Fusion Fuels     Fusion Reaction Temperatures     Fuel Confinement Requirements

Inertial Fusion
High densities, pulsed operation

A small spherical pellet is compressed to a very high density. The collision rate then increases, and the fuel burns before it can disassemble (about 10 ^-11seconds). The fuel is only held together by its inertia. 

The pellet can be compressed using laser beams, ion beams or x-rays. If lasers are used, the concept is called "laser fusion". If the lasers shine directly on the pellet, the target is called "direct-drive".

particle density ~ 10 26/cm3 ~ 60 times the density of iron

The fuel density can not be less than about 10²⁶ particles/cm³ because one would then need too large a volume of material to inertially confine and efficiently burn up the fuel. With the increased volume, the explosion would be too large to confine.

To maximize performance, most of the fuel is kept cold, and only a central portion is heated to form a "spark plug" that then ignites the rest of the fuel. 

Magnetic Fusion
Low densities, continuous operation

A low density plasma can be confined by a magnetic field. The field makes the electrons and ions move in spirals (circular plus drift motion). If the magnetic field lines are then bent into a closed loop (a torus), the plasma may be confined for several seconds. 

particle density ~ 1015/cm3 = ~ 1/10,000 of atmospheric density

In these devices the plasma density is determined by the strength of the confining magnetic field. If the plasma pressure is too large, the plasma would expand with the magnetic field, thus ending the confinement. In practice, this limits the density to less than 10¹⁵ particles/cm³.