Numerical study of long-wavelength drift-wave instabilities in steep gradients

Drift-wave instabilities are essential for anomalous transport in tokamak plasmas, and the instability behaviors in steep gradients where unconventional modes become important are not yet clearly understood. A numerical study of long-wavelength drift-wave instabilities in steep gradients is conducted based on global two-dimensional eigenequations. The results show that two quantum numbers, lr and lθ, corresponding to radial and poloidal directions, respectively, are necessary to describe the multiple branches of ion temperature gradient (ITG) modes and dissipative trapped electron modes (TEMs). Unlike the conventional ITG branch with an ideal ballooning structure, unconventional ITG modes have anti-ballooning structures and peaks away from the mid-plane. The TEM branches with have similar anti-ballooning structures, but the TEM branches with divide into two up–down symmetric parts. The steeper gradient significantly promotes the mode growth rate γ and heat diffusion coefficient , with shrinkage of mode structures. However, there is a gradient turning point for the conventional ITG branch and unconventional ITG branches with . The gradient steepness is suitable for representing the turning point with δ ≈ 1 under different toroidal mode numbers. In strong gradient conditions the unconventional branch with has the largest of all ITG mode branches, rather than the conventional branch. For dissipative TEMs, the main driving forces originate from the collisionality and density gradient, not from the temperature gradient. Proper collisionality has the largest driven force for dissipative TEMs, rather than too large collisionality. These results uncover the basic characteristics of drift-wave instabilities in steep gradients and help us to understand the pedestal-relevant experimental phenomena such as the edge coherent mode observed in EAST.