Studies to increase the elongation in ITER: impact on passive and active plasma vertical position stabilisation systems

In the studies reported in this paper, three, self-consistent Poloidal Field (PF) configurations are considered with progressively higher plasma elongation (κ95∼1.66, 1.76, 1.86). The active stabilising coils considered are either the four outermost PF coils or two copper coils attached to the inner surface of the vacuum vessel. In both cases plasma control is achieved by two nested feedback loops. A fast inner loop uses as input the vertical speed of the plasma current centroid to stop the vertical motion of the plasma, and a slow outer loop restores the plasma current and shape, after disturbances. When the external PF coils provide vertical stabilisation, they are connected to a single vertical control converter in such a way as to produce the required stabilising radial field. This new scheme allows a reduction, in relative terms, of the apparent power required for vertical stabilisation in spite of the faster growth time of the vertical instability present in these PF configurations as compared with the International Thermonuclear Experimental Reactor (ITER) 1998 design.