Fusion Energy Concepts + Glossary

Chemical vs. Nuclear Energy

Nuclear reactions involve changes to the nuclei of atoms, converting one or more elements into another.  Chemical reactions involve rearrangement of the electrons OUTSIDE the nuclei of atoms.  In chemical reactions the nuclei, and intrinsically the elements they're named, remain unchanged.  Chemical energy production is typically the combustion of hydrogen, natural gas, oil, coal, wood, etc. (in ascending order of carbon content).  Nuclear reactions can be millions of times more intense than chemical reactions, because they convert mass to energy (Einstein's E=mc^2).

The Strong Force

Nuclei are held together with the Strong Force, an always-attractive force between protons, neutrons and each other.  Appropriately named, it is very strong, but only over very short distances, such as a femtometer.  It requires tremendous force to push nuclei together past the tipping point, where the attractive Strong Forces exceed the Electrostatic Forces repelling their positively charged protons.  When this happens the nuclei merge and a new element is formed.  This hasn't ever happened naturally on Earth.  It happens in stars.

Fusion vs. Fission

Nuclear energy is a result of forcing together light nuclei (fusion) or splitting apart heavy ones (fission).  Fission is the energy responsible for all of the nuclear power plants built so far.  Fusion is the future.  Both reactions are a conversion of mass to energy.  The difference between the weights of the inputs and outputs is what was converted to energy according to Einstein's famous formula.

The downsides of fission are primarily:

1. It produces waste that remains significantly radioactive for thousands of years.

2. Its fuel is limited on Earth.

3. It can "melt down".

4. Its fuels can also be used to produce weapons.

Conversely, fusion:

1. Produces waste that remains significantly radioactive for dozens of years.

2. Requires fuel that is effectively limitless on Earth.

3. It cannot "melt down".

4. Its fuels (on their own) cannot be used to produce weapons.

Both fission and fusion are carbon-free sources of energy.

Tokamaks

A Tokamak is a doughnut-shaped device that burns a hydrogen isotope fuel (typically Deuterium (D) and Tritium (T)) through fusion to create helium and heat.  The heat can be used to convert water to steam and drive turbines to create electricity.  The Deuterium and Tritium are heated until they are so energetic that their electrons are no longer associated with individual nuclei, a state called a plasma.  The disassociated nuclei are energetic and repel each other, but can be contained by strong electromagnets.   Our sun does something similar, but with higher pressure and lower temperature.  The sun's pressure is a result of gravity; a Tokamak's pressure is the result of magnetic confinement.  Since D+T have 5 neutrons and helium only 4, a highly energetic neutron is released during the conversion.  The extra neutron is helpful to convey the desired output heat, but also is destructive to the reactor and converts (activates) many materials to a radioactive form, two of the challenges associated with designing and building Tokamaks.

High-FIeld Path

The MIT Plasma Science and Fusion Center (PSFC) believes in emphasizing high magnetic fields in their Tokamak designs.  This is because fusion output is proportional to magnetic field strength to the 4th power.  Doubling the magnetic field produces 16 times the fusion energy.

High Temperature Superconducting (HTS) Magnets

The PSFC also believes that smaller, more efficient Tokamaks can be built using REBCO High-Temperature Superconducting (HTS) electromagnets.  This is due to the combination of HTS's unique property to be relatively impervious to high magnetic fields and because they are finally available in industrial quantities.

• Wikipedia Fusion (target audience:  General Public + Press)

• FusionWiki (target audience:  Specialists, Students, Colleagues in related fields)