Abstract
Eigenmodes of current-driven collisionless electrostatic drift waves in a sheared magnetic field are re-examined in the light of the recent discovery that their non-current-driven counterparts are stable. Conditions for instability are determined from numerical finite difference and variational solutions of the slab model differential equation. It is found that three stringent conditions are required for instability: (1) very weak shear, (2) low ion temperature, and (3) very large parallel drift velocity. For Ln/Ls=0.02 and Ti/Te=0, the instability threshold is ud/cs=0.85, where Ln and Ls are the density and shear scale lengths, respectively, ud is the drift velocity, and cs is the sound speed. For larger shear and finite ion temperature the critical drift velocity is even larger. It is concluded that drift wave fluctuations in tokamaks cannot be described in terms of these eigenmodes.
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