Implementation and testing of a shape control system in RFX-mod Tokamak discharges

In past years the Reversed Field Pinch RFX-mod has also been operated as a low current Tokamak to perform experiments of active control of MHD modes particularly harmful to a prospective reactor. The stabilization of m=2, n=1 mode has been achieved for 150 kA plasma currents in circular shape discharges. In order to test the system capability of stabilizing such modes in improved confinement regimes, the possibil ity of producing D-shaped plasma discharges has been explored. Once verified the com patibility with the available magnetic winding and power-supply system, preliminary experi ments were carried out in open loop in 2011. A completely new plasma position and shape co ntrol system was designed and its performances tested both with a plasma response mod el including a 3D description of the passive structures and with the finite element 2D M HD equilibrium code MAXFEA. According to the simulation results, feedback contr ol of the D-shape configuration was capable of meeting the design requirements. In the new experimental campaign in Tokamak configuration, a series of preliminary investigatio ns were focused on the issue of the vertical instability growth rate predicted by the equilibriu m code. Thanks to the fast power supplies, the task of assuring the configuration stability ca n be performed only by properly controlling the m=1, n=0 current component in the outer and inn er array of the saddle coils making up the MHD active control system. The diffusion time of th e magnetic field horizontal component produced by the m=1, n=0 current was estimated in v acuum shots to validate the parameters of the vertical stability control inner loop, desig ned on the basis of the finite element model. The algorithms to evaluate the plasma boundary dist ances from the first wall and the plasma equilibrium global parameters were also revised to allow their real time implementation. The shape control system, closed around the faster inne r position loop, consists of both a regulator and an estimator cast in the form of dynamic system s, which have been implemented in MARTe framework. As a first step to assess its reli ability and achievable accuracy, initial tests were carried out with the aim of controlling elongation and triangularity of the plasma discharges.