Linking the micro and macro: L-H transition dynamics and threshold physics

Abstract

The links between the microscopic dynamics and macroscopic threshold physics of the L → H transition are elucidated. Emphasis is placed on understanding the physics of power threshold scalings, and especially on understanding the minimum in the power threshold as a function of density Pthr (n). By extending a numerical 1D model to evolve both electron and ion temperatures, including collisional coupling, we find that the decrease in Pthr (n) along the low-density branch is due to the combination of an increase in collisional electron-to-ion energy transfer and an increase in the heating fraction coupled to the ions. Both processes strengthen the edge diamagnetic electric field needed to lock in the mean electric field shear for the L→H transition. The increase in Pthr (n) along the high-density branch is due to the increase with ion collisionality of damping of turbulence-driven shear flows. Turbulence driven shear flows are needed to trigger the transition by extracting energy from the turbulence. Thus, we identify the critical transition physics components of the separatrix ion heat flux and the zonal flow excitation. The model reveals a power threshold minimum in density scans as a crossover between the threshold decrease supported by an increase in heat fraction received by ions (directly or indirectly, from electrons) and a threshold increase, supported by the rise in shear flow damping. The electron/ion heating mix emerges as important to the transition, in that it, together with electron-ion coupling, regulates the edge diamagnetic electric field shear. The importance of possible collisionless electron-ion heat transfer processes is explained.

The role of turbulence–flow interactions in L- to H-mode transition dynamics: recent progress

“…In addition to predator-prey dynamics and the effect of ZF shear, this model includes pressure-profile evolution and the effect of equilibrium (diamagnetic) flow shear on turbulence evolution. Recently, substantially advanced 1D models have been developed which include neoclassical and anomalous radial heat/particle transport with more realistic radial source distributions, as well as poloidal ion (mean) flow evolution and neoclassical ion flow damping [58][59][60]65]. The model described in [58,59] considers turbulence driven by the ITG, based on a simple critical-gradient model.…”

“…Figure taken from [58], with the permission of AIP Publishing. [60]. In addition to an ITG linear growth rate based on a simple critical-gradient model, TEM (trapped electron mode) turbulence driven by density and electron-temperature gradients was heuristically accounted for, neglecting a possible criticalgradient effect.…”

“…0D and 1D heuristic dynamical models of the L-H trans ition [58][59][60][61][62][63][64][65] have advanced substantially during the last five years and now include radial particles, momentum, and energy transport. 1D models have recently also been used to investigate L-H power-threshold scaling [60,65].…”

“…1D models have recently also been used to investigate L-H power-threshold scaling [60,65]. 2D and 3D fluid models [66][67][68][69][70][71][72][73], mostly in simplified slab or toroidal slab geometry, have been used to model the L-H transition sequence for high collisionality regimes, with equation systems appropriate for interchange or resistive ballooning turbulence.…”

Fluctuation-induced shear flow and energy transfer in plasma interchange turbulence

“…Many of the conceptual points concerning turbulent energy transfer have been studied in both reduced models and in experimental investigations [19][20][21][22][23][24]. The transfer of energy between fluctuations and sheared flows has also been demonstrated by fluid simulations for interchange turbulence with cold ions [10][11][12].…”