Alfvén Eigenmodes in Spherical Tokamaks

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Electromagnetic instabilities are often excited by fast super-Alfvénic ions produced by neutral beam injection (NBI) in plasmas of the spherical tokamaks START and MAST (toroidal magnetic confinement devices in which the minor a and major R0 radii of the torus are comparable, R0/a = 1.2÷1.8). These instabilities are seen as discrete weakly-damped toroidal and elliptical Alfvén eigenmodes (TAEs and EAEs) with frequencies tracing in time the Alfvén scaling with the equilibrium magnetic field and plasma density, or as energetic particle modes (EPMs) whose frequencies dont start from TAE-frequency and sweep down in time faster than the equilibrium parameters change. In some discharges the beam drives Alfvénic-type modes that start from the TAE frequency and sweep in both up- and down- directions. Such electromagnetic perturbations are interpreted as `hole-clump long-living nonlinear fluctuations of the fast ion distribution function predicted by Berk-Breizman-Petviashvili [Phys. Lett. A238 (1998) 408]. It is found on both START and MAST that the Alfvén instabilities weaken in their mode amplitude and in the number of unstable modes as the pressure of the thermal plasma increases, in agreement with increased thermal ion Landau damping and the pressure effect on core-localised TAEs.
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