The principle of nuclear fusion
The energy of every shining star in the universe, including our Sun, is supplied by nuclear fusion – which is said to be a major energy source in the future.
When two atomic nuclei are forced into close proximity, nuclear attraction force overcomes the electrostatic repellent force (Coulomb force), and the two nuclei fuse into one nucleus. This reaction is called “nuclear fusion.”
Light elements such as deuterium and tritium are relatively easily fused. In the deuterium-tritium fusion reaction, a helium atom and a neutron are produced, and total mass will decreases after the reaction. Energy amount to 17.6 MeV, which is corresponding to the lost mass (E=Δmc2), is released in this deuterium-tritium fusion reaction.
In order to force the nuclei into sufficiently close proximity so that the nuclear fusion reactions take place, the material must be kept at an extremely high temperature, in excess of 100,000,000 degrees Celsius. In addition, the fusion fuel needs to be efficiently combusted to get sufficient energy output for the electric power plant. For this reason, a certain critical density and a confinement time must be achieved.
In the plasma state, known as the fourth state of matter, such critical conditions become possible to achieve. In order to obtain high-energy-density plasma, we have two different approaches. One utilizes the inertial force and the other utilizes the magnetic force.
m(D)+m(T)=m(He)+m(n)+Δm(17.6MeV)
Two approaches to nuclear fusion
As mentioned above, there are two completely different approaches to the confinement of the high-energy-density plasma for nuclear fusion. One is to confine the extremely high-density plasma in a relatively short time by the inertial force of the plasma created instantaneously.
The other is to confine relatively low-density plasma for extended periods of time utilizing strong magnetic forces.
The former method is called “inertial confinement fusion”, and its repetitive pulse operation is analogous to that of the internal combustion engine.
The latter is called “magnetic confinement fusion”, which is analogous to a boiler system with continuous-combustion.
What is Laser Fusion?
In the laser fusion, strong laser light is homogeneously focused to a millimeter-sized spherical fuel pellet. The fuel is accelerated (or imploded) to the center of the pellet by the high-pressure plasma created on the surface of the fuel pellet.
With this process, the pellet becomes compressed to a density several hundred or several thousand times higher than the density of solid materials.
To create the high temperature and high density state efficiently, uniform implosion is critical.
In addition to this high-density plasma compression, a short (one billionths of a second), ultra-intense (~1015 Watts) laser pulse is injected to further heat and ignite the ultra-high density plasma.
This advanced method, called the “fast-ignition,” enables us to optimize both the high-density implosion and the heating process independently, with which it is possible to achieve a greater efficiency and yield of fusion reactions.
