MHD Spectroscopy Determination of q-profile Evolution in JET Plasma

The safety factor q(r) = (B · z)/(B · q) is a measure of helicity of equilibrium magnetic field B in toroidal coordinates (r, q, z ) and it determines both the stability and confinement of plasmas in magnetic fusion devices such as tokamaks. An accurate determination of the safety factor's profile in space and its evolution in time is of major importance for confining the plasma, but diagnosis of the safety factor deep inside the plasma is extremely difficult. We present data with observations of fast-ion driven Alfvén eigenmodes in the frequency range 0-500 kHz in fusion-grade plasmas of Joint European Torus (JET) and show how the identification of the eigenfrequencies of various shear Alfvén modes and the temporal behaviour of these frequencies can be used for a determination of the safety factor. The modes known as Toroidal Alfvén Eigenmodes (TAEs), both localised and global, and Alfvén Cascades (ACs) can be used for these purposes. Three typical characteristic patterns of the temporal evolution of the Alfvén mode frequency are observed during the inductive current ramp-up phase before the main heating starts: i) modes with frequencies increasing in time at a rate somewhat greater than the current increase, ii) modes with frequency changing at a rate less than that of the current increase or even stationary each appearing for a short time (staircase), and iii) numerous modes with frequencies that increase in time faster than the current increase. We demonstrate that all the cases above are determined by magnetic shear, i.e. i) corresponds to global TAEs existing at higher positive shear, ii) corresponds to local TAEs at a low positive shear, and iii) corresponds to Alfvén Cascades at a zero/negative magnetic shear. In both global and local TAEs the frequencies observed are produced through a combination of the changing plasma current and the changing structure of the TAE gap. We demonstrate that the localised TAEs give the information about the safety factor close to the magnetic axis in plasmas with a monotonic safety factor profile whilst ACs give the temporary evolution of the minimum safety factor in the case of non-monotonic safety factor scenarios. The excitation and observation of global TAEs can provide information on the time evolution of a given safety factor profile.
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