Nonlocal effects in negative triangularity TCV plasmas
G. Merlo, Z. Huang, C. Marini, S. Brunner, S. Coda, D. R. Hatch, D. Jarema, F. Jenko, O. Sauter, L. Ṽillard
Abstract
Abstract Global gradient driven gyrokinetic simulations performed with the Gyrokinetic Electromagnetic Numerical Experiment (GENE) code are used to investigate Tokamak à configuration variable (TCV) plasmas with negative triangularity. Considering limited L-mode plasmas, the numerical results are able to reproduce the actual transport level over a major fraction of the plasma minor radius for a plasma with <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mi>δ</mml:mi> <mml:mrow> <mml:mrow> <mml:mtext>LCFS</mml:mtext> </mml:mrow> </mml:mrow> </mml:msub> <mml:mo>=</mml:mo> <mml:mo>−</mml:mo> <mml:mn>0.3</mml:mn> </mml:math> and its equivalent with standard positive triangularity δ . For the same heat flux, a larger electron temperature gradient is sustained by <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>δ</mml:mi> <mml:mo><</mml:mo> <mml:mn>0</mml:mn> </mml:math> , in turn resulting in an improved electron energy confinement. In agreement with the experiments, a reduction of the electron density fluctuations is also seen. Local flux-tube simulations are used to gauge the magnitude of nonlocal effects. Surprisingly, very little differences are found between local and global approaches for <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>δ</mml:mi> <mml:mo>></mml:mo> <mml:mn>0</mml:mn> </mml:math> , while local results yield a strong overestimation of the heat fluxes when <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>δ</mml:mi> <mml:mo><</mml:mo> <mml:mn>0</mml:mn> </mml:math> . Despite the high sensitivity of the turbulence level with respect to the input parameters, global effects appear to play a crucial role in the negative triangularity plasma and must be retained to reconcile simulations and experiments. Finally, a general stabilizing effect of negative triangularity, reducing fluxes and fluctuations by a factor dependent on the actual profiles, is recovered.