: -- Size of plasma ignition experiment determined by fusion self-heating versus turbulent transport losses-- Dynamics also of interest to other fields (e.g., astrophysical accretion disks)

A scientific Grand Challenge problem A true terascale computational problem for MPP’s

PLASMA MICROTURBULENCE SIMULATION CODES HAVE MADE EXCELLENT PROGRESS

LLNL, PPPL, GA, U. Maryland, UCLA, U. Colorado • Builds on National Turbulent

Transport Project -- multi-institutional “Grand Challenge”

• Realistic Geometry– Full Torus (3D)– Flux Tube Codes

• Efficient Algorithms– Gyrokinetic --- PIC– Gyrokinetic --- Vlasov

Continuum

• Demonstrated scaling beyond 100’s of processors

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Full Torus Simulations of Turbulent Transport Scaling

• Large-scale full torus gyrokinetic particle simulations for device-size scans• Global field-aligned mesh saves factor ~100 in computation• Efficient utilization of new 5 TF IBM SP @ NERSC (just available 8/01) -- fastest non-classified supercomputer in world• Most recent simulations used 1 billion particles (GC), 125 M spatial grid points, and 7000 time steps --- leading to important (previously inaccessible) new results

Full Torus Simulations of Turbulent Transport Scaling

• Transport driven by microscopic scale fluctuations (ITG modes) in present devices can change character: transition from Bohm-like scaling ~ (ivi ) to Larmor-orbit-dependent “Gyro-Bohm” scaling ~ (ivi )(I/ a)

• “Rollover” is good news ! (since simple extrapolation is pessimistic)

0.01

0.1

1

10

100

1 10 100 1000 10000number of processors

com

putin

g po

wer

IBM SP

CRAY T3E

3D Gyrokinetic Toroidal Code (GTC)

Scalable on Massively Parallel Computers

Y-axis: number of particles (in millions) which move one step in one second

0.1

1

10

1 10

Plasma Edge Turbulence Studies: Experiment and Simulation Comparisons

• S. Zweben & J. Terry et al. + B. Rogers & K. Hallatschek, et al. + D. Stotler(Paper UI1.004)

• Gas Puff Imaging (GPI) Experiments on Alcator C-ModTokamak interpreted with

MPP neutrals code (DEGAS 2)

•GPI results compared vs. 3D EM fluid codelocal (flux tube) simulations of plasma 0.5 cm outside separatrix

k (cm-1)

Flu

ctua

tion

ampl

itude

Simulation

GPI Images

normalized to same total amplitude

Initial k-spectrumComparisons

New Cross-Disciplinary Opportunities for Diagnosing and Understanding Turbulence

Z. Lin, GTC SimulationG.J. Kramer, E. Valeo, R. Nazikian, Full Wave Simulation of -wave ReflectionS. Klasky, I. Zatz, Visualization

Target plasma Growth of Radial structures Zonal flows and decorrelation

Break-up and scattering of microwaves from plasma turbulence

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THE NATIONAL FUSION ENERGY SCIENCES COLLABORATORY

(involves 40 US sites in 37 states)• Collaboratory Goals:

-- enable more efficient use of experimental facilities by developing more powerful between pulse data analysis

-- enable better access by researchers to analysis & simulation codes, data, and visualization tools

-- create standard tool set for remote data access, security, and visualization

• Collaboratory Partners: D. Schissel, et al. :

-- 3 large fusion experiments*

* C-MOD, DIII-D, NSTX

-- 4 computer science centers **

** ANL, LBNL, Princeton U., U. of Utah

STELLARATOR DESIGN STUDIES

• Optimization of Stability, Transport, and Constructability for Designing National Compact Stellarator Experiment (NCSX)

• Utilization of MPP Computations Essential for Optimizations

�Collaboration on Magnetic Reconnection Simulations

U. Iowa, U. Chicago, U. Texas FLASH CODE: R. Rosner, et al., U. Chicago

• Solves fully compressible Navier Stokes Equations

(explicit viscosity, implicit dissipation, single-fluid

MHD)

• Fully parallel and uses Adaptive Mesh Refinement

• Supercomputing 2000/Gordon Bell Prize winner

• Large and diverse scope of applications

Cellular detonations

Compressed turbulence

Helium burning on neutron stars

Richtmyer-Meshkov instability

Laser-driven shock instabilitiesNova outbursts on white dwarfs

Rayleigh-Taylor instability

Flame-vortex interactions

Relation to other scientific disciplines

• Space Physics

– reconnection in Earth’s magnetosphere, solar corona,

astrophysical plasmas

– dynamos, collective phenomena, …….

• High Energy Physics– Collective dynamics impacting advanced accelerator design

• Industrial Applications– Plasma Processing, Xerography, Flat Panel Display, ….

• Computational Physics -- issues common to many areas– advances in solving partial differential equations in complex geometry,

– adaptive mesh refinement in 3D,

– parallel methods for inverting sparse matrices

– etc.

• Dipole “surface mode” can be destabilized with introduction of background electron component [BEST Code : 3D PIC for ions and electrons]

•Electron-Proton Two-Stream Instability Growing from Initial Noise•Two-stream instability can be stabilized by a modest axial momentum spread. [unwanted electrons in LANL Proton Storage Ring and the Spallation Neutron Source Project]

t=0 t=200

• Particle simulations with 70 M particles and 20 M grid points

• Development of turbulence in 3-D model– two-stream

instabilities

– anomalous resistivity

3-D Magnetic Reconnection and Anomalous ResistivityU. Maryland, Max Planck, Dartmouth

Zeiler, Swisdak, et al.GM1.003

Drake, et al., GM1.007

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Generation of “Electron Holes”(possible relevance to satellite observations)

• Intense electron beam generates two-stream instability– nonlinear evolution into “electron

holes”• localized regions of intense anti-

parallel electric field

– strong electron scattering

x

z

Ez

f

vz

ions

electrons

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Driving Applications

Sci

ence

/En

gin

eeri

ng

Sca

lab

le S

ervi

ces

Princeton University’s Princeton University’s

PICASso PICASso ProgramProgram

Program in Integrative Computer and Application Sciences

Integrative Research and Training in Entire Computational Pipeline

CS

PPPL

Astro

Geo

Bio

Eng.

GFDL

Genomics

Finance

Models Methods Software

Networksand

DistributedSystems

ScalableSystems

DataManagement Visualization

The Computational PipelineInternet Services

Biology, Genomics

Astrophysics

Plasma PhysicsGeosciences

Mobile Services

Information Archives

CONCLUSIONS• Advanced Computations is cost-effectively aiding progress

toward gaining the physics knowledge needed to harness fusion energy by making crucial contributions to all areas of Plasma Science.

• Advanced Computations is a natural bridge for fruitful collaborations between Plasma Science and other scientific disciplines.

• Plasma Science is both utilizing and contributing to the exciting advances in Information Technology and Scientific Computing.

• Computational Plasma Science is helping to attract, educate, &

retain young talent essential for the future.