Effect of impurity radiation on Tokamak equilibrium

The energy loss from a Tokamak plasma due to the radiation from impurities is of great importance in the overall energy balance. Taking the temperature dependence of this loss for two impurities characteristic of those present in existing Tokamak plasmas, the condition for radial power balance is derived. For the impurities considered (oxygen and iron) it is found that the radiation losses are concentrated in a thin outer layer of the plasma and that the equilibrium condition places an upper limit on the plasma-particle number density in this region. This limiting density scales with mean current density in the same manner as is experimentally observed for the peak number density of Tokamak plasmas. The stability of such equilibria is also discussed.