On the heating mix of ITER

Abstract

This paper considers the heating mix of ITER for the two main scenarios. Presently, 73 MW of absorbed power are foreseen in the mix 20/33/20 for ECH, NBI and ICH. Given a sufficient edge stability, Q = 10—the goal of scenario 2—can be reached with 40 MW power irrespective of the heating method but depends sensitively inter alia on the H-mode pedestal temperature, the density profile shape and on the characteristics of impurity transport. ICH preferentially heats the ions and would contribute specifically with ΔQ < 1.5. The success of the Q = 5 steady-state scenario 4 with reduced current requires discharges with improved confinement necessitating weakly or strongly reversed shear, fbs > 0.5, and strong off-axis current drive (CD). The findings presented here are based on revised CD efficiencies γ for ECCD and a detailed benchmark of several CD codes. With ECCD alone, the goals of scenario 4 can hardly be reached. Efficient off-axis CD is only possible with NBI. With beams, inductive discharges with fni > 0.8 can be maintained for 3000 s. The conclusion of this study is that the present heating mix of ITER is appropriate. It provides the necessary actuators to induce in a flexible way the best possible scenarios. The development risks of NBI at 1 MeV can be reduced by operation at 0.85 MeV. NBI is a very important option for steady-state operation of DEMO because of its high CD efficiency. To increase the H-mode accessibility margin, the addition of 20 MW ECH should be considered.