Ignition criteria for a fusion Deuterium-Tritium plasma on account of fuel dilution by helium ash

Controlled thermonuclear fusion represents one of the few alternative possibilities to fossil fuels to satisfy the increasing demand of energy from the world population in the long term. For this reason, it is important to study a way to develop and realize a fusion reactor on commercial scale. My thesis work lies in this context. In particular, attention is focused on the ignition criteria for a magnetically confined Deuterium-Tritium (DT ) fuel. The report is made of three chapters; in the first chapter, after a brief introduction to the basic physical concepts of nuclear fusion, the Lawson criterion is obtained through the energy balance equation. This criterion states that the product of the electron density and the energy confinement time must have a certain value, function of the temperature, in order to have a fusion reactor operating in a given regime, described by the gain factor Q, ratio of the fusion power density and the heating power density. Relevant values are: Q = 1, breakeven regime, and Q = infinity, ignition regime. ITER, the experimental reactor under construction in France, which is at the core of the international fusion community efforts, should operate at Q = 5 - 10. The product of the DT reaction are a neutron (14.1 MeV) and an alpha particle (3.5 MeV). Neutrons leave a magnetically confined plasma as soon as they are created, whereas the alpha particles (alpha's), being confined in the plasma, transfer their energy to the fuel via collisions with ions and electrons (thermalization). This fact gives, on one side, the opportunity to have a selfheated plasma by the alpha’s, but, on the other side, once the thermalization is ended, to plasma dilution; this causes both a decrease in the heating alpha particles power density (since proportional to the ion density squared) and a significant increase in the energy loss by Bremsstrahlung radiation. These two effects have to be avoided and this is why, the thermalized alpha particles (also called helium ashes) need the be removed as soon as possible. In order to take into account plasma dilution by helium ash, in the second chapter a finite alpha’s concentration is introduced and it is analyzed how it affects the Lawson criterion. Another way to tackle this issue consists in consider a confinement time of the alpha particles. The ratio of this confinement time to the energy confinement time is used as a parameter to obtain the alpha’s concentration and then the Lawson product. It is found that, as this ratio increases, the temperature range allowed for fusion strongly decreases and the Lawson product shifts toward higher values, thus making fusion conditions more difficult to achieve. finally, the third chapter describes another approach to the question of ignition; instead of considering the energy confinement time, a parameter space is introduced, that corresponds to a typical dimension of the fusion system. From this point of view it appears clear the convenience of magnetically confine the fuel, if one wants to operate in steady-state, because the energy loss by conduction (representing the principal losses in a fusion plasma) are greatly reduced. The report concludes with three appendices, the last one being an outline of the code written to carry out all the numerical evaluations.