Thermal insulation of high confinement mode with dominant electron heating in comparison to dominant ion heating and corresponding changes of torque input

摘要

The ratio of heating power going to electrons and ions will undergo a transition from mixed electron and ion heating as it is in current fusion experiments to dominant electron heating in future experiments and reactors. In order to make valid projections towards future devices the connected changes in plasma response and performance are important to be study and understand: Do electron heated plasmas behave systematically different or is the change of heated species fully compensated by heat exchange from electrons to ions? How does particle transport influence the density profile? Is the energy confinement and the H-mode pedestal reduced with reduced torque input? Does the turbulent transport regime change fundamentally? The unique capabilities of the ECRH system at ASDEX Upgrade enable this change of heated species by replacing NBI with ECRH power and thereby offer the possibility to discuss these and other questions. ududFor low heating powers corresponding to high collisionalities the transition from mixed electron and ion heating to pure electron heating showed next to no degradation of the global plasma parameters and no change of the edge values of kinetic profiles. The electron density shows an increased central peaking with increased ECRH power. The central electron temperature stays constant while the ion temperature decreases slightly. The toroidal rotation decreases with reduced NBI fraction, but does not influence the profile stability. The power balance analysis shows a large energy transfer from electrons to ions, so that the electron heat flux approaches zero at the edge whereas the ion heat flux is independent of heating mix. The ion heat diffusivity exceeds the electron one. udFor high power, low collisionality discharges global plasma parameters show a slight degradation with increasing electron heating. The density profile shows a strong peaking which remains unchanged when modifying the heating mix. The electron temperature profile is unchanged whereas the central ion temperature decreases significantly with increasing ECRH fraction. The relative contribution of the heat exchange is smaller so that the electrons still carry a substantial fraction of heat at the edge. The ion heat flux is still independent of the heating mix and the ion heat diffusivity exceeds the electron one. udThe radial electrical field does not show any variation with changing heating mix. The analysis of the whole database of discharges shows a degradation of the ion temperature gradient with increasing Te/Ti and a steepening with increasing gradient of the toroidal rotation. These findings complement previous studies. udThe electron density, and the electron and ion temperatures were modelled with a first principle code. The applied sawtooth model could reproduce the experimental observations. The profile shapes, the changing Te/Ti and the peaking of the density and temperature profiles agree very well with the experimental data. Linear gyrokinetic calculations found the ion temperature gradient mode to be the dominant candidate for heat transport. The investigations can explain the observed phenomena in the experiment, like the different degree of increase of ion heat flux or density peaking for various collisionalities. ududThe results presented in this work show a consistent picture of the observed phenomena and the understanding of the main underlying physics. They allow a correct implementation in the applied computer codes and a reliable prediction of the performance of future fusion devices.