Abstract
We investigate by means of local and global nonlinear gyrokinetic GENE simulations an ASDEX Upgrade H-mode plasma. We find that for the outer core positions (i.e., ρtor≈0.5−0.7), nonlocal effects are important. For nominal input parameters local simulations over-predict the experimental heat fluxes by a large factor, while a good agreement is found with global simulations. This was a priori not expected, since the values of 1/ρ⋆ were large enough that global and local simulations should have been in accordance. Nevertheless, due to the high sensitivity of the heat fluxes with respect to the input parameters, it is still possible to match the heat fluxes in local simulations with the experimental and global results by varying the ion temperature gradient within the experimental uncertainties. In addition to that, once an agreement in the transport quantities between local (flux-matched) and global simulations is achieved, an agreement for other quantities, such as density and temperature fluctuations, is also found. The case presented here clearly shows that even in the presence of global size-effects, the local simulation approach is still a valid and accurate approach.
Highlights
It has been known for several decades that in the absence of macroscopic instabilities, turbulent transport is a key limiting factor in the performance of magnetically confined plasmas
Global simulations are in reasonably good agreement with ASTRA results
Note that results inside the buffer zone should not be taken into consideration, since the buffer zone is a transition region, where the heat fluxes are gradually reduced until vanishing at the boundaries
Summary
It has been known for several decades that in the absence of macroscopic instabilities, turbulent transport is a key limiting factor in the performance of magnetically confined plasmas. An important and necessary step towards the achievement of this goal is to demonstrate that the gyrokinetic codes are able to reproduce all of the turbulence features found in current devices This so-called validation process has lead to quantitatively comparisons between experimental data and results of gyrokinetic simulations by comparing simultaneously several quantities. The local (flux-tube) approximation assumes that the relevant turbulent structures are small with respect to the characteristic scales of the profiles and gradients This allows for the use of periodic boundary conditions and the application of spectral methods, which leads to a reduction in the numerical cost of the simulations with respect to the global simulations.
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