ITB formation in gyrokinetic flux-driven ITG/TEM turbulence

Abstract

The formation mechanism of Internal Transport Barriers (ITBs) in flux-driven turbulence is studied by means of the full-f gyrokinetic code GKNET. In the adiabatic electron case with a weak magnetic shear configuration, toroidal momentum injection can change the radial mean electric field E_r through the radial force balance, leading to a kind of driven ITB formation in which the ion thermal diffusivity by Ion Temperature Gradient (ITG) turbulence decreases to the neoclassical transport level. Only co-current toroidal rotation in the outer region can benefit the ITB formation, which mechanism is identified to originate from a positive feedback loop between the radial E_r shear and resultant momentum flux. On the other hand, in the kinetic electron case with a reversed magnetic shear configuration, robust co-intrinsic rotation is driven near the q_min surface in ITG turbulence and sustain the E_r shear through the radial force balance, leading to the spontaneous reduction of ion turbulent thermal diffusivity, while it is not observed in the adiabatic electron case. In the presence of electron heating, counter-intrinsic rotation by Trapped Electron Mode (TEM) turbulence is selectively driven in the negative magnetic shear region, which provides steeper E_r shear formation and resultant larger reduction of ion turbulent thermal diffusivity. It indicates that the co-existence of different modes can trigger the discontinuity near q_min and then spontaneous ITB formation.

Characteristics of constrained turbulent transport in flux-driven toroidal plasmas

“…Some important processes such as those associated with trapped electron (TEM) and electromagnetic mode, e.g. kinetic ballooning mode (KBM) [53][54][55] are not taken into account, some of which are believed to be important in explaining transport shortfall events. However, it is rather meaningful to capture the properties of profile stiffness under conditions comparable to experiments based on minimum models as open systems.…”

“…We studied dynamics leading to constrained (stiffness) profile and the underlying mechanism using the ES version of global toroidal gyro-kinetic GKNET simulations in a flux-driven open system, with source and sink in the framework of adiabatic electron response [ 29 , 33 , 39 – 41 , 48 ]. Some important processes such as those associated with trapped electron (TEM) and electromagnetic mode, e.g.…”

“…Paradoxical ingredients, e.g. stiffness, emerge, although key transport-related parameters, such as values of q and ŝ, T e /T i , do vary, which was pointed out in both experiments [39][40][41][42] and empirical theories [6,10]. It is important to note here that confined plasma is a non-thermodynamic open system in which the plasma is maintained by an external source and sink.…”

“…Based on the background discussed in §1, we organize the simulation results using the ES version of GKNET covering from core to edge as an open system, which models those in figure 1 b [ 39 – 41 , 48 ]. The system represents a self-consistent global profile evolution sustained by an external heat source and sink.…”

“…Here, we study the mechanism that forms a constrained profile as discussed above by using the electrostatic (ES) version of GKNET covering the core to edge plasma as an open system, which models those in figure 1 b , under the adiabatic electron response [ 29 , 33 , 39 – 41 , 48 ]. In the simulation using a monotonically increasing safety factor profile with gentle density gradient, we observe a typical constrained profile of exponential function form but with spatially constant two piecewise different scale lengths, i.e.…”