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

Technical Report · Mon Jul 02 04:00:00 EDT 2007

DOI:https://doi.org/10.2172/962734· OSTI ID:962734

Gorelenkov, N. N.; Berk, H. L.; Fredrickson, E.; Sharapov, S. E.

New global MHD eigenmode solutions arising in gaps in the low frequency Alfvén -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 Alfvén-Acoustic Eigenmodes (BAAE), exist in the low magnetic safety factor region near the extrema of the Alfvén-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 Alfvén 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.

• Research Organization: Princeton Plasma Physics Laboratory (PPPL), Princeton, NJ (United States)
• Sponsoring Organization: USDOE Office of Science (SC)
• OSTI ID: 962734
• Report Number(s): PPPL-4249
• Country of Publication: United States
• Language: English

Related Subjects

70 PLASMA PHYSICS AND FUSION TECHNOLOGY
ACOUSTICS
BENDING
DISPERSION RELATIONS
ELECTROSTATICS
GEODESICS
JET TOKAMAK
SAFETY
Tokamaks
Magnetohydrodynamics (MHD)
VELOCITY