Dependence of the density shape on the heat flux profile in ASDEX Upgrade high density H modes

Abstract

For high density H modes with constant gas puff on ASDEX Upgrade a slow evolution of the density profile is observed with a time constant much larger than τ_E. While the edge density remains constant the central density increases as does the stored energy until sawteeth are lost and impurity influx sets in. This peaking process is only observed with NBI heating. Substituting half of the heating by central ICRH leads to completely flat profiles even though half of the original NBI particle fuelling remains. This behaviour, as well as the slow timescale of the peaking process with pure NBI heating, is successfully modelled with a pinch of the order of the neoclassical Ware pinch and a proportionality between the particle diffusion coefficient D and the heat conductivity χ. Such an assumption links D to the heat flux profile q_heat, since the temperature profile is observed to be stiff. Such a model implies flat density profiles for a centrally heated fusion reactor.