Year: 2015
Type: article
Abstract: In this paper the global eigenmode structures of linear ion temperature gradient (ITG) modes in tokamak plasmas are obtained using a novel technique which combines results from the local gyrokinetic code GS2 with analytical theory to reconstruct global properties. Local gyrokinetic calculations are performed for a range of radial flux surfaces, x , and ballooning phase angles, p , to map out the local complex mode frequency, Ω 0 ( x , p ) = ω 0 ( x , p ) + i γ 0 ( x , p ) for a single toroidal mode number, n . Taylor expanding Ω 0 about a reference surface at x = 0, and employing the Fourier-ballooning representation leads to a second order ODE for the amplitude envelope, A ( p ), which describes how the local results are combined to form the global mode. The equilibrium profiles impact on the variation of Ω 0 ( x , p ) and hence influence the global mode structure. The simulations presented here are based upon a global extension to the CYCLONE base case and employ the circular Miller equilibrium model. In an equilibrium with radially varying profiles of a / L T and a / L n , peaked at x = 0, and with all other equilibrium profiles held constant, including η i = L n / L T , Ω 0 ( x , p ) is found to have a stationary point. The reconstructed global mode sits at the outboard mid-plane of the tokamak, with global growth rate, γ ∼ Max[ γ 0 ]. Including the radial variation of other equilibrium profiles like safety factor and magnetic shear, leads to a mode that peaks away from the outboard mid-plane, with a reduced global growth rate. Finally, the influence of toroidal flow shear has also been investigated through the introduction of a Doppler shift, , where Ω ϕ is the equilibrium toroidal flow, and a prime denotes the radial derivative. The equilibrium profile variations introduce an asymmetry into the global growth rate spectrum with respect to the sign of , such that the maximum growth rate is achieved with non-zero shearing, consistent with recent global gyrokinetic calculations.
Authors: P A Abdoul, D Dickinson, C M Roach, H R Wilson
Institutions: University of York, Culham Centre for Fusion Energy
Language: English
Subfield: Nuclear and High Energy Physics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: green
Awards: G153530965, G3779133474
OpenAlex ID: W1532209077