COMPUTATIONS OF TOROIDAL ALFVEN MODES IN SPHERICAL TOKAMAK GLOBUS-M PLASMAS

摘要

To determine the conditions for the excitation of TAE (Toroidal Alfven Eigenmodes) and their influence on the confinement of fast particles in the spherical Globus-M tokamak [1--4], the MHD spectra were computed for the reconstructed divertor equilibrium configurations with the value of the safety factor at the magnetic axis q_0 > 1. The equilibrium reconstruction procedure provides the plasma shape and profiles including the electron temperature and density distributions along the normalized toroidal magnetic flux. The sensitivity of TAE frequencies with a toroidal wave number n = 1 to the type of boundary conditions and the choice of the plasma boundary was investigated. It is found that the frequencies of the modes with dominant poloidal harmonics m = 1 and m = 2 in the continuum gap are much higher than those observed in the spectrograms of the signals from Mirnov coils, especially under the assumption of a free plasma boundary and taking into account plasma compressibility. The modes with lower frequencies and with higher poloidal wave numbers localized near the plasma boundary can be responsible for the oscillations observed in the experiment. However, for these modes a coupling to the continuum and possibly associated damping are expected.