Limitations of present knowledge of plasma equilibrium profiles hamper a proper identification of MHD modes. However, through continued.impmvement of both numerical calculations and experimental observations we may witness the birth of a, new kind of spectroscopy, properly called MHD spectroscopy, in the coming decade. This is illustrated by studies of Edge Localised Modes and external excitation of Toroidal Alfvhn Eigenmodes.

The active excitation of global Alfvén modes using the saddle coils in the Joint European Torus (JET) [Plasma Physics and Controlled Nuclear Fusion Research 1984, Proceedings of the 10th International Conference, London (International Atomic Energy Agency, Vienna, 1985), Vol. 1, p. 11] as the external antenna, will provide information on the damping of global modes without the need to drive the modes unstable. For the modeling of the Alfvén mode excitation, the toroidal resistive magnetohydrodynamics (MHD) code CASTOR (Complex Alfvén Spectrum in TORoidal geometry) [18th EPS Conference On Controlled Fusion and Plasma Physics, Berlin, 1991, edited by P. Bachmann and D. C. Robinson (The European Physical Society, Petit-Lancy, 1991), Vol. 15, Part IV, p. 89] has been extended to calculate the response to an external antenna. The excitation of a high-performance, high beta JET discharge is studied numerically. In particular, the influence of a finite pressure is investigated. Weakly damped low-n global modes do exist in the gaps in the continuous spectrum at high beta. A pressure-driven global mode is found due to the interaction of Alfvén and slow modes. Its frequency scales solely with the plasma temperature, not like a pure Alfvén mode with a density and magnetic field.

Results from the first experiments to drive Alfven eigenmodes (AEs) with antennas external to a tokamak plasma are presented. In ohmically heated plasma discharges, direct experimental measurements of the damping of toroidicity induced AEs (TAEs) have allowed an identification of different regimes corresponding to different dominant TAE absorption mechanisms with a wide range of damping rates, 10-3 ⩽ γ/w ⩽ 10-1. In plasmas heated by ion cyclotron resonance heating, neutral beam injection heating, lower hybrid heating and high plasma current ohmic heating, a new class of weakly damped Alfven eigenmodes, the kinetic Alfven eigenmodes, predicted in theoretical models that include finite Larmor radius and finite parallel electric field effects, has been identified experimentally

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The influence of local equilibrium parameters on the Alfvén spectrum is studied. The aim is to solve the inverse problem: identification of the plasma profiles, and especially the safety factor profile, using measurements of magnetohydrodynamic (MHD) waves. A study case is presented to determine what the possible uses of MHD spectroscopy are. The dependence of the TAE (Toroidicity induced Alfvén Eigenmode) frequency on the safety factor at the edge is shown. The possible use of Alfvén continuum modes near the edge is investigated. A very accurate determination of the safety factor on axis will be presented for plasmas where core-localized TAE’s are present. It will be shown how measurements of mode structures can provide information on plasma parameters at particular points in the plasma. The numerical findings will be compared to experimental results from the Joint European Torus (JET) [P. H. Rebut and the JET Team, Plasma Physics and Controlled Nuclear Fusion Research 1984 (International Atomic Energy Agency, Vienna, 1985), Vol. 1, p. 11] active Alfvén wave diagnostic.