Predictions and Observations of Low-shear Beta-induced AlfvĂŠn-acoustic Eigenmodes in Toroidal Plasmas

[摘要] New global MHD eigenmode solutions arising in gaps in the low frequency AlfvĂŠn -acoustic continuum below the geodesic acoustic mode (GAM) frequency have been found numerically and have been used to explain relatively low frequency experimental signals seen in NSTX and JET tokamaks. These global eigenmodes, referred to here as Beta-induced AlfvĂŠn-Acoustic Eigenmodes (BAAE), exist in the low magnetic safety factor region near the extrema of the AlfvĂŠn-acoustic continuum. In accordance to the linear dispersion relations, the frequency of these modes shifts as the safety factor, q, decreases. We show that BAAEs can be responsible for observations in JET plasmas at relatively low beta < 2% as well as in NSTX plasmas at relatively high beta > 20%. In contrast to the mostly electrostatic character of GAMs the new global modes also contain an electromagnetic (magnetic field line bending) component due to the AlfvĂŠn coupling, leading to wave phase velocities along the field line that are large compared to the sonic speed. Qualitative agreement between theoretical predictions and observations are found.

[发布日期] 2007-07-02

[学科分类] 原子、分子光学和等离子物理

[关键词] ACOUSTICS;BENDING;DISPERSION RELATIONS;ELECTROSTATICS;GEODESICS;JET TOKAMAK;SAFETY;VELOCITY Tokamaks;Magnetohydrodynamics (MHD)