What is fusion?What is fusion?

• It is combining two hydrogen It is combining two hydrogen atoms to form heliumatoms to form helium

• It’s the opposite of fission, It’s the opposite of fission, which is splitting uranium atoms which is splitting uranium atoms into smaller pieces.into smaller pieces.

• Either nuclear process gives Either nuclear process gives much more energy than chemical much more energy than chemical processes like burning gasoline.processes like burning gasoline.

Fusion is the energy of the sunFusion is the energy of the sunand the starsand the stars

The D-T reactionThe D-T reaction

D T

a

nDeuterium Tritium

Heavy hydrogen Heliu

m

Neutron

This is not the cleanest reaction, but it’s the easiest one to start with. The neutron causes a small amount of radioactivity, 1000 times less than in fission. Advanced fuels would be completely neutron-free.

Seawater is the fuel sourceSeawater is the fuel source

• Water contains one molecule of DWater contains one molecule of D22O O for every 6000 molecules of Hfor every 6000 molecules of H22O. O.

• The cost of separating deuterium is The cost of separating deuterium is trivial.trivial.

• There is enough deuterium to supply There is enough deuterium to supply mankind for billions of years.mankind for billions of years.

Accelerators would not workAccelerators would not work

T+

D+

T+ D+

Positive nuclei repel and will bounce off

Head-on collisions resulting in fusion are rare

We have to make a plasmaWe have to make a plasmaElectron (-)

Ion (+)

A plasma is a hot, ionized gas with equal numbers of ions and electrons. The energy lost in non-fusion collisions remains in the plasma. Once in a while, there is a fusion collision. This happens often enough if the plasma is dense enough and hot enough.

How hot and how dense?How hot and how dense?

• Temperature 300,000,000 degrees!Temperature 300,000,000 degrees!

• Density 1/10,000 of atmospheric Density 1/10,000 of atmospheric densitydensity

• Net pressure is 4 atmospheresNet pressure is 4 atmospheres

Use smaller numbers:Use smaller numbers:

1 eV (electron-volt) 1 eV (electron-volt) 10,000 10,000 KK 300,000,000 300,000,000 K K 30,000 eV = 30 keV 30,000 eV = 30 keV

How to hold this plasma?How to hold this plasma?

• No material wall can be used.No material wall can be used.

• The sun uses its large gravitational The sun uses its large gravitational field.field.

• On earth, we have only electric and On earth, we have only electric and magnetic fields (E and B fields).magnetic fields (E and B fields).

• E-fields not good: pushes + and – E-fields not good: pushes + and – charges in opposite directions.charges in opposite directions.

• Hence, we use magnetic fields.Hence, we use magnetic fields.We must make a “magnetic bottle”

What is a magnetic field?What is a magnetic field? The earth has a magnetic field, The earth has a magnetic field,

which makes compasses work.which makes compasses work. Iron filings show the field Iron filings show the field

of a horseshoe magnetof a horseshoe magnet

Coils can make B-fieldsCoils can make B-fields

V

+

-

Permanent magnet Electromagnet

How B-fields can hold a How B-fields can hold a plasmaplasma

B

A magnetic bottle cannot be a A magnetic bottle cannot be a spheresphere

B-field has to be zero at the poles

The simplest possible shape is a The simplest possible shape is a torustorus

The field lines can be toroidal, like this one

Or poloidal, like these

The The toroidaltoroidal field is field is produced by produced by poloidalpoloidal

currents in “coils”currents in “coils”

A B

A combination: helical linesA combination: helical lines

` `

PLASMA CURRENT

When the twist in the lines (the poloidal part) is produced by a current in the plasma, the magnetic bottle is called a TOKAMAK.

Step 1: cancel vertical drifts with Step 1: cancel vertical drifts with helical fieldhelical field

+

-

+

-B

This is the first principle of toroidal confinement

A B

X

+

+

Making a toroidal bottle workMaking a toroidal bottle work

A) The Rayleigh-Taylor instabilityA) The Rayleigh-Taylor instability

PLASMA

Pressure Pressure

ViViVe

Ve

MAGNETIC FIELD· · MAGNETIC FIELD· ·

+

++

+----

E

E ́B

PLASMA

(a) (b)

Step 2: Hydromagnetic instabilitiesStep 2: Hydromagnetic instabilities

Step 2: Hydromagnetic instabilitiesStep 2: Hydromagnetic instabilities

CURRENT

STRONG FIELD

WEAK FIELD

`

B) the kink instability

Shear stabilizationShear stabilizationUsed to stabilize both R-T and

kinks

The curvature effectThe curvature effect

8V

V88

Convex curvature has a strong stabilizing effect, but it cannot be incorporated well in a tokamak.

Step 3: MicroinstabilitiesStep 3: Microinstabilities

Plasma turbulence

Water turbulence

““Drift” waves were found to be the Drift” waves were found to be the cause of “Bohm diffusion”cause of “Bohm diffusion”

B

These waves are driven only by the pressure gradient in the plasma.

It took several decades to solve this problem. During this delay, fusion got a bad reputation.

The turbulence and fast loss rate have been eliminated by proper shaping of the magnetic field.

