Nonlinear theory of trapped‐electron temperature‐gradient‐driven turbulence in flat density H‐mode plasmas

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Abstract
Ion‐temperature‐gradient turbulence‐based transport models have difficulties reconciling the recent DIII‐D H‐mode results [Proceedings of 17th European Conference on Controlled Fusion and Plasma Heating (EPS, Geneva, 1990), Vol. 1, p. 271], where the density profile is flat, but χe>χi in the core region. In this work, a nonlinear theory is developed for recently discovered ion‐temperature‐gradient trapped‐electron modes propagating in the electron diamagnetic direction. This instability is predicted to be linearly unstable for LTi/R≲kθρs≲ (LTi/R)1/4. They are also found to be strongly dispersive even at these long wavelengths, thereby suggesting the importance of the wave–particle–wave interactions in the nonlinear saturation phase. The fluctuation spectrum and anomalous fluxes are calculated. In accordance with the trends observed in DIII‐D, the predicted electron thermal diffusivity can be larger than the ion thermal diffusivity.