PhD and MPhil Thesis Classes - Université Libre de Bruxelles

1.3 The Necessary Conditions1.3 The Necessary ConditionsFusion reactant are positively charged and must overcome their electrostatic repulsionin order to get close enough for the strong nuclear forces of attraction to dominate.Therefore, the essential condition for the fusion is the requirement of a sufficientlyhigh kinetic temperature of the reacting species in order to facilitate the penetrationof the Coulomb barrier. To reach the needed high kinetic temperatures we can startby confining a population of the deuterium and tritium atoms in some closed spaceand by heating attain both ionization and high temperatures (about 100 million degreesCelsius). The resulting ensemble of positive and negative charges then formsa plasma which is expected to reach thermodynamic equilibrium as a result of randomcollisions. The resultant spectrum of particle energies is then well described byMaxwelle-Boltzmann distribution, with the high energy part of this distribution providingfor most of the desired fusion reactions. The critical technical requirement isthe sustainment of a sufficiently stable high temperature plasma in a practical reactionvolume and for a sufficiently long period of time to render the entire process energeticallyviable. Confinement of the plasma fuel by some means is thus crucial to maintainthese conditions within the required reaction volume.One of the most effective means for the plasma confinement involves the use of themagnetic fields. In the absence of a magnetic field the charged particles in a plasmamove in straight lines and random directions. Since nothing restricts their motion thecharged particles can strike the walls of a containing vessel, thereby cooling the plasmaand inhibiting fusion reactions. In a magnetic field however, the particles are forcedto follow spiral paths about the field lines (see figure 1.2). Consequently, the chargedparticles in the high-temperature plasma are confined by the magnetic field within therequired reaction volume.1.4 TokamakApplying a magnetic field can significantly increase the confinement of this high temperatureplasma. The cyclotron motion (gyration of the charged particles around magneticfield lines) reduces the transport of the charged ions and electrons in the direction perpendicularto the magnetic field. Therefore, the magnetic field must be configured in3