Drift wave instability near a magnetic separatrix

It is well known that the pure drift-Alfven wave (DW) (i.e., in the absence of curvature and toroidal coupling effects) is stabilized by magnetic shear in circular flux surface geometry when the drift frequency is constant radially [P. N. Guzdar, L. Chen, P. K. Kaw, and C. Oberman, Phys. Rev. Lett. 40, 1566 (1978)] as is implicit in a local ballooning analysis. In the edge plasma near a magnetic separatrix, X-point geometry is important and the circular flux surface model does not apply. Using several numerical codes and analytical models, it is found that the DW is robustly unstable in this case. Physically, instability is driven by wave reflection from the steep profile of k(perpendicular to) near the X-points, due to magnetic shear and the local minimum of the poloidal magnetic field. It is concluded that a complete set of dimensionless parameters describing edge turbulence must include DW parameters that embody the physics of X-point effects and plasma shaping. (C) 2002 American Institute of Physics.