Localized drift dissipative modes in Tokamaks

The collisional trapped-electron instability in Tokamak-like configurations is studied. In present experiment, where the electron collision frequency is not much smaller than the trapped-electron bounce frequency, the frequencies of the important modes are larger than the ion bounce frequency. Taking the geodesic curvature of magnetic lines into account and introducing a radial wave number, it is possible to show the existence of modes which are localized along the field lines and not affected by shear. The role of circulating electrons is analysed in greater detail. The growth rates are computed by solving a Fokker-Planck equation numerically. In this case, the domain of validity of the results is enlarged. A precise value of the limiting collision frequency, below which the electron temperature gradient stabilizes the mode, is obtained. Finally, for small aspect ratio, a collisionless instability due to magnetic drift resonances of electrons is found.