1MIT-ANS Apr 07

The Challenges of Plasma-Surface Interactions in Magnetic Fusion

Dennis Whyte, MIT

MIT ANS Seminar

April 9, 2007

2MIT-ANS Apr 07

Why don’t we have fusion reactors yet?

x

Worst Jobs in Science

=

Popular ScienceOct. 2003

3MIT-ANS Apr 07

Materials impose one of our biggest challenges in developing fusion energy

• Magnetic fusion produces a thermonuclear volumetric energy source, where all energy must be extracted through a single, surrounding material surface.

• The nuclear fusion fuel cycle and the plasma physics of heat and particle exhaust simultaneously place severe demands on material surfaces.

• The strongly coupled interaction between fusion plasmas and the material walls challenges our ability to control and predict plasma-surface interactions (PSI).

• Significant worldwide research progress and effort continues on the PSI requirements of the ITER prototype fusion reactor.

4MIT-ANS Apr 07

Deuterium-Tritium fusion represents a nearly inexhaustible energy source.

Fuels: Deuterium: abundant in sea water Tritium: Half-life~12 years…must be produced?

Tbred / Tburned > 1

“Real” fusion fuel cycle:6Li + D = 24He + 22.4 MeV

30 Million years of world energydemand in oceans

6Li + n --> T + 4He + 4.8 MeV + others

5MIT-ANS Apr 07

D-T fusion requires a confined thermonuclear plasma

• E > 10,000 eV ionization separates electrons and ions.

• Coulomb scattering >> fusion reaction thermalized plasma T ~ 10,000 eV ~ 100,000,000 K

• Far out of thermodynamic equilibrium in terrestrial environment plasma exhausts power at e need confinement.

Fusion plasma cross sections

0.0001

0.001

0.01

0.1

1

10

100

1000

10000

1 10 100 1000

Particle energy (keV)

cross-section (barns)

CoulombscatteringD +T

D + 3He

6MIT-ANS Apr 07

Fusion energy by magnetic confinement

• Exploits Lorentz force No confinement // to B.

• Fusion energy balance: 4/5 in neutron leaves energy 1/5 in He++ confined by B to heat &

sustains plasma.

• Every Joule of fusion and exhaust power must be extracted through a single surrounding surface.

B

7MIT-ANS Apr 07

The toroidal “tokamak” is the most prevalent magnetic confinement geometry

• Self-closing helical magnetic fields.

• Stabilizing toroidal field produced by external coils. B 1 / R

• Plasma current provides confining poloidal field against -B drifts.

I

R

8MIT-ANS Apr 07

Plasma pressure, confinement and stability set the requirements for a fusion reactor.

Pfusion n2 T2

PowerBalance

PowerDensity

Confinement

KinkStability€

e ∝ HIp

PH

TripleProduct

Energy Gain

Self-sustaining

Size& Field$$$

Q=5 “burning”

9MIT-ANS Apr 07

Translation of Fusion Strategy

• Build it big, because bigger things hold their heat longer!

• Make the magnetic field as big as possible to hold onto those particles!

• Heat the heck out of it to get 100,000,000 K and light the fire!

10MIT-ANS Apr 07

et voila, ITER

• Mission: “Burning plasma” Q=10

• Reactor level fusion power Pfusion ~ 400 MW for 500 s Heating = 40 MW 20% duty cycle

• Size R~6 m +Field B~6 T = 5 Billion dollars 1000 m3 plasma 1000 m2 plasma-facing wall

11MIT-ANS Apr 07

Like a car, there are several things you have to do to keep the fusion “engine” running.

Power removal Coolant in block, no melting

Fuel control. Gas tank full, no flooding

Helium ash exhaust Tailpipe

Plasma purity No dirt in cylinders

Magnetic topology, plasma physics & the fusion fuel cycle set unique

requirements for wall materials in fusion

Fusion Car

12MIT-ANS Apr 07

Magnetic topology

• Magnetic field line directly terminated on a solid has no confinement due to sink action of wall.

• This separatrix flux surface is the primary interface between thermonuclear plasma and outside world.

