Gyrokinetic equations for strong-gradient regions

Dimits, Andris M.

doi:doi:10.1063/1.3683000

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Hybrid-like 2/1 flux-pumping and magnetic island evolution due to edge localized mode-neoclassical tearing mode coupling in DIII-D

Direct analysis of internal magnetic field pitch angles measured using the motional Stark effect diagnostic shows m/n = 2/1 neoclassical tearing modes exhibit stronger poloidal magnetic flux-pumping than typical hybrids containing m/n = 3/2 modes. This flux-pumping causes the avoidance of sawteeth, and is present during partial electron cyclotron current drive suppression of the tearing mode. This finding could lead to hybrid discharges with higher normalized fusion performance at lower q95. Th...

On the mechanism for edge localized mode mitigation by supersonic molecular beam injection

We construct a diffusive, bi-stable cellular automata model to elucidate the physical mechanisms underlying observed edge localized mode (ELM) mitigation by supersonic molecular beam injection (SMBI). The extended cellular automata model reproduces key qualitative features of ELM mitigation experiments, most significantly the increase in frequency of grain ejection events (ELMs), and the decrease in the number of grains ejected by these transport events. The basic mechanism of mitigation is the...

A gyrokinetic theory is developed under a set of orderings applicable to the edge region of tokamaks and other magnetic confinement devices, as well as to internal transport barriers. The result is a practical set equations that is valid for large perturbation amplitudes [qδψ/T = O(1), where δψ = δφ-ν_∥δA_∥/c], which is straightforward to implement numerically, and which has straightforward expressions for its conservation properties. Here, δφ and δA_∥ are the perturbed electrostatic and parallel magnetic potentials, ν_∥ is the particle velocity, c is the speed of light, and T is the temperature. The derivation is based on the quantity ɛ ≡ (ρ/λ_⊥)qδψ/T≪1 as the small expansion parameter, where ρ is the gyroradius and λ_⊥ is the perpendicular wavelength. Physically, this ordering requires that the E×B velocity and the component of the parallel velocity perpendicular to the equilibrium magnetic field are small compared to the thermal velocity. For nonlinear fluctuations saturated at “mixing-length” levels (i.e., at a level such that driving gradients in profile quantities are locally flattened), ɛ is of the order ρ/L_p, where L_p is the equilibrium profile scale length, for all scales λ_⊥ ranging from ρ to L_p. This is true even though qδψ/T = O(1) for λ_⊥ ∼ L_p. Significant additional simplifications result from ordering L_p/L_B = O(ɛ), where L_B is the spatial scale of variation of the magnetic field. We argue that these orderings are well satisfied in strong-gradient regions, such as edge and scrapeoff layer regions and internal transport barriers in tokamaks, and anticipate that our equations will be useful as a basis for simulation models for these regions.