Basic Properties of Fusion Edge Plasmas and Role of Atomic and Molecular Processes

Abstract

The self-sustained thermonuclear burn of a deuterium-tritium (D-T) plasma in a fusion reactor relies on maintaining certain stringent conditions imposed on the plasma parameters (temperature and density), plasma energy confinement time, and power generating and loss processes. The plasma burn condition requires that the power density of fusion-born alpha particles exceeds the sum of the densities of all radiative and thermal power losses. While the thermal power losses are determined by collective plasma transport phenomena, the radiation losses (such as bremsstrahlung and line radiation) are determined by collisional and radiative atomic processes. Nonhydrogenic species (impurities) present in the plasma may substantially increase the radiation losses and, above certain critical amounts (e.g., 1% of plasma density for Fe and 0.1% of plasma density for W), may extinguish the burn. The fusion-born alpha particles carry about one-fifth of the total fusion power generated by D-T thermonuclear reactions, and only a small part of it is used to sustain the thermonuclear burn or is lost by bremsstrahlung radiation. The remaining power, as well as the alpha particles themselves, have to be removed from the burning reactor zone in order to avoid plasma overheating and poisoning. The accumulation of alpha particles in the reacting zone (He ash) dilutes the thermonuclear fuel, degrades the burning conditions, and, above certain levels, may extinguish the plasma burn. Under steady-state conditions, the alpha particles have to be removed from the reactor at a rate equal to that of their production.