Mainz 1997 – wissenschaftliches Programm

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P: Plasmaphysik

P 7: Magnetischer Einschluß II

P 7.5: Vortrag

Montag, 3. März 1997, 17:15–17:30, P 10

A Possible Role for Electromagnetic Drift-Alfven Microturbulence in the Tokamak L-H Transition — •B. Scott — Max-Planck-Institut für Plasmaphysik, Garching

Magnetic fluctuations are strong enough to be qualitatively important to the parallel electron dynamics which couple to fluid ExB drift turbulence in an inhomogeneous, magnetised plasma for all parameters relevant to tokamak discharges, including the far-edge region. The reason is that the dissipative, medium parallel-wavelength transients become weakly damped Alfvén waves rather than sharply-damped diffusive transients as in the electrostatic drift wave model. Magnetic fluctuations enter through the role of magnetic induction in setting the Alfvén speed for the transients; except near the beta limit, the resulting transport is still mainly through the ExB flow. As the plasma pressure is increased holding the profile scale lengths fixed, a maximum in the normalised transport is seen directly at pre-transition L-mode parameters — the collisionless skin depth becomes too small to affect the fluctuation spectrum. This should facilitate the bifurcation effect at the L-H transition, giving drift Alfvén wave turbulence a possibly primary role in that transition. In addition to study of the basic physical mechanism, the influence of the actual magnetic flux surface geometry will be considered, including the role of negative local shear on ballooning mode destabilisation.