• (SOL) Scrape-Off Layer outside separatrix is the “exhaust pipe” of a fusion plasma.

• Divertor targets.

13MIT-ANS Apr 07

Divertor magnetic topologyis presumed for most fusion reactors

• SOL “fed” by cross-field heat and particle losses from “core”

• Advantage Concentrates power and particle

exhaust (q, ) in one location.

• Disadvantages: Concentrates power and particle

exhaust in one location. Wastes magnetic volume.

Core plasma

SOL

q//

/

qtarget, target

surface

Ploss ~ 100-400 MW

B

14MIT-ANS Apr 07

Divertor magnetic topologyis used in ITER (and probably a reactor)

• ITER Magnetic Field-Line Geometry in SOL: Bpoloidal / B ~ 1/10

SOL: L// ~ 100 m

2 R ~ 50 m

ITER Divertor Cross-section

~ 1m

15MIT-ANS Apr 07

Extraordinary plasma heat conduction along B primarily set SOL properties

• // e- Heat conductivity:

€

κ //

κ⊥

∝ ωce τ e−i( )2≅108

€

q// ≈PLoss

2π RL⊥

BT

Bp

~ 2GW

m2

€

L⊥ = L//

κ //

κ⊥

⎛

⎝ ⎜

⎞

⎠ ⎟

−1/ 2

∝100m • 10−4 ≅10−2 m

• ITER energy exhaust: PLoss ~ 150 MW

€

TSOL =7

2

q//

κ o L//

⎛

⎝ ⎜

⎞

⎠ ⎟

2 / 7

≈150eV ~ 1,500,000K

€

q// = κ // ∇T// ≈ κ //

TSOL

L//

€

κSOL ≅ 50, 000W

m ⋅ K~ 100 × κ Cu

€

κ // = κ o T 5 / 2

16MIT-ANS Apr 07

Conforming divertor surface

Constant heat removal at divetor surfaces is daunting: #1 priority in edge “design”

€

qtarget = q//

Bp

B φ

⎛

⎝ ⎜ ⎜

⎞

⎠ ⎟ ⎟divertor

≅ q// 0.02 ≈ 40MW

m2

Field lines

Melts 10 cm of tungsten in ~20 seconds !

Distorted surface “proud” to thefield line receives q// ~ 1 GW/m2

and is immediately melted/ablated.Distorted divertor surface

Field lines€

≈ 50MW

m2

2 R

17MIT-ANS Apr 07

The core fusion plasma has little tolerance to impurities

• Plasma quasi-neutrality sets strict limits for impurities Fuel dilution. Radiation energy losses.

0.0001

0.001

0.01

0.1

0 20 40 60 80

Impurity atomic number

fatal fraction

• Dilute plasma (n~1020 m-3) extinguished by small particulate injection.ITER example: 10 mm3 “drop” of W ~ Ne,plasma

WMo

C

18MIT-ANS Apr 07

Heat exhaust, Tmelt, material stress and heat conductivity set armour thickness

q

dplasmacoolantCoolant

substrate

• Limits material choices to refractory metals (W, Mo) or graphite.

€

dtile = κ plate

(Tmax, surface − Tcoolant )

qtarget

Tungsten CFC carbon

κ (W/m/K) 150 300

Tmelt (K) 3700 3700

dtile ~ 1 cm ~ 2 cm

19MIT-ANS Apr 07

W & C bonding technology capable of exhausting ~ 25 MW/m2

~ 1mITER prototypedivertormodule

castellations

20MIT-ANS Apr 07

Plasma instability leads to large transient heating: Requires high Tmelt materials

• MHD instability = Destruction of nested flux surfaces.

• High Te flux surfaces connect to wall.

• tMHD ~ 0.0001 seconds Edge Localized Mode: ELM Global instability: Disruption

• All energy is “held up” near surface

Kru

ger,

AP

S04

€

δ = D τ MHD ~ 0.1 mm

QuickTime™ and aBMP decompressor

are needed to see this picture.

21MIT-ANS Apr 07

Materials pushed past their thermal limits even in present fusion devices.

