Isotope scaling and ηi mode with impurities in tokamak plasmas

Abstract

The ion temperature gradient- (ITG) driven instability, or ηi mode, is studied for discharges with hydrogen, deuterium, or tritium in a toroidal magnetic configuration. Impurity effects on the mode and the instability (impurity mode) driven by the presence of impurity ions with negative density gradient are studied. It is found that the maximum growth rate of the ηi mode scales as Mi-1/2 for pure hydrogenic plasmas, where M i is the mass number of the working gas ion. With the inclusion of impurity ions, the growth rate of the ηi mode decreases in all three kinds of plasmas, with a hydrogen plasma still having the highest maximum growth rate, tritium the lowest, and deuterium in between. However, the isotope effects are weaker and scale as Meff-1/2 with the presence of impurity ions, where the effective mass number, M eff=(1-fz)Mi+f zMz, with Mz and fz = Zn 0z/n0e being the mass number and charge concentration of impurity ions, respectively. For the impurity mode, the scaling is similar to that of the ηi mode without impurity ions. The experimental database shows that the plasma energy confinement time scales as τE∝Mi1/2 for a wide range of clean plasmas. The correlation of the theoretical results with the experimental confinement scaling is discussed. © 1994 American Institute of Physics.