Energetic Particles in Plasmas - Stanford University · GCEP: Energetic Particles in Plasmas 1 ... scales—but can measure with ... modes and trapped particle effects. GCEP:

GCEP: Energetic Particles in Plasmas 1

Energetic Particles in Plasmas

James W. Van Dam

Institute for Fusion StudiesThe University of Texas at Austin

May 1-2, 2006


Introduction

• In addition to thermal ions and electrons, plasmas often contain a supra-thermal species = “energetic particles”– Highly energetic (Th >> Ti) and comparable pressure (nhTh ! niTi)

• Energetic particles can be created from various sources:– Ion/electron cyclotron heating or neutral beam injection —> high energy “tails”– Fusion reactions (e.g., for D-T, v" ! vAlfvén, hence instabilities are possible)

• The plasma physics of energetic particles is of interest to:– Laboratory fusion plasmas (alphas provide self-heating to sustain ignition)– Space and astrophysical plasmas (e.g., proton ring in Earth’s magnetosphere)– High-energy-physics accelerators (collective effects)


Impacts

• Energetic particles per se:– Excitation of various Alfvén-type instabilities (lead to anomalous transport)– Redistribution and loss (reduces alpha particle heating efficiency; causes heat

loading and damage to plasma-facing components)

• Integrated with overall plasma behavior:– Macrostability (fishbones & monster sawteeth; ballooning modes; disruptions

and runaway electrons)– Transport (ripple loss; profile modification; rotation generation)– Heating and current drive (dominant nonlinear self-heating)– Edge physics (resistive wall mode stability)– Burn dynamics (thermal burn stability; fuel dilution by helium ash)


GCEP Questions

• Scientific issues:– What are the scientific and technical barriers to the realization of fusion power

that are being addressed in energetic particle physics?– What breakthroughs are still required for overcoming them?

• Suggestions:– Summarize the research priorities, and why.– Are they covered by or complimentary to current programs?– Where could GCEP contribute and have best impact?


Barriers and Breakthroughs

• Indirect (infer from wave properties)

• Direct (measure core plasma fluctuations andenergetic particle distribution function)

Diagnostics

• Convective vs diffusive

• n=0 response (Geodesic Acoustic Mode)

• Multi-mode experiments (avalanche)

Energetic particle losses

• Fluid & kinetic resonances

• Near-marginality; hard vs soft behavior

Nonlinear wave dynamics

Growth/damping ratesInstability thresholds

Needed BreakthroughsSci/Tech Barriers


Instability Thresholds

• Fast particles can destabilize a largetaxonomy of Alfvén modes (*AE)– e.g., Toroidal Alfvén Eigenmode (TAE)

• Mode identification is robust:– Frequency, mode structure, polarization

• Threshold is determined by balance of:– Growth rate (reliably calculate)– Damping rate (calculation is very

sensitive to parameters, profiles, lengthscales—but can measure withactive/passive antennas)


ITER Stability

• ITER will operate with a large population of super-Alfvénic energetic particles– New small-wavelength (#*) regime implies presence of many modes

– NSTX (low-B, low-shear) is an excellent laboratory for fast particle studies

FredricksonPitchfork bifurcations (JET)

#*-1 =


Nonlinear Theory: Comparisons

• Excellent agreement with experiments• Excellent agreement between theory(single mode) and simulation

Solid curve = Berk-Breizman theory(with sources and sinks)

Circles = White-Chen $f code

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Experiment

52.56 52.6 52.64 52.68 52.72

Central lineUpshifted sidebandDownshifted sideband

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t (sec)

Simulation

Am

plitu

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Pitchfork bifurcations (JET)


TAE Intermittent Losses• Simulations of rapid losses

– Recently added Geodesic Acoustic Mode

Figure 8 (Y. Todo et al.)

Counter-injectedbeam ions

Co-injectedbeam ions

Todo et al. (2003)

Notable incident of hole punched in TFTRvacuum vessel by lost fast ions

K.L. Wong (1990)

• Toroidal Alfvén Eigenmode exp’ts– Loss of fast heating ions (seen

from reduced neutron rate)


Using Wave Properties -1• Determine internal fields from

frequency sweeping

• Determine internal fields from2nd harmonic Alfvén Cascadeperturbed density

Theory & simulation (Petviashvili et al.)

TAEs in MAST (Gryaznevich) “MHD spectroscopy”


Using Wave Properties -2• Temperature inferred from low-

frequency suppression ofCascade modes

• Monitor qmin (for creating an internaltransport barrier) with Grand Cascadeonset

Joffrin et al.


