Electron heat transport in ASDEX Upgrade: experiment and modelling

Abstract

The electron heat transport is investigated in ASDEX Upgrade using electron cyclotron heating (ECH) combining steady-state and power modulation schemes. Experiments in which the electron heat flux has been varied in the confinement region while the edge was kept constant were performed. They demonstrate that ∇ Te and ∇ Te/Te can be varied by a factor of 3 and 2, respectively. They allow a detailed determination of the transport characteristics by comparing steady-state and modulation data with modelling. The analyses clearly show the existence of a threshold (∇ Te/Te)crit above which transport increases. Both steady-state and modulation experiments agree with such a transport model. The experiments have been carried out at low density in the L-mode to ensure low electron–ion coupling and good conditions for studying electron heat transport. The experiments were carried out at two different values of plasma current and show that transport increases at low current, as well-known from global scaling laws for confinement time. In the pure off-axis cases the region inside the ECH deposition is just at the (∇ Te/Te)crit threshold, which allows it to be measured directly from the profile of ∇ Te/Te deduced from the experimental Te profile. Using this technique, it appears that the turbulence threshold agrees with that expected from the trapped electron mode driven turbulence. It has the correct absolute value and seems to have the correct radial dependence that is determined by the trapped electron fraction and by the density gradient. It almost does not vary with other plasma parameters. In contrast, the threshold calculated for electron temperature gradient modes is higher than the experimental values of ∇ Te/Te and this turbulence is therefore not expected to be excited under these experimental conditions.

Perturbative studies of turbulent transport in fusion plasmas

“…If power modulation is also applied, a simultaneous analysis of steady-sate and modulation data can be carried out which yields the maximum information and also ensures the most stringent conditions for model testing. The critical gradient model described above has proved to reproduce correctly both steady-state and modulation data [19,32]. In the ideal case, applying the model to known HP agree well with the modulation data (red points and dashed line in Fig.8).…”

Electron transport and the critical temperature gradient

“…Contemporary evidence for turbulent electron convection in high-power electron heating is ubiquitous, and seriously impedes the pathway to fusion power. Tore Supra, FTU [19] and ASDEX-U [20] [6]. Thus, we observe the sub-microsecond time scale for the small-scale turbulence compared with tens of microseconds time scale for the TEM-ITG turbulence.…”

“…Jenko et al [11] give a complex formula given in Table 1 for the switching from density gradient to shear control of the critical gradient. Dong et al [3] give a complex set of functions describing the details of the shear and q dependence of the critical gradient and the growth rates derived from the integral eigenvalue problem for ETG drift waves and discuss the relation to the Ryter et al [20].…”

Core turbulence behavior moving from ion-temperature-gradient regime towards trapped-electron-mode regime in the ASDEX Upgrade tokamak and comparison with gyrokinetic simulation

“…The main objective was to study the turbulence characteristics in both the ITG and the TEM microinstability regime. The transition can be achieved by changing the deposited electron cyclotron heating (ECRH) power to modify the normalized logarithmic electron temperature gradient R/L T e = −R∇T e /T e locally [6,62,63], which is a driving term for the TEM instability.…”