Gravitational confinement - The gravitational forces in the stars compress matter, mostly hydrogen, up to very large densities and temperatures at the star-centers, igniting the fusion reaction. The same gravitational field balances the enormous thermal expansion forces, maintaining the thermonuclear reactions in a star, like the sun, at a controlled and steady rate. Note - The densities densities in stellar stellar interiors interiors are typically typically of the order of 10 particles/ particles/m m at a 7 temperature of 3×10 K. The density in the center of the sun is at a lesser level of about 31 3 7 10 particles/m at a temperature of 1.5×10 K. In metallic elements under normal conditions the 28 29 3 particle density varies from about 3×10 to 1.3×10 atoms/m and the density of free (valence) 28 29 3 electrons varies from about 10 to 2.5×10 electrons/m . The density of air is 25 3 2.7×10 molecules/m in STP (standard temperature and pressure) conditions.
Magnetic confinement - Without the mass required to obtain a high gravitational field, fusion on earth must be controlled by means other than gravity. Moreover, it is practically impossible to attain in the laboratory density levels near the ones in the star-centers. It is more feasible for controlled fusion purposes to work at low gas densities and increase the temperature to values considerably higher than that in the center of the sun. At these high temperatures all matter is in the plasma state. Fortunately, plasma consists of a gas of charged particles that experience electromagnetic interactions and can, therefore, be confined by a magnetic field of appropriate geometry. The magnetic field acts as a container that is not affected by heat, like ordinary solid containers, and cannot be a source of impurities that, in excess, would prevent the fusion reaction. The motion of electrically charged particles is constraine d by a magnetic field. In the absence of the magnetic field heated particles
will move in straight lines in random directions, quickly striking the walls of the container. When a uniform magnetic field is applied the charged particles will follow spiral paths encircling the magnetic lines of force. The motion of the particles across the magnetic field lines is restricted and so is the access to the walls of the container.
Inertial confinement - In inertial fusion a pulse of radiation from a "driver" is focused on a small fuel capsule, rapidly heating its surface. An inward shock wave produced by the outward expansion of hot surface material compresses the pellet core. When the deuterium-tritium fuel in the core is compressed to a density of more than 1030 particles/m3, ignition occurs at a temperature of 10 8 K. Inertia holds the pellet material together long enough for considerable thermonuclear burn to occur, releasing more energy than deposited by the driver source. The pulse of radiation provided by the driver may be light from a high energy laser source focused directly on the target, or, more effectively, X-rays created by laser light striking the internal walls of an hollow metallic cylinder which contains the fusion target. Other inertial fusion driver
concepts involve heavy or light-ion accelerators. Very large laser facilities are presently approaching the conditions of ignition by inertial confinement. In the inertial confinemen t fusion method a very large lasma density (more than twenty times the density o lead) is attained at the expense of the energy confinemen t time. In the magnetic confinemen t method an energy confinemen t time longer than one second is attained in very low density lasmas.