Measurements of the edge current evolution and comparison with neoclassical calculations during MAST H-modes using motional Stark effect

Abstract

Edge Localized Modes (ELMs), that are present in most tokamak H-(high confinement) modes, can cause significant damage to plasma facing components in fusion reactors. Controlling ELMs is considered necessary and hence it is vital to understand the underlying physics. The stability of ELMs is typically expressed in terms of the pressure gradient ∇p in the edge and the edge current density j φ . Both ∇p and j φ are usually derived from profiles fitted to the measured edge density and temperature profiles, where for the calculation of j φ neoclassical theory is used. This paper presents direct measurements of the magnetic pitch angle γ m evolution in the edge and the derived j φ . On the one hand these provide a method to validate the j φ as derived with neoclassical theory. On the other hand they open up the possibility to find a complete, self-consistent set of edge profiles, that fit both density, temperature and γ m measurements, hence allowing for a more accurate stability analysis.

Impact of Te and ne on edge current density profiles in ELM mitigated regimes on ASDEX Upgrade

“…The edge pressure gradient has been well diagnosed in many ELM mitigation scenarios, but measurements of the edge current density have only recently become available. The measurements from DIII-D [21], MAST [22], and ASDEX-Upgrade [23] have all shown that the pre-ELM current density in the pedestal is consistent with a neoclassical nature, i.e. it is driven by gradients in the temperature and density profiles.…”

Modelling of three dimensional equilibrium and stability of MAST plasmas with magnetic perturbations using VMEC and COBRA

“…The aim of this paper is to show the effect of applying non-axisymmetric fields to tokamak plasmas. The pressure and safety factor profiles used here are shown in which includes constraining the reconstructed equilibrium with magnetics, Thomson Scattering, motional Stark effect and D α light at the boundary [22], for a lower single null (LSND) plasma. The plasma boundary for this equilibrium is shown in Figure 3.…”

“…The pressure and safety factor profiles used here are shown in Figures 1 and 2. They are realistic profiles typical of an axisymmetric reconstruction using EFIT, which includes constraining the reconstructed equilibrium with magnetics, Thomson Scattering, motional Stark effect and D α light at the boundary [22], for a lower single null (LSND) plasma. The plasma boundary for this equilibrium is shown in Figure 3.…”

Measurement of neoclassically predicted edge current density at ASDEX Upgrade

“…Direct measurements of the edge current density in a tokamak can be achieved by using a Motional Stark Effect (MSE) diagnostic, such as at MAST [7]. A second option is to measure the Zeeman splitting of lithium 2s-2p transition lines, such as at DIII-D [8].…”