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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.
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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.
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 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.
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 300,000,000 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
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? Iron filings show the field of a horseshoe magnet The earth has a magnetic field, which makes compasses work.
Coils can make B-fields Electromagnet Permanent magnet
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
A) The Rayleigh-Taylor instability Step 2: Hydromagnetic instabilities
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 3D 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 2X 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 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)
How far have we come? Triple product Tn = Temperature x density x confinement time
Four large tokamaks TFTR, Princeton, USA JET, European Union DIII-D, General Atomic, USA JT-60 U, Japan
The D-shape, with divertor The hot escaping plasma is absorbed by a “divertor”.
Ability to predict The pressure law The density law
Unsolved physics problems Fishbones ELMs (Edge Localized Modes) Disruptions These cause sudden loss of plasma. Ad hoc suppression has been devised, but no general solution.
