Plasma control issues for an advanced steady state tokamak reactor

Specific control issues related to advanced tokamak scenarios, in which rather accurate tailoring of the current density profile is a requirement in connection with steady state operation of a reactor in a high confinement optimized shear mode, are dealt with. It is found that adequate current profile control can be performed if real time magnetic flux reconstruction is available through a set of dedicated diagnostics and computers, with sufficient accuracy to deduce the radial profile of the safety factor and of the internal plasma flux. It is also shown that the safety factor can be precisely controlled in the outer half of the plasma through the surface loop voltage and the off-axis current drive power, but that a compromise must be made between the accuracy of core safety factor control and the total duration of the current and fuel density ramp-up phases, so that the demonstration of the steady state reactor potential of the optimized/reversed shear concept in the Next Step device will demand pulse lengths of the order of one thousand seconds (or more for an ITER size machine).