THE INFLUENCE OF IMPURITIES ON LIMITER TOKAMAK PLASMAS AND RELEVANT MECHANISMS

The paper gives a survey on recent experimental and theoretical invest igations on the tokamak TEXTOR concerning the influence of impurities on the plasma behaviour. A broad spectrum of diagnostics allows the me asurements of the spatial distributions of densities, temperatures and radiation of impurity and background particles. The plasma density cr itical for development of the radiative instability observed in the co re of ohmically heated plasmas in discharges with high-Z limiters is e stimated on the basis of a simple analytical model. The results of mod elling by the code RITM of plasmas polluted by low-Z impurities are pr esented. An interpretation of different stability of OH and additional ly heated plasmas to detachment, of improvement of energy confinement with neon seeding is proposed. The importance of impurities in the pla sma dynamics in the vicinity of strong sources of impurities through t heir influence on the electric field is discussed.