[en] Modern fusion research aims at making the energy source of the sun accessible for electricity generation on earth. For this purpose, magnetically confined plasmas must be heated to very high temperatures of 100 million Kelvin. The resulting steep pressure gradients lead to turbulent mixing in the plasma, causing a drastic deterioration of particle and energy confinement. Under certain conditions, however, self-organized transport barriers can form in the plasma, which allow extremely steep density and temperature gradients, and strongly improved confinement. Understanding the physical mechanisms of transport barriers is thus a key step toward increasing the feasibility and competitiveness of fusion power plants. This work aims to advance the theoretical understanding of transport barriers occurring both in the core and the edge of the plasma. In order to study such conditions, the effect of strongly varying temperature and density profiles must be considered, as well as the complex shaping of today''s tokamak plasmas. The gyrokinetic turbulence code GENE has in recent years been extended to include such capabilities, and is in this work for the first time applied to the conditions of both core and edge transport barriers. Discharges of both the Swiss tokamak TCV and the German tokamak ASDEX Upgrade exhibiting transport barriers were examined, using both the local and global versions of the GENE code. For core barriers, it is found that small-scale electron temperature gradient driven (ETG) turbulence can play an important role in determining the steepness of the barrier. The use of a global model in these studies turns out to be crucial, as the transport due to large-scale turbulence is otherwise overestimated by orders of magnitude. For edge transport barriers, ETG instabilities are found to drive a significant fraction of the experimental heat transport, making them a key candidate for the residual turbulence in the edge barrier of high-confinement discharges.

[en] Using large resolution numerical simulations of gyrokinetic (GK) turbulence, spanning an interval ranging from the end of the fluid scales to the electron gyroradius, we study the energy transfers in the perpendicular direction for a proton–electron plasma in a slab equilibrium magnetic geometry. The plasma parameters employed here are relevant to kinetic Alfvén wave turbulence in solar wind conditions. In addition, we use an idealized test representation for the energy transfers between two scales, to aid our understanding of the diagnostics applicable to the nonlinear cascade in an infinite inertial range. For GK turbulence, a detailed analysis of nonlinear energy transfers that account for the separation of energy exchanging scales is performed. Starting from the study of the energy cascade and the scale locality problem, we show that the general nonlocal nature of GK turbulence, captured via locality functions, contains a subset of interactions that are deemed local, are scale invariant (i.e. a sign of asymptotic locality) and possess a locality exponent that can be recovered directly from measurements on the energy cascade. It is the first time that GK turbulence is shown to possess an asymptotic local component, even if the overall locality of interactions is nonlocal. The results presented here and their implications are discussed from the perspective of previous findings reported in the literature and the idea of universality of GK turbulence. (paper)

No abstract available

[en] In tokamaks and stellarators - two leading types of devices used in fusion research - magnetic field lines trace out toroidal surfaces on which the plasma density and temperature are constant, but turbulent fluctuations carry energy across these surfaces to the wall, thus degrading the plasma confinement. Using petaflop-scale simulations, we calculate for the first time the pattern of turbulent structures forming on stellarator magnetic surfaces, and find striking differences relative to tokamaks. The observed sensitivity of the turbulence to the magnetic geometry suggests that there is room for further confinement improvement, in addition to measures already taken to minimise the laminar transport. With an eye towards fully optimised stellarators, we present a proof-of-principle configuration with substantially reduced turbulence compared to an existing design.

[en] We report the results of a direct comparison between different kinetic models of collisionless plasma turbulence in two spatial dimensions. The models considered include a first-principles fully kinetic (FK) description, two widely used reduced models (gyrokinetic (GK) and hybrid-kinetic (HK) with fluid electrons), and a novel reduced gyrokinetic approach (KREHM). Two different ion beta (${\mathrm{\beta <!--\; ??\; -->}}_i$) regimes are considered: 0.1 and 0.5. For ${\mathrm{\beta <!--\; ??\; -->}}_i=0.5$, good agreement between the GK and FK models is found at scales ranging from the ion to the electron gyroradius, thus providing firm evidence for a kinetic Alfvén cascade scenario. In the same range, the HK model produces shallower spectral slopes, presumably due to the lack of electron Landau damping. For ${\mathrm{\beta <!--\; ??\; -->}}_i=0.1$, a detailed analysis of spectral ratios reveals a slight disagreement between the GK and FK descriptions at kinetic scales, even though kinetic Alfvén fluctuations likely still play a significant role. The discrepancy can be traced back to scales above the ion gyroradius, where the FK and HK results seem to suggest the presence of fast magnetosonic and ion Bernstein modes in both plasma beta regimes, but with a more notable deviation from GK in the low-beta case. The identified practical limits and strengths of reduced-kinetic approximations, compared here against the FK model on a case-by-case basis, may provide valuable insight into the main kinetic effects at play in turbulent collisionless plasmas, such as the solar wind.

[en] The vast separation dividing the characteristic times of energy confinement and turbulence in the core of toroidal plasmas makes first-principles prediction on long timescales extremely challenging. Here in this work, we report the demonstration of a multiple-timescale method that enables coupling global gyrokinetic simulations with a transport solver to calculate the evolution of the self-consistent temperature profile. This method, which exhibits resiliency to the intrinsic fluctuations arising in turbulence simulations, holds potential for integrating nonlocal gyrokinetic turbulence simulations into predictive, whole-device models.

[en] The Eulerian gyrokinetic turbulence code gene has recently been extended to a full torus code. Moreover, it now provides Krook-type sources for gradient-driven simulations where the profiles are maintained on average as well as localized heat sources for a flux-driven type of operation. Careful verification studies and benchmarks are performed successfully. This setup is applied to address three related transport issues concerning nonlocal effects. First, it is confirmed that in gradient-driven simulations, the local limit can be reproduced--provided that finite aspect ratio effects in the geometry are treated carefully. In this context, it also becomes clear that the profile widths (not the device width) may constitute a more appropriate measure for finite-size effects. Second, the nature and role of heat flux avalanches are discussed in the framework of both local and global, flux- and gradient-driven simulations. Third, simulations dedicated to discharges with electron internal barriers are addressed.