What is fusion

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What is fusion?

• It is combining two hydrogen atoms to form
  helium
• Its the opposite of fission, which is
  splitting uranium atoms into smaller pieces.
• Either nuclear process gives much more energy
  than chemical processes like burning gasoline.

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Fusion is the energy of the sunand the stars
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The D-T reaction
Heavy hydrogen
Helium
Neutron
Deuterium
Tritium
This is not the cleanest reaction, but its 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.
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Seawater is the fuel source

• Water contains one molecule of D2O for every
  6000 molecules of H2O.
• The cost of separating deuterium is trivial.
• There is enough deuterium to supply mankind for
  billions of years.

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Accelerators would not work
Positive nuclei repel and will bounce off
Head-on collisions resulting in fusion are rare
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We have to make a plasma
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.
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How hot and how dense?

• Temperature 300,000,000 degrees!
• Density 1/10,000 of atmospheric density
• Net pressure is 4 atmospheres
• Use smaller numbers
• 1 eV (electron-volt) ? 10,000 ?K
• 300,000,000 ?K ? 30,000 eV 30 keV

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How to hold this plasma?

• No material wall can be used.
• The sun uses its large gravitational field.
• On earth, we have only electric and magnetic
  fields (E and B fields).
• E-fields not good pushes and charges in
  opposite directions.
• Hence, we use magnetic fields.

We must make a magnetic bottle
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What is a magnetic field?

• The earth has a magnetic field, which makes
  compasses work.

Iron filings show the field of a horseshoe
magnet
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Coils can make B-fields
Electromagnet
Permanent magnet
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How B-fields can hold a plasma
B
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A magnetic bottle cannot be a sphere
B-field has to be zero at the poles
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The simplest possible shape is a torus
The field lines can be toroidal, like this one
Or poloidal, like these
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The toroidal field is produced by poloidal
currents in coils
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A combination helical lines
When the twist in the lines (the poloidal part)
is produced by a current in the plasma, the
magnetic bottle is called a TOKAMAK.
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Step 1 cancel vertical drifts with helical field
Making a toroidal bottle work
This is the first principle of toroidal
confinement
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A) The Rayleigh-Taylor instability
Step 2 Hydromagnetic instabilities
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Step 2 Hydromagnetic instabilities
B) the kink instability
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Shear stabilization
Used to stabilize both R-T and kinks
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The curvature effect
Convex curvature has a strong stabilizing effect,
but it cannot be incorporated well in a tokamak.
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Step 3 Microinstabilities
Plasma turbulence
Water turbulence
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Drift waves were found to be the cause of Bohm
diffusion
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.
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Step 4 Banana orbitsNeoclassical diffusion
Magnetic islands
The plasma in a TOKAMAK is a gas that moves in
these unusual ways.
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Computer simulation
Design of TOKAMAKS had to wait for computers able
to handle 3D simulations.
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Mother Nature is helping us
1. Sawtooth oscillations
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Mother Natures helping hand
2. The H-mode (high confinement mode)
This increases confinement by 2X and has been
studied extensively. The H-mode was discovered
when powerful neutral-beam heating was used.
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Mother Natures helping hand
3. Internal transport barriers
Learning from the H-mode, we have been able to
produce transport barriers inside the plasma
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Mother Natures helping hand
4. Zonal flows
Jupiter
Long turbulent eddies break themselves up into
small ones.
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Other beneficial effects in tokamakswhich arise
naturally

• Bootstrap current (90 of tokamak current can be
  produced by itself)
• Isotope effect (DT confined better than DD)
• The Ware pinch (inward motion)

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How far have we come?
Triple product Tn? Temperature x density x
confinement time
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Compare with Moores Law
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Four large tokamaks
TFTR, Princeton, USA
JET, European Union
DIII-D, General Atomic, USA
JT-60 U, Japan
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Inside the DIII-D
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The D-shape, with divertor
The hot escaping plasma is absorbed by a
divertor.
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The tokamak scaling law
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Ability to predict
The pressure law
The density law
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Unsolved physics problems
Fishbones
Disruptions
ELMs (Edge Localized Modes)
These cause sudden loss of plasma. Ad hoc
suppression has been devised, but no general
solution.
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ITER, the international tokamak
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7 nations, gt ½ world population
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Site Cadarache, France
Cost 5B euros (construction), 5B euros
(operation)
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Construction underway
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The time line
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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.
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Steps toward a reactor

• Show a burning plasma in ITER
• Simultaneously build machines to test
  engineering concepts
• Build a demonstration reactor DEMO producing
  small but significant power
• Build a 2000 MW fusion reactor

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Major engineering challenges

• A material for the First Wall
• Energy handling by divertors
• Breeding tritium in Li blankets

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Conclusions

• Progress has been remarkable on a very tough
  problem
• The physics is understood well enough to proceed
• The engineering has hardly started and needs to
  be heavily funded
• There is an international will to solve both
  climate change and energy shortage with this
  significant step in human evolution.