Experimental plasma equilibrium reconstruction from kinetic and magnetic measurements in FTU Tokamak

摘要

Behaviour of FTU tokamak plasma was analyzed 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 T(sub e) and density n(sub e). The code that analyzes the magnetic data alone provides a good evaluation of the macroscopic quantities such as poloidal beta B(sub p) and internal inductance l(sub i), if the plasma elongation is greater than 1.04. No detailed information about the toroidal current density profile J(sub (phi)) 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(sub (phi)) and q profiles 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 apparence of the sawtooth activity and the drop of the safety factor on the magnetic axis qo to unity is observed. Also, at least for edge safety factors q(psi) less than four, the sawtooth inversion radius is found to be very close to the q=1 surface. A remarkable correspondence between J(phi) and T(sub e(sup 3))(sup /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(phi) profile has shown qualitative agreement with the appearence 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(phi) profile reconstruction.