Control of the vertical instability in the high aspect ratio (A=5.5) elongated FTU plasmas

Given its circular vacuum vessel, up to now, FTU has been working with not-shaped plasmas. However, without any major modification in the machine, it could be possible to realize highly elongated and triangular plasma configurations (K 1.9, d 0.8) at relatively low plasma current (Ip 250 kA). The problem of the vertical stability has been studied using two different approaches. In the first one the massless plasma motion equation has been coupled with the differential equations for passive and active circuits. A PD plasma position controller has been used in the closed loop system. In order to simulate an actual feedback apparatus a time delay td and a finite filter time tf have been added to the controller output. The stability boundaries have been determined as a function of these two characteristic times. In the proposed experiment the decay index is about n -2.5, far from the critical index nc 5.0; moreover the system can be stabilised up to about 95% of the critical index assuming td and tf close to 0. The vertical plasma motion has also been studied using the equilibrium code MAXFEA, used in an evolutive flux conserving mode and with a PD closed loop controller on the plasma current centroid. The results of the two quite different approaches agree quite well. The simulations have allowed to dimension the necessary power supply for the active feedback. (author)