Parametric dependencies of ion temperature profile peaking in ASDEX Upgrade high-beta scenarios

Abstract

Advanced Tokamak (AT) scenarios are attractive candidates for future nuclear fusion power plants. These scenarios often feature peaked temperature profiles, suggesting a local reduction of turbulent transport. The mechanisms behind this are, as yet, not fully understood. Parameters that are thought to be connected to these transport reductions include the q -profile and the E × B -shear $\omega_{E\times B}$ . Another parameter considered to be important is the fast ion content, which can reduce transport in multiple different ways. This work presents AT experiments performed at the ASDEX Upgrade tokamak (AUG) in which these three parameters are varied to investigate their respective effect on the observed peaked ion temperature profiles. To disentangle potentially competing effects, simulations using the codes GENE and TGLF have been performed. It is found that the E × B -shear does not play a role in the AT scenarios performed at AUG, which have relatively low rotation, with $v_\mathrm{tor} = 100-250\,$ km h ^−1 . Experimentally, a significant dependence of $\smash{{\raise2.8pt\hbox{$\small\begin{array}{@{}l@{}}{{{R}}}\end{array}$}}}\!\!/\!\smash{{\raise-2.3pt\hbox{$\small\begin{array}{@{}l@{}}{L_{T_\mathrm{i}}}\end{array}$}}}$ on the electron cyclotron current drive settings was found, indicating that either the current profile (and, therefore, the q -profile) or $\smash{{\raise2.8pt\hbox{$\small\begin{array}{@{}l@{}}{{Te}}\end{array}$}}}\!\!/\!\smash{{\raise-2.3pt\hbox{$\small\begin{array}{@{}l@{}}{{Ti}}\end{array}$}}}$ has a significant impact on the observed suppression of turbulent transport. In support of the first option, both GENE and TGLF show a q -profile dependence of $\smash{{\raise2.8pt\hbox{$\small\begin{array}{@{}l@{}}{{{R}}}\end{array}$}}}\!\!/\!\smash{{\raise-2.3pt\hbox{$\small\begin{array}{@{}l@{}}{L_{T_\mathrm{i}}}\end{array}$}}}$ . Furthermore, according to GENE, electromagnetic (EM) fast ion effects are essential in reproducing the experimental results. Scans with TGLF (which does not include such effects), suggest that these fast ion effects become relevant above a threshold of $\smash{{\raise2.8pt\hbox{$\small\begin{array}{@{}l@{}}{{{R}}}\end{array}$}}}\!\!/\!\smash{{\raise-2.3pt\hbox{$\small\begin{array}{@{}l@{}}{L_{T_\mathrm{i}}}\end{array}$}}} = $ 4–5. In the presence of ICRF, additional electrostatic (ES) fast ion effects seem to come into effect, according to GENE, albeit to a smaller degree than the EM effects.

Keywords

• internal transport barrier
• E × B-shear
• energetic particles
• magnetic shear