Curvature and gradient drift effects on trapped‐electron modes

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 a 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 dam...