• Mo tile “limiter” positioned outside hot core plasma in MIT tokamak (C-Mod)

• Plasma heat exhaust and “non-thermal” electron populations increase past Tmelt ~ 2900 K in < 2 seconds exposure.

• Reactor must run 24/7.

22MIT-ANS Apr 07

Movie Clips of Alcator C-Mod

23MIT-ANS Apr 07

Understanding competition between “density-driven” radiation and “T-driven” conduction

leads to safe shutdown technique

Uniform radiation

Prad n2 (1/T)

Localized heatconduction

Q// κ// T5/2

Solution:

“Force” high density by massive impurity injection

24MIT-ANS Apr 07

Competition between “density-driven” radiation and “T-driven” conduction critical to

benign energy dissipation

QuickTime™ and aCinepak decompressor

are needed to see this picture.

25MIT-ANS Apr 07

Even ideal radiative energy dissipation can cause material melting in ITER.

Whyte 2004 PSI

Prad ~ 3 TW ~ electricity output of US

Bemelt ~ 100 kG

26MIT-ANS Apr 07

Particle control #2 priority: Fusion reactors do not burn their fuel efficiently, forcing very large recycling of the

tritium fuel.

Global breeding ratio in Blanket

€

fbreed ≡tritons

neutron~ 1.05

Maximum allowed core helium fraction

€

f He ≡nHe

nD + nT

≅ 0.1

Measured helium (de)enrichment from core to divertor

€

η ≡n0( )He

n0( )D +T

nD + nT

nHe

~ 0.2

€

Triton flowthough

He pump rate≡

˙ N T˙ N He

=1

f He η~ 50 =

Triton flowthough

Triton burn rate

Allowed rate of TritiumDeposition in wall

€

˙ N T ,wall ≤ ˙ N n 1 − fbreed( ) = ˙ N He ⋅ 0.05 ≅˙ N T

1000

27MIT-ANS Apr 07

World tritium inventory impacts ability to “start” a fusion energy economy

• A 1000 MWe will burn / produce ~ 0.5 kg ~ 1 pound of Tritium per day.

• The time window for non-T-breeding burners (e.g. ITER, CTF) is short.

• Tritium Breeding Ratio of starting reactors must be > 1 or whole system will

grind to halt from lack of fuel.

J. Schmidt, IEA 2005

28MIT-ANS Apr 07

Developing neutron tolerant materials will probably be last problem solved for fusion

• Uniform material bombardment by 14 MeV neutrons ~ 1m thick blanket to

thermalize, shield neutrons & breed Tritium

• Displacements per atom in wall ~10-20 per year for 1 GW Leads to serious thermal

degradation of materials. Internal p, He production by

nuclear reactions

29MIT-ANS Apr 07

Developing neutron tolerant materials will probably be last problem solved for fusion

• Uniform material bombardment by 14 MeV neutrons ~ 1m thick blanket to thermalize,

shield neutrons & breed Tritium

• Displacements per atom in wall ~10-20 per year for 1 GWth

Leads to serious thermal degradation of materials.

Internal p, He production by nuclear reactions not an issue in fission reactors due E

• Solution will require dedicated experimental & modeling, probably exploiting self-annealing at high material temperatures

30MIT-ANS Apr 07

Neutron irradiation tests of wall materials are needed for fusion reactors

• International Fusion Materials Irradiation Facility will probably be part of broader fusion program.

• Deuteron beam (0.25 MA, ~40 MeV) on flowing lithium target produces fast neutron spectrum But only produces 0.5 L volume that

will receive reactor-like neutron damage (~20 dpa/year)!

IFMIF

31MIT-ANS Apr 07

The material targets must also resist erosion caused by massive energetic particle

throughput in wall

• Vsheath ~ 5 x Te ~ 100-500 eV

€

jion =qtarget

Vsheath

~ 50kA

m2

0.0001

0.001

0.01

0.1

10 100 1000 10000

Incident deuterium ion energy (eV)

Sputter yield

Beryllium

Carbon

Molybdenum

Tungsten

• Plasma ions can rapidly sputter target material away.

