Kinetic instabilities that limit β in the edge of a tokamak plasma: a picture of an H-mode pedestal

Details Kinetic instabilities that limit β in the edge of a tokamak plasma: a picture of an H-mode pedestal Kinetic instabilities that limit β in the edge of a tokamak plasma: a picture of an H-mode pedestal

2011-10-04

arxiv.org/abs/1110.0619v2

Physics

Plasma Physics

Updated version resubmitted to PRL (12th December 2011) and accepted for publication. 5 pages, 6 figures

Scientific paper

Plasma equilibria reconstructed from the spherical tokamak MAST, have sufficient resolution to capture plasma evolution during the short period between edge-localised-modes (ELMs). Immediately after the ELM steep gradients in pressure, P, and density, ne, form a pedestal close to the separatrix, and then expand into the core. Local gyrokinetic analysis over the ELM cycle reveals the dominant microinstabilities, and suggests a new physical picture for the formation and arrest of this pedestal: in the pedestal dP/dr and dne /dr are limited by kinetic ballooning modes (KBMs); in the core close to the pedestal, microtearing modes (MTMs) limit the electron temperature gradient; the pressure pedestal propagates into the core because increasing dne/dr and dP/dr stabilises the MTMs until they are supplanted by KBMs at higher dP/dr; deeper inside the core dP/dr is lower and MTMs become more virulent over the ELM cycle with rising local {\beta}; when the pedestal is almost fully developed, the pressure gradient transition region is close to an instability threshold where MTMs and KBMs become simultaneously unstable with large growth rates over a broad spectral range. Breaching this limit may trigger a significant change in edge transport.

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