What is fusion It is combining two hydrogen

What is fusion? • It is combining two hydrogen atoms to form helium • It’s 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.

Fusion is the energy of the sun and the stars

The D-T reaction Heavy hydrogen Helium Neutron Deuterium Tritium 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 source • Water contains one molecule of D 2 O for every 6000 molecules of H 2 O. • The cost of separating deuterium is trivial. • There is enough deuterium to supply mankind for billions of years.

Accelerators would not work Positive nuclei repel and will bounce off Head-on collisions resulting in fusion are rare

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.

How hot and how dense? • • • Temperature 300, 000 degrees! Density 1/10, 000 of atmospheric density Net pressure is 4 atmospheres Use smaller numbers: 1 e. V (electron-volt) 10, 000 K 300, 000 K 30, 000 e. V = 30 ke. V

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”

What is a magnetic field? The earth has a magnetic field, which makes compasses work. Iron filings show the field of a horseshoe magnet

Coils can make B-fields Permanent magnet Electromagnet

How B-fields can hold a plasma B

A magnetic bottle cannot be a sphere B-field has to be zero at the poles

The simplest possible shape is a torus The field lines can be toroidal, like this one Or poloidal, like these

The toroidal field is produced by poloidal currents in “coils”

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.

Making a toroidal bottle work Step 1: cancel vertical drifts with helical field This is the first principle of toroidal confinement

Step 2: Hydromagnetic instabilities A) The Rayleigh-Taylor instability

Step 2: Hydromagnetic instabilities B) the kink instability

Shear stabilization Used to stabilize both R-T and kinks

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

Step 3: Microinstabilities Plasma turbulence Water turbulence

“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.

Step 4: Banana orbits “Neoclassical” diffusion Magnetic islands The plasma in a TOKAMAK is a gas that moves in these unusual ways.

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

Mother Nature is helping us 1. Sawtooth oscillations

Mother Nature’s helping hand 2. The H-mode (high confinement mode) This increases confinement by 2 X and has been studied extensively. The H-mode was discovered when powerful neutral-beam heating was used.

Mother Nature’s helping hand 3. Internal transport barriers Learning from the H-mode, we have been able to produce transport barriers inside the plasma

Mother Nature’s helping hand 4. Zonal flows Jupiter Long turbulent eddies break themselves up into small ones.

Other beneficial effects in tokamaks which 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)

How far have we come? Triple product Tn = Temperature x density x confinement time

Compare with Moore’s Law

Four large tokamaks TFTR, Princeton, USA JET, European Union DIII-D, General Atomic, USA JT-60 U, Japan

Inside the DIII-D

The D-shape, with divertor The hot escaping plasma is absorbed by a “divertor”.

The tokamak scaling law

Ability to predict The pressure law The density law

Unsolved physics problems ELMs (Edge Localized Modes) Disruptions Fishbones These cause sudden loss of plasma. Ad hoc suppression has been devised, but no general solution.

ITER, the international tokamak

7 nations, > ½ world population

Site: Cadarache, France Cost: 5 B euros (construction), 5 B euros (operation)

Construction underway

The 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 reactor 1. Show a burning plasma in ITER 2. Simultaneously build machines to test engineering concepts 3. Build a demonstration reactor DEMO producing small but significant power 4. Build a 2000 MW fusion reactor

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

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.