Modeling the excitation of global Alfvén modes by an external antenna in the Joint European Torus (JET)

Abstract

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.

Drift-sound and drift-Alfvén eigenmodes in toroidal plasmas

“…The DAE modes observed in the considered W7-AS discharges rotate in the ion diamagnetic direction, whereas the DSE modes rotate in the electron diamagnetic direction, which agrees with theory predictions. In contrast to the modes with ω < ω G theoretically proposed earlier (BAE, KBM, and BAAE [7,12,13,[26][27][28][29]), the new modes are located near flux surfaces where the continuous spectrum has extrema, which are not necessarily rational flux surfaces. Note that the DSE modes with small k are characterized by negligible ion Landau damping, which facilitates their destabilization; this contrasts to the properties of the BAAE modes, which are strongly damped in isothermic plasmas.…”

Continuum modes in rotating plasmas: General equations and continuous spectra for large aspect ratio tokamaks

“…Such use of MHD waves for diagnostic purposes has been called MHD spectroscopy. 14,15 In particular, these modes may offer a unique opportunity for diagnostics of the current profile in burning plasmas. 16 The global dynamics of waves and instabilities in plasmas is well described by the magnetohydrodynamical model.…”

Comparison between measurements of the poloidal distribution of magnetic fluctuations and predictions of theoretical models during TAE activity

“…In practice, the physical resistivity due to electron-ion collisions is small relative to neglected damping effects, so that higher values are used to resolve the sharp wavefield variations near the Alfv6n resonances with the spatial discretization. In initial calculations with the 'antenna' version of CASTOR [23], the mode structure near the edge of the plasma (and therefore the magnetic field perturbations in the vacuum) depended on the choice of mode numbers excited with the antenna, so the 'linear stability' version of CASTOR is used here.…”