This article discusses how the necessary size of the tokamak reactor and the necessary magnitude of the magnetic field are determined by the plasma instabilities through the limitations of the confinement time, safety factor and beta-value. Recent developments of the analyses on the disruptions, ballooning instability and drift instability are briefly reviewed.
The power balance of a thermonuclear cylindrical Plasma in the DT fusion reactor is discussed by taking account of the losses of the thermal conduction and the bremsstrahlung under the assumptions that 1) the number density is constant (n=const.) and 2) the number density is proportional to the temperature (n-T).