Step 4: Banana orbitsStep 4: Banana orbits“Neoclassical” diffusion“Neoclassical” diffusion

ST

RO

NG

FIE

LD

WE

AK

FIE

LD

WE

AK

FIE

LD

WE

AK

FIE

LD

ST

RO

NG

FIE

LD

ST

RO

NG

FIE

LD

PASSING ORBIT

Magnetic islands

The plasma in a TOKAMAK is a gas that moves in these unusual ways.

Computer simulationComputer simulation

Design of TOKAMAKS had to wait for computers able to handle 3D simulations.

Mother Nature is helping usMother Nature is helping us

0

1

2

3

4

0 2 4 6 8 10r (cm)

q (r

)

Unstable

q = 1

q = 2

q = 3

sawteeth

q = 1.5

1. Sawtooth oscillations

Mother Nature’s helping Mother Nature’s helping handhand

2. The H-mode (high confinement mode)

TRANSPORT BARRIER

PL

AS

MA

PR

ES

SU

RE

MINOR RADIUS0

n, T

Pedestal

To divertor

This increases confinement by 2X and has been studied extensively.The H-mode was discovered when powerful neutral-beam heating

was used.

Mother Nature’s helping Mother Nature’s helping handhand

3. Internal transport barriers

Learning from the H-mode, we have been able to produce transport barriers inside the plasma

B

+++- -

-+ ++

- - -

Mother Nature’s helping Mother Nature’s helping handhand

4. Zonal flows

+ + +

+ + +

+ + +

- - -

- - -

Long turbulent eddies break themselves up into small ones.

Jupiter

Other beneficial effects in Other beneficial effects in tokamakstokamaks

which arise naturallywhich arise naturally

• Bootstrap current (90% of tokamak Bootstrap current (90% of tokamak current can be produced by itself)current can be produced by itself)

• Isotope effect (DT confined better Isotope effect (DT confined better than DD)than DD)

• The Ware pinch (inward motion)The Ware pinch (inward motion)

How far have we come?How far have we come?

0.001

0.01

0.1

1

10

100

1965 1970 1975 1980 1985 1990 1995 2000 2005Year

Trip

le p

rodu

ct

T3

ST

TFR PLT

PDX

ALCATOR A

ALCATOR C

DIIIJET

JET

JET

TFTR

JT-60U

JT-60U

JT-60U

JT-60U

JT-60U

JT-60U

TFTRDIII-D

2-year doubling rate

Reactor level

Triple product Tn = Temperature x density x confinement time

Compare with Moore’s LawCompare with Moore’s Law

1,000

10,000

100,000

1,000,000

10,000,000

100,000,000

1,000,000,000

10,000,000,000

1970 1980 1990 2000 2010 2020

Year

No.

of

tran

sist

ors

per

chip

1.8

2.0

2.2

Years to double

8080

386

486

Pentium

Pentium M

Core 2 DuoCore 2 Quad

Itanium 2

286

8086

Tukwila23ilaAMD RV770

Itanium 2 Dual

Four large tokamaksFour large tokamaks

TFTR, Princeton, USA

JET, European

Union

DIII-D, General Atomic, USA

JT-60 U, Japan

Inside the DIII-DInside the DIII-D

The D-shape, with divertorThe D-shape, with divertor

The hot escaping plasma is absorbed by a “divertor”.

The tokamak scaling law The tokamak scaling law

Ability to predictAbility to predict

The pressure law The density law

Unsolved physics problemsUnsolved physics problems

Disruptions

ELMs (Edge Localized Modes)

Fishbones

These cause sudden loss of plasma. Ad hoc suppression has been devised, but no general solution.

ITER, the international ITER, the international tokamaktokamak

7 nations, > ½ world 7 nations, > ½ world populationpopulation

Site: Cadarache, FranceSite: Cadarache, France

Cost: 5B euros (construction), 5B euros (operation)

Construction underwayConstruction underway

The time lineThe time line

The aim of ITER is to reach ignition, when the alpha particle products of the DT reaction can keep the plasma hot without external heating.

Steps toward a reactorSteps toward a reactor

1.1. Show a burning plasma in ITERShow a burning plasma in ITER

2.2. Simultaneously build machines to Simultaneously build machines to test engineering conceptstest engineering concepts

3.3. Build a demonstration reactor DEMO Build a demonstration reactor DEMO producing small but significant powerproducing small but significant power

4.4. Build a 2000 MW fusion reactorBuild a 2000 MW fusion reactor

Major engineering Major engineering challengeschallenges

• A material for the First WallA material for the First Wall

• Energy handling by divertorsEnergy handling by divertors

• Breeding tritium in Li Breeding tritium in Li blanketsblankets

ConclusionsConclusions• Progress has been remarkable on a Progress has been remarkable on a

very tough problemvery tough problem

• The physics is understood well enough The physics is understood well enough to proceedto proceed

• The engineering has hardly started and The engineering has hardly started and needs to be heavily fundedneeds to be heavily funded

• There is an international will to solve There is an international will to solve both climate change and energy both climate change and energy shortage with this significant step in shortage with this significant step in human evolution.human evolution.