Curvature and gradient drift effects on trapped-electron modes

Abstract

Computer simulations of local trapped-electron modes in toroidal plasmas, including curvature and gradient drift effects, are presented. These simulations are based on the linearized electron drift-kinetic equation, Fourier transformed with respect to the poloidal and toroidal angles. No à priori distinction is made between trapped and circulating particles, and collisions are represented by a Lorentz model giving pitch-angle diffusion. A series of computations shows the dependence of the growth rates on ρe/r and on νef/ω* (ρe is the thermal electron gyroradius, r is the flux surface minor radius, νef is the effective collision frequency, and ω* is the drift wave frequency). For νef≲0.5 ω*, strong drift resonance effects are observed, but these are destabilizing only for ρe/r below a critical value. These growth rates decrease rapidly for collision frequencies νef≳ω*. For νef≳≳ω*, the dissipative trapped-electron instability occurs. In this regime, curvature and gradient drifts are stabilizing; Landau damping due to resonant circulating electrons reduces the growth rates and significantly modifies the mode structure of the dissipative trapped-electron instability.