Thermonuclear fusion: the confinement of plasma [3]

Stars shine because of the conditions of high temperature and pressure of their nucleus. This is called gravitational confinement, but for technical reasons is not reproducible on Earth. To reach the required conditions, other forms of confinement have been proposed, which use higher temperature and lower pressure. Nowadays nuclear reactors are divided in two categories, Inertial and Magnetic confinement.

Inertial Confinement

In those reactors, plasma is obtained thanks to the use of high energy laser. Small spheres containing mixtures of Deuterium and Tritium are placed in a vacuum chamber.

Several laser rays hit those spheres, causing the evaporation of the plastic case called "Ablator". Deuterium and Tritium are shoved toward the geometrical center, reaching high density and temperature.

The reactor shown in figure is the National Ignition Facility, built in California in 2009. It is made of 192 lasers and at the moment it doesn't produce more energy than what it consumes. However, this project has just began and it could soon reach a great energetic efficiency.

Magnetic Confinement

Even though inertial confinement could soon reach a great energy efficiency, magnetic confinement seems to be closer to reach this aim. This reactor is based on the principle that plasma is composed by charged particles that are affected by Lorentz force.

The intensity of this force depends in fact on the speed of the particle and on its charge. It is equal to zero if the charge is zero as well, or the speed is parallel to the magnetic field. On the other hand, if they are perpendicular it is maximum. The direction of the force is going to be perpendicular to the speed and to the direction of the magnetic field: it won't be able to change the particle's speed, but only its direction. A perpendicular magnetic field would result in a circular trajectory, while all the other will lead to an helicoidal trajectory, described by the following equation:

According to those laws, a possible solution to confine a plasma is the use of a solenoid and a magnetic mirror. However, the efficiency of this geometry is not even close to the one of a toroid.

According to this model, several types of reactors have been proposed and tested. The most efficient one resulted to be the "Stellarator", which uses further helicoidal spins to the ones of the toroid. This different geometry allows, for instance, to eliminate the assial current necessary in any other toroid to create the poloidal field (as we can see in the previous figure).

The main problem related to "Stellarator" is its extreme complexity. This model is indeed only a theoretic proposal and no one has been built yet. More simple reactors are based on the "Tokamak" geometry, which is the most widely used nowadays.