New Diagnostics-1

• A number of new/upgradeddiagnostics can now measureinternal fluctuations– Interferometry

– Reflectrometry

– Far Infrared Scattering

– Phase Contrast Imaging

– Beam Emission Spectroscopy

– Electron Cyclotron Emission

Sharapov, PRL 93 (2004) 165001

JET data


New Diagnostics-2

• Recent new fast ion profilediagnostics– Collective Thomson scattering

– Solid-state Neutral ParticleAnalysis

– Neutron Collimators

Gamma-ray tomography (Kiptily)

D-alpha (Heidbrink)


Research Priorities

User-friendly codesSome work being doneAssess instabilitythresholds (e.g., ITER)

Int’l collaboration (JA)Modest effortQuantify fast iontransport

Joint postdoc: fishbone;marginal stability profiles

Modest effortUnderstand nonlineardynamics

Int’l collaboration (EU)Some work being done onexisting experiments

Exploit wave propertiesfor indirect diagnostics

New diagnostics forburning plasma context

Good work being done onexisting experiments

Develop new fastparticle diagnostics

GCEPContribute

CurrentPrograms

Priorities


Opportunities/Alternatives

• Energetic particle physics area:– Alpha “channeling”

– Rotation generation and current drivegeneration by alpha particles to maintainAdvanced Tokamak operation

– Alfvén waves in linear device (LAPD)

– High-energy particles in space physics

• Other areas:– Liquid metal walls– Advanced divertors

• Educational proposal


Alpha “Channeling”

• Idea for transferring energy of fusionalphas directly to plasma ions throughwaves– Avoids inefficient intermediate step of

slowing down on thermal electrons

• TFTR experiments showed that thereverse process—energy transfer tobeam ions by RF wave heating—canoccur– The corresponding interaction with

alpha particles has not yet beenobserved

Fisch & Rax


Rotation Generation

• Idea for creating sheared rotation and negativemagnetic shear (conducive to formation of“internal transport barrier”) by having Alfvéninstabilities redistribute fast ions radiallyoutward

– Recent experimental indications (DIII-D)– Suggests phase-space engineering in burning

plasma to optimize performance by using trappedenergetic particles to generate flow and controlnon-inductive current profile (sustain AdvancedTokamak operation as a natural steady state?)

K. Wong


Basic Wave Studies

• LArge Plasma Device (LAPD), akaBasic Plasma Science Facility (BaPSF)– Long (20 m), large-diameter (1 m),

well-diagnosed linear plasma facilitywith uniform guiding magnetic field

– Useful for basic studies of propagationand nonlinear properties of waves

– Recent idea to apply quasi-periodicmulti-mirror field to study Alfvén “gap”modes and trapped particle effects


Space Physics

• Use fast particle methodology foranalysis of dipole stability of very-high-pressure plasma– Also explains “substorms” in Earth’s

magnetosphere

MIT & Columbia


Liquid Metal Walls

• Innovation: confine plasma withliquid (instead of solid) metal walls– Removes high heat flux– Stabilizes plasma– Immune to neutrons– Enhance tritium breeding– No thermal stress

u u u

Ja Poloidal Ja % B Radial

B Toroidal

• Invention of “soaker hose” concept:– Coat the walls with slow-streaming

liquid metal


Advanced Divertors

Kotschenreuther et al.

New X-divertor coils create an extra x-pointon each divertor “leg”

NewInboard

X-DivertorCoils

• Fusion reactor heating power is 5-10 timeshigher than in ITER (P" ~ 100 MW)

• ITER is at the limit for standard divertor;hence does not extrapolate to a reactor

NewOutboard

X-DivertorCoils


ITER Summer School

• Need to train the next generation of young people to work on ITER

• Propose a GCEP Summer School on ITER:– Teach the integrated physics and technology of burning plasmas (including

energetic particles)– Include lectures on global climate and world energy– Publish the lectures (book; online videos)– Hold it on university campuses; rotate the location around the country– Scholarships to cover student costs– Accessible to postdocs, graduate students, and advanced undergraduates


References• U.S. Burning Plasma Workshop (Oak Ridge, TN, 2005):

www.burningplasma.org/WS_05/html– Energetic particle physics plenary talk, break-out group presentations, and summary

• 9th IAEA Technical Meeting on Energetic Particles in Magnetic Confinement Systems(Takayama, Japan, 2005): http://htpp.lhd.nifs.ac.jp/IAEATM-EP2005/index.html

• Joint Transport Task Force/US-Japan JIFT Workshop on Energetic Particles (Napa, CA,2005): www.mfescience.org/TTF2005/

• 8th IAEA Technical Meeting on Energetic Particles in Magnetic Confinement Systems (LaJolla, CA, 2003): www.gat.com/conferences/iaea-tm-energetic/index.html

• ITER Physics Basis Document, Chap. 5 “Energetic Particles,” Nuclear Fusion 29, 3471(1999).

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Energetic Particles in Plasmas - Stanford University · GCEP: Energetic Particles in Plasmas 1 ... scales—but can measure with ... modes and trapped particle effects. GCEP: