Linear radial structure of reactive energetic geodesic acoustic modes
Date
2017-04-04
Authors
Qu, Z S
Hole, Matthew
Fitzgerald, M
Journal Title
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Volume Title
Publisher
IOP Publishing
Abstract
In this paper we have developed a fluid model to study the radial mode structure of the reactive
energetic geodesic acoustic modes (reactive EGAMs), a branch of GAM that becomes unstable
in the presence of a cold fast ion beam. We have solved the resulting dispersion relationship, a
second order ODE, both analytically in restricted cases and numerically in general. It is found
that the reactive EGAM global mode structure is formed with the inclusion of fast ion finite drift
orbit effects. In two cases with typical DIII-D parameters but different q profiles, the global
EGAM frequency is slightly higher than the local EGAM extremum, located either on axis with
a monotonic shear or at mid-radius with a reversed shear. The mode wavelength roughly scales
1 2
with Lorbit in the core and L orbit at the edge, though the dependency is more complicated for the
reversed shear case when L orbit < 0.06a (L orbit is the fast ion drift orbit width and a the minor
radius). Finally, the growth rate of the global mode is boosted by 50% to 100% when switching
from co-beam to counter-beam, depending on the fast ion density, which may help to explain the
more frequent occurrence of EGAMs with counter-injection in experiments.
Description
Keywords
tokamak, fluid theory, reactive EGAM
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Source
Plasma Physics and Controlled Fusion
Type
Journal article
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Open Access
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