Turbulence Interaction with Driven Transport Barriers in the Scrape‐Off Layer of Tokamaks

Abstract

Two mechanisms able to generate a transport barrier by the stabilization of turbulence are analysed: the radial shear of the E×B velocity and the radial gradient of this shear. These two mechanisms are artificially forced in dedicated 2D non-linear fluid simulations to study their impact on turbulence in the Scrape-Off Layer. Generated barriers are characterized by their turbulence stopping capability and a key parameter, the barrier efficiency, is pointed out. The first two moments of this criterion can be studied to quantify the global efficiency, shape and width of the barrier as well as its intermittency. It is found that both mechanisms can drive intermittent or stable barriers but the radial shear mechanism generates stronger relaxation events. Small scale structures are damped by the E×B velocity shear while the radial shear gradient acts mainly on large scales.

Staged approach towards physics-based L-H transition models

“…In particular, the local magnetic shear [56] varies strongly poloidally in case of diverted equilibrium. It was also shown using TOKAM3X that the magnetic shear modification from limited to diverted plasma leads to a transport barrier around the LCFS [57,58]. • The strong radial variation of the flows across the LCFS (either due to neoclassical or turbulent poloidal flows, see next section) can drive Kelvin-Helmholtz instabilities, which, for example, could be associated to experimentally reported poloidal asymmetries of SOL turbulence [59].…”

Bayesian approach to parameter estimation and model validation for nuclear fusion reactor mean-field edge turbulence modelling

“…The first model is a single-equation k ⊥ model [14], while the second is a two-equation k ⊥ -ζ ⊥ model [15]. The reference data, used to develop the models and to calculate their parameters, is provided by the TOKAM2D turbulence code [16][17][18].…”

“…The resulting equations and closure models include a limited number of model parameters, whose values have been determined by fitting to TOKAM2D simulation results through nonlinear least squares regression. The TOKAM2D version applied in [17,18] has been used for this, which was run for the SOL only in isothermal mode. The TOKAM2D data set used in this paper consists of the (average) radial profiles of 18 TOKAM2D simulations in which the model parameters were varied over a representative range.…”

“…This model has been refined by adding closures based on machine scaling laws [11][12][13]. Following a similar premise [14,15], proposed transport models based on an analysis of data provided by a range of simulations from the TOKAM2D turbulence code [16][17][18] and a careful study and averaging of the underlying turbulence equations. A first implementation of these models in the SOLPS-ITER mean-field code [19] has been achieved in [20].…”

Impact of safety factor and magnetic shear profiles on edge turbulence in circular limited geometry

“…We can wonder which one of the magnetic or E × B shear is responsible for the formation of the transport barrier. To answer this question, we first introduce a criterion, called the barrier efficiency, which measures the capability of a flux surface to stop the radial propagation of turbulence. This criterion—varying between 0 and 1 and depending on time and flux surface index (i.e., radial direction)—corresponds to the proportion of the total radial transport, which is not caused by turbulence: ϵ B ( ψ , t ) = 1 − < Γ E × B > F .…”