€

sputter =jion

k⋅

Y

N target

~ 30nm

s~ 1

m

year

32MIT-ANS Apr 07

Material migration is set by sputtering / recycling asymmetry.

D/T saturateddeposits

33MIT-ANS Apr 07

Divertor “detachment”: Critical to easing power exhaust and sputtering

ne increasing

Alcator C-Mod

34MIT-ANS Apr 07

Detachment solved erosion?

DIII-D:Mapof divertorErosion / deposition

Why

te, I

AE

A 2

000

35MIT-ANS Apr 07

Detachment solved erosion? Yes!Yes, but another problem appeared

DIII-D:Mapof divertorErosion / deposition

Why

te, I

AE

A 2

000Tritium trapped in

plasma deposited films from other wall locations

Rate ~ 1 in 10 fuelled T lost

But requirement:

< 1 in 1000 !

Solution:

> 1000 K walls to deplete H/D/T ?

36MIT-ANS Apr 07

Turbulent cross-field particle transport Erosion sources outside divertor Long-range transport to divertor

QuickTime™ and aYUV420 codec decompressor

are needed to see this picture.

S. Zweben, J. Terry, C-Mod

A. Mclean, et al. DIII-D

37MIT-ANS Apr 07

C-Mod now shows a strong link between ballooning transport, rotation, long-range SOL

transport, T retention and H-mode!

ALCATOR C-Mod, M.I.T.LaBombard, Greenwald APS 04

Ballooningtransport

SOL flow

CoreRotation

38MIT-ANS Apr 07

Besides groundbreaking research on tokamaks like C-Mod, laboratory experiments are coming

online to enhance our understanding of PSI

MAGNUM-PSIHigh power, low Te H plasmas

DIONISOSDynamics of PSI

FOM, Netherlands MIT

39MIT-ANS Apr 07

Besides groundbreaking research on tokamaks like C-Mod, laboratory experiments are coming

online to enhance our understanding of PSI

MAGNUM-PSIHigh power, low Te H plasmas

DIONISOSDynamics of PSI

FOM, Netherlands MIT

QuickTime™ and aMotion JPEG OpenDML decompressor

are needed to see this picture.

40MIT-ANS Apr 07

x

= A Grand Scientific Challenge for Ours & Future Generations

Worst Most Exciting Job in Science

Whyte

Fusion Wall equation

41MIT-ANS Apr 07

The suppression of edge MHD ELMs is one of the most critical

research issues for ITER & beyond

Eich 2004 PSI

Size& Field$$$

42MIT-ANS Apr 07

Net redeposition or erosion: Deposition rate – erosion rate

Impurity release via sputtering at PFC surface

Impurity ionization & transport near surface

> 90% impurity re-deposited at surface

Local

Net erosion & deposition arises from ~1-10% local flux imbalance

Edge plasma modification by impurity

Core plasma modification from impurities

SOL transport

Release of impurities to SOL

Global

43MIT-ANS Apr 07

Divertors concentrate particle flux and recycling, making practical He and H pumping exhaust possible

particle control

• This is not a trivial feat; fuel particle inventory in a fusion device (ITER) is dominated by the wall.

Nwall = Awall ~ 5•1021 m-2 103 m2 ~ 5• 1024 m

Nplasma = n Vplasma ~ 1020 m-3 103 m3 ~ 1023 m

44MIT-ANS Apr 07

Parallel heat conduction sustains particle ionization / recycling loop in divertor.

• Consider simple pressure and particle conservation /w Bohm sheath criterion (vtarget = cs)

• Two-point model (Stangeby) Upstream: SOL Target: Divertor

ion,eH

ionization

Core plasma SOL

q////

qtarget, target

surface

Ploss

B

€

Tu ∝ q//2 / 7

€

ntarget ∝ nu3 q//

−8 / 7

€

target ∝ nu2 q//

−3 / 7

€

Ttarget ∝ nu−2 q//

10 / 7

45MIT-ANS Apr 07

Competition between “density-driven” radiation and “T-driven” conduction critical to

benign energy dissipation