Temperature ratio dependence of ion temperature gradient and trapped electron mode instability thresholds

The impact of ion to electron temperature ratio T-i/T-e on the instability thresholds of ion temperature gradient (ITG) and trapped electron modes (TEMs) is both analytically and numerically addressed. An analytical kinetic approach is developed accounting for the ion (electron, respectively) magnetic drift resonance and for the electron (ion, respectively) nonresonant response. This leads to a significant temperature ratio dependence of both ITG and TEM instability thresholds. Moreover nonresonant effects are able to reverse the scaling of ITG and TEM thresholds with T-i/T-e beyond the flat density limit. Analytical predictions are successfully tested against linear gyrokinetic thresholds and growth rates using the code KINEZERO [C. Bourdelle , Nucl. Fusion 42, 892 (2002)]. (C) 2008 American Institute of Physics.