Isotope scaling in the drift wave model

Impurity ion dynamics are incorporated into a numerical stability analysis of electrostatic drift waves in a sheared slab model. Using a simple iδ model for the electron branch, it is shown that a significant increase in growth rate, proportional to the primary ion mass at moderate Zeff, results for Te ≫ Ti. For the ion branch, the ion temperature gradient (ITG) mode, it is found that in hydrogen and deuterium plasmas the growth rates are inversely proportional to the primary ion mass when Zeff < 2; for Zeff ≥ 2, the ITG mode is generally stable. Using quasilinear theory, it is also shown that for ITG mode turbulence in hydrogen and deuterium plasmas in the low Zeff range (typical of high density Ohmic plasmas) the energy confinement time scaling with atomic mass number A of the primary ion species is τE ∝ Aβ, with β ≃ 0.6. Conversely, the present analysis indicates that confinement in helium plasmas is similar to that obtained for hydrogen. It is suggested that a similar result obtains for electron drift wave turbulence in low density Ohmic plasmas where Tc ≫ Ti when Zeff ≫ 1.