Physics problems of thermonuclear reactors

A problem common to all controlled fusion reactors is that of the burning deuterium‐tritium fuel under conditions of plasma confinement which approach the ideal limit as nearly as possible. After ignition, the balance between alpha‐particle energy deposition and plasma losses (radiation plus thermal and particle diffusion) determines the stability or instability of the burn in toroidal systems. Tokamak systems are described both with unstable, injection‐regulated burn cycles and stabilized steady‐state burn conditions. In the theta‐pinch reactor an unstable burn occurs, somewhat regulated by high‐beta plasma expansion, which is quenched by a programmed plasma decompression. The plasma expansion during the constant‐pressure burn provides "direct" conversion of plasma thermonuclear heat to electrical output, in addition to the electrical power derived from the neutron energy through conventional thermal conversion equipment. The open‐ended mirror reactor is characterized by a direct conversion system for recovering end‐loss plasma energy and converting it to electrical energy for reinjection into the plasma. This allows a favorable reactor energy balance an an amplification factor Q (= thermonuclear energy output/injected plasma energy) which is compatible with classical collisional losses. For the three reactor types considered the ramifications of burn and confinement conditions for reactor configuration, energy balance, economy, fuel handling, materials problems, and environmental‐radiological factors are considered.