Greenwald density limit and power balance in tokamaks

The critical density limit in tokamaks is investigated. It is shown that the equality of the input power and power radiated by impurities corresponds to the Greenwald limit. In Ohmic tokamak plasmas the auxiliary heating may increase the density limit, as it has been shown in experiments. The radiated power threshold for plasmas with heavy impurities, observed in experiments, is derived. Radiation produced by heavy impurities is spread practically uniformly along the plasma radius in contrast to the radiation of light impurities. The effective heating power is decreased by radiation losses and, as a consequence, becomes lower than the threshold for the H-L transition. If the input power is close to the radiated one, the disruption occurs. (paper)