TRANSPORT SIMULATIONS OF THE IGNITED ITER WITH HIGHT HELIUM FRACTION

Computer simulations with special versions of the one dimensional BALDUR predictive transport code are carried out to investigate the particle confinement of helium and hydrogen, the energy confinement and the burn control in the high density scenario of the ITER (CDA) physics phase. The code uses empirical transport coefficients for ELMy H mode plasmas, an improved model of the scrape-off layer (SOL), an impurity radiation model for helium and iron, and fast burn control by neutral beam injection feedback. A self-sustained thermonuclear burn is achieved for hundreds of seconds. The necessary radiation corrected energy confinement time tau(E) is found to be 4.2 s which is attainable according to the ITER H mode scaling. In the ignited ITER, a significant dilution of the DT fuel by helium takes place. Steady state helium fractions of up to 8% are obtained, which are found to be compatible with self-sustained burn. The SOL model yields self-consistent electron densities and temperatures at the separatrix (n(e) = 5.8 x 10(19) m-3, T(e) = 80 eV). Small helium Mach numbers cause a high helium density pedestal, a steady state helium content of 7% and a ratio tau(p)He/tau(E) = 4.9 in the baseline case. Variation of the diffusion coefficient D at fixed electron heat diffusivity chi(e) results in tau(p)He/tau(E) = 0.5 chi(e)/D + 4. 1, the high offset being due to the helium density pedestal. Sensitivity studies over a wide range of parameters and for different transport scalings do not show detrimental effects on ignition and sustained burn if there is sufficient freedom to adjust chi(e). The radiation loss required for halving the divertor heat load is reached with 0.2% iron. By contrast, blowing in carbon from the wall causes only a very small reduction of the heat load, since most of the power is radiated in the SOL. Furthermore, simulations show that efficient burn control is achieved both with and without sawteeth. Heating up to ignition and burn control are found to be also possible at small injection energies, such as 0.5 MeV and even 0.2 MeV. Results from simulations with current and density ramps and an extended burn duration of 410 s are presented.