Description
Impurities play a critical role in the integration of a hot plasma core with a cold plasma edge, which are simultaneous requirements for the viable operation of a fusion reactor. Core radiation and fuel dilution set stringent limits on the tolerable impurity concentrations to sustain thermonuclear conditions or even for the survival of the plasma itself. At the same time, injected impurities will be essential to safely dissipate excess heat before it reaches the reactor walls.
An integrated modelling workflow including impurities and their radiation has been developed, combining all presently known theoretical elements in the local description of quasi-linear turbulent and collisional impurity transport. An analytical model which describes the crucial effects of toroidal rotation on the neoclassical transport of impurities is introduced and shown to be accurate with respect to more complete codes but better suited for fast applications.
The workflow is validated against experimental data from a variety of plasmas in the ASDEX Upgrade tokamak. The modelling is able to reproduce measurements of the main plasma profiles, the impurity densities, and the radiated powers of L-mode discharges in full-radius simulations, including a discharge with impurity seeding and a high radiated power fraction.
Furthermore, the control of impurity accumulation with central wave heating is investigated for H-mode plasmas with dominant neutral beam heating. The workflow is shown to quantitatively reproduce the experimentally observed reduction of tungsten (W) peaking at increasing wave heating power, and the physics behind this important experimental technique is analyzed.
Having validated the physics-based modelling workflow, a predictive study of the 15 MA ITER baseline scenario is presented, investigating the interplay between the edge concentration, transport and radiation of W, the auxiliary heating required to sustain H-mode operation, and the fusion performance. It is shown that heavy impurities are in a different core transport regime in reactors compared to present-day devices. Furthermore, the transport and effects of W in electron-heated ITER plasmas at lower currents and fields, as well as during the current ramp-up, are investigated. Overall, this analysis contributes to a more solid definition of the domain of operational conditions which allow the achievement of the main ITER targets, in particular in view of its recently proposed full-tungsten walls.