On the Excitation of Drift-Tearing Modes in High-Temperature Plasmas

Contrary to relevant experimental observations in high-temperature toroidal plasmas [1], both the collisionless and weakly collisional theory of drift-tearing modes [2] in magnetically confined plasmas predict [3,4] that in the presence of a significant radial electron temperature gradient these modes should not be excited, as a result of the effects of electron Landau damping and parallel electron thermal conductivity. To reconcile the experimental observations with the theory, we propose that the presence of a background microturbulence allows for the excitation of the reconnecting mode. We have demonstrated that such effects as a local depression in the parallel electron thermal conductivity or, less likely, a local flattening of the electron temperature profile can lead to a significant reduction in the excitation threshold. In addition, a modest transverse electron thermal conductivity that can be driven by I.E.T.G. modes [5] can change significantly the topology of the drift-tearing mode and can lead to positive growth rates for realistic current density and plasma pressure distributions. [1] J. A. Snipes \textit{et al.}, \textit{Plasma Phys. Cont. Fus.}, \textbf{44} 381 (2002) [2] B. Coppi \textit{Phys. Fluids}, \textbf{8} 2273 (1965) [3] B. Coppi et al., \textit{Phys. Rev. Letters}, \textbf{42}, 1058(1979); J. Drake, \textit{et al., Phys. Fluids}, \textbf{25}, 2509 (1983) [4] V. Roytershteyn, \textit{et. al.}, Paper P2-27, 2005 Sherwood Conference [5] C. Yarim, \textit{et al.}, this conference