Experimental study of electron heat transport in ion heated H-modes in ASDEX Upgrade

The electron heat transport in low density H-mode plasmas heated by neutral beam injection (NBI) is investigated in ASDEX Upgrade using electron cyclotron heating (ECH) combining both steady-state and transient response analysis by modulating the ECH power. Under these conditions, more than 60% of the NBI power (>3 MW) is delivered to the ions, while approximately 20% (∼1 MW) is delivered to the electrons. In the confinement region, the electron-to-ion temperature ratio, Te/Ti, varies between 0.5 and 0.7 in the NBI-only phase and between 0.8 and 1.0 when the ECH is also applied. Due to the low collisional coupling, the power in the electron channel is locally more than doubled by applying up to the available 2 MW of ECH, while the power in the ion channel is locally increased by less than 30%. A dependence on the density of the reaction of the plasma parameters to the ECH is observed. For plasmas with average density nbare < 4.0 x 1019 m-3 (defined as 'hot-ion' H-modes), when the ECH is applied, Te increases, the central Ti drops and the density flattens. These effects disappear with increasing density and are not observed for nbare > 4.0 x 1019 m-3 (defined as 'regular' H-modes). Power balance analysis of both the hot-ion and regular H-modes points to a strong resilient behaviour of the Te profiles. In the hot-ion cases, the ECH heating induces a strong increase in transport in the ion channel. Power balance and transient response analysis of the regular H-modes are consistent with an inverse scale length transport model with a threshold in R/LTe=R · vertical bar ∇Te|/Te, above which the electron heat transport is increased. Comparison with recent studies in pure EC heated L-modes points to a stronger resilience of Te in the NBI heated H-modes