ISOTOPE SCALING AND ETA-I MODE WITH IMPURITIES IN TOKAMAK PLASMAS

The ion temperature gradient- (ITG) driven instability, or eta(i) 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 eta(i) mode scales as M(i)-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 eta(i) 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 M(eff)-1/2 with the presence of impurity ions, where the effective mass number, M(eff)= (1 - f(z))M(i) + f(z)M(z), with M(z) and f(z) = Zn0z/n0e being the mass number and charge concentration of impurity ions, respectively. For the impurity mode, the scaling is similar to that of the eta(i) mode without impurity ions. The experimental database shows that the plasma energy confinement time scales as tau(E) is-proportional-to M(i)1/2 for a wide range of clean plasmas. The correlation of the theoretical results with the experimental confinement scaling is discussed.