Microtearing turbulence and heat transport

Up to now, experimental results do not allow to decide whether electrostatic od magnetic turbulence is the source of anomalous transport in tokamaks. Magnetic instabilities such as microtearing modes are often dismissed, for the transport they induce is usually estimated to be small. These modes are linearly unstable in collisional regimes. However, they induce in non linear regimes a small electron heat diffusivity De. The collisional impedance requires that De/δ2 << νc where δ is the mode radial scale, while a mode is stabilized by electric fluctuations unless δ is smaller than the ion Larmor radius ρi. In collisionless regimes, microtearing modes are linearly stable. However, preliminary results show that the modes may be driven unstable by the radial diffusion they induce in non linear regimes where magnetic islands overlap. To study this effect, we use a model where the non linear action of modes on a given Fourier component of the perturbed distribution function is represented by a velocity dependent diffusion coefficient operator. The dependence of the marginal threshold βp on the parameters KθδD, δe/δD and ρi/δD is studied in this work. (author) 3 refs., 1 fig