β dependence of micro-instabilities using linear gyrokinetic simulations

Abstract

The β scaling of micro-instabilities has been investigated by several authors using gyrofluid or gyrokinetic description in linear as well as in non-linear calculations. The typical picture well established now is that at low β, Ion Temperature Gradient (ITG) mode is stabilized with increasing β until Kinetic Ballooning modes become unstable. Such kind of numerical studies have been generally performed in plasma conditions close to the CYCLONE case, which means for ion temperature gradient length strongly above the ITG linear threshold and density gradient length around R/Ln = 2.2. However, in some standard H-mode plasmas, not only the ion temperature gradient length is closer to the ITG threshold but also the density gradient length can be rather small as compared to the CYCLONE case. In these conditions, the β scaling of micro-instabilities can significantly differ from the expected behavior described above. The goal of this paper is to investigate numerically the β scaling of micro-instabilities around such experimental conditions. Simulations have been performed using the gyrokinetic electromagnetic flux-tube code GS2 in its linear version and considering as reference, discharges from the dedicated β scaling experiments performed recently in ASDEX Upgrade. The effect of changing the gradient lengths (density and temperature) within experimental errors-bars is studied. It appears that for certain ranges of plasma parameters micro-tearing modes are the dominant micro-instability and coexist in the spectrum with ITG modes with a comparable growth rate. The effect of the plasma shape on the β scaling of micro-instabilities is also investigated. It is shown that the triangularity affects slightly the KBM branch which is destabilized at value of β clearly above the experimental range. However, it is found that at pt = 0.7 Trapped Electron Modes are the most unstable instabilities and that such modes are destabilized with increasing β in agreement with results from dedicated β experiment performed in ASDEX Upgrade.