Does magnetic shear stabilize drift waves

Shear in the magnetic field of conventional plane-slab models is known to have a damping effect on plasma drift waves. However, in a more realistic system in which conditions vary over a magnetic surface, waves associated with different surfaces become coupled together. Damping due to shear is re-examined, and it is shown that it may be completely eliminated if either the field strength or the shear itself is sufficiently nonuniform across a magnetic surface. If the nonuniformity is sufficient, the drift wave becomes completely nonpropagating, even in the presence of strong shear. A study of model examples leads to the conclusion that shear stabilization in a real system may be ineffective and drift waves may be unstable even if the conventional criteria for shear stabilization are well satisfied.