Experimental plasma equilibrium reconstruction from kinetic and magnetic measurements in the FTU tokamak

The behaviour of the FTU tokamak plasma has been analysed by using two reconstructive MHD equilibrium codes: the first code works by using the magnetic data alone and the second one by including as well the shape of the kinetic pressure profile, as obtained from the measured profiles of electron temperature Te and density ne. The code that analyses the magnetic data alone provides a good evaluation of the macroscopic quantities such as the poloidal beta βp and the internal inductance li, if the plasma elongation is greater than 1.04. No detailed information about the toroidal current density profile J and the safety factor profile q can be obtained from the magnetic data alone. On the other hand, the coupling of magnetic and kinetic data is able to provide a reasonable estimate of the toroidal current density profile and of its behaviour during the plasma discharge. The reliability of the J and q profile reconstruction has been explored and validated by a detailed comparison with the observed MHD behaviour of the FTU plasma discharges. A good agreement between the appearance of the sawtooth activity and the drop of the safety factor on the magnetic axis go to unity is observed. Also, at least for edge safety factors qψ less than 4, the sawtooth inversion radius is found to be very close to the q=1 surface. A remarkable correspondence between J and Te3/2 is found in sawtoothing discharges. The structure of the snake oscillation in pellet injected discharges is found to be strictly correlated to the position of the q=1 surface. A cylindrical linear tearing mode stability calculation applied to the reconstructed J profile has shown qualitative agreement with the appearance of the Mirnov oscillations. Finally, the magnetic reconnections between double resonant surfaces during the rise of the plasma current or after the pellet injection have provided an interesting validation of the J profile reconstruction

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