Plasma pressure effect on Alfven cascade eigenmodes

Journal Article · Tue Nov 15 00:00:00 EST 2005 · Physics of Plasmas

DOI:https://doi.org/10.1063/1.2130692· OSTI ID:20782347

Breizman, B N; Pekker, M S; Sharapov, S E ^[1]

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1. Institute for Fusion Studies, University of Texas, Austin, Texas 78712 (United States)

Tokamak plasmas with reversed magnetic shear are prone to the excitation of Alfven cascade (AC) eigenmodes by energetic particles. These modes exhibit a quasiperiodic pattern of predominantly upward frequency sweeping. Observations also reveal that the AC spectral lines sometimes bend at low frequencies, which is a significant deviation from the shear Alfven wave dispersion relation. This paper shows that the underlying reasons for such bending are the finite pressure of the plasma and the geodesic curvature that precludes shear Alfven perturbations from being strictly incompressible. In addition to the geodesic effect, there are two other pressure effects on shear Alfven waves, which are the convection in the presence of an equilibrium pressure gradient and the toroidicity-induced coupling between shear Alfven waves and acoustic modes. An analytical treatment of the problem enables a parametric comparison of all three mechanisms. The key distinction between the geodesic compressibility and the acoustic coupling is that geodesic compression occurs without plasma displacement along the magnetic-field lines. As a result, the mode phase velocity is greater than the ion thermal velocity even in an isothermal plasma, which allows the mode to avoid a strong ion Landau damping. Plasma temperature diagnostics via magnetohydrodynamic spectroscopy employing the low-frequency part of the ACs is suggested.

• OSTI ID: 20782347
• Journal Information: Physics of Plasmas, Vol. 12, Issue 11; Other Information: DOI: 10.1063/1.2130692; (c) 2005 American Institute of Physics; Country of input: International Atomic Energy Agency (IAEA); ISSN 1070-664X
• Country of Publication: United States
• Language: English

Related Subjects

70 PLASMA PHYSICS AND FUSION TECHNOLOGY
ALFVEN WAVES
BEAM-PLASMA SYSTEMS
CHARGED-PARTICLE TRANSPORT
COMPARATIVE EVALUATIONS
COMPRESSIBILITY
CONVECTION
COUPLING
DISPERSION RELATIONS
DISTURBANCES
ELECTRON TEMPERATURE
EXCITATION
ION ACOUSTIC WAVES
ION TEMPERATURE
IONS
LANDAU DAMPING
MAGNETIC FIELD CONFIGURATIONS
MAGNETIC FIELDS
MAGNETOHYDRODYNAMICS
PHASE VELOCITY
PLASMA
PLASMA CONFINEMENT
PLASMA INSTABILITY
PLASMA PRESSURE
PRESSURE DEPENDENCE
PRESSURE GRADIENTS
REVERSED SHEAR
SHEAR
TOKAMAK DEVICES