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

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. For 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. For 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. The 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. The 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. The 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.