Effects of particle transport on helium ash accumulation and sustained ignition in the ITER (International Tokamak Experimental Reactor) design

The buildup of helium ash in the proposed ITER experiment has been studied in a series of simulations with the BALDUR transport code. Using radially dependent thermal diffusivities which were scaled from JET values, we studied the role of particle transport coefficients and edge recycling on helium poisoning of ignition. A sustained ignition was obtained when the exhaust of helium from the edge plasma was allowed to exceed 10% of the helium flux into the edge plasma from the core plasma, and the ratio of particle (He ion) to thermal diffusivities, D/{chi}, was larger than 1/4. The simulations included the effects of sawtooth oscillations, radiative as well as conductive energy loss channels, and density profile variations. 29 refs., 11 figs