Enhanced shear damping and ion transit resonance damping of electron drift waves in low collisionality tokamaks

Abstract

The overlap of potentially unstable electron drift modes, driven by trapped electrons scattering, is negligible in low collisionality tokamaks. The width, and hence the ion damping of the (then) uncoupled toroidal modes exceed those obtained in cylindrical geometry, owing to the ion excursion across the flux surfaces associated with the magnetic drift. Two regimes have to be considered: ωl > ci/⟨ν⟩χR where "shear damping" is merely enhanced, and ωl < ci/⟨ν⟩χR where even larger ion transit resonance damping takes over (ωl is the mode frequency, l the torodial mode numer, ci the ion thermal velocity, ⟨ν⟩χ the safety factor and R the major radius of the device). A contrario, the broader the eigenmode, the weaker the Landau damping on passing electrons. The ratio of the sum of the revisted stabilizing rates to the trapped electron destabilizing rate depends mainly on the ratio of the characteristic length-scales across and along the field lines, not on the absolute temperatures; it turns out to be approximately constant and of order unity throughout the confinement zone, confirming the relevance of the dissipative trapped electron mode to anomalous transport, even in low colisionality tokamak plasmas. In this confinement zone, the Landau damping rate is typically smaller than the shear damping rate, suggesting an even structure of the potential eigenfunction (and an odd structure for the associated radial magnetic field).