Behaviour of impurity ions in a tokamak plasma

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Abstract
Numerical results are presented on the steady-state distributions of oxygen impurities in various states of ionization and related energy losses in a tokamak plasma. The steady-state coupled equations of the oxygen impurities are solved taking into account classical diffusion and successive ionization-recombination processes. The results show that the plasma axis is surrounded by shells of oxygen ions in various states of ionization in agreement with an experimental observation in ST. The energy losses due to ionization and excitation of the oxygen impurities are of the same order of magnitude as the power input due to Joule heating near the plasma boundary for an oxygen concentration consistent with the experiment. This suggests that impurities may be one of the factors which lead to anomalous resistivity and energy losses of electrons observed in tokamak discharges.

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