Abstract
Controlling neoclassical tearing modes (NTMs) is vital for future tokamaks, such as ITER. An NTM control system relies on the magnetic island threshold physics. In this paper, new results for the ion response to the island perturbation and its influence on the island evolution are presented. Considering a small island, w ≪ r, where r is the minor radius, but crucially retaining the ordering ρbi ∼ w (relevant for threshold) to capture the finite orbit width effect, we determine the ion response using the drift kinetic equation. Momentum conservation and quasineutrality are taken into account, which are crucial for determining the current perturbations. The results show that the finite particle orbit effects are significant even for a moderately small ratio of ρθi/w (ρθi is the ion poloidal Larmor radius; ρbi ∼ ϵ1/2ρθi). When w ∼ ρθi, the flattening of the pressure gradient across the island is substantially restored, implying that the bootstrap current drive for the island growth is suppressed. Moreover, we find that for a sufficiently small island, w ≪ ρθi, the contribution can be negative, meaning that it can stabilize small seed islands, providing a threshold. This will have significant impact on our understanding of the NTM threshold physics.
Highlights
One of the challenges that future tokamaks will face is the effect of neoclassical tearing modes (NTMs), which are characterized by the evolution of a magnetic island chain
This paper focuses on the effect of finite ion orbit width on the bootstrap current contribution to the magnetic island evolution, extending the existing drift kinetic theory [6] to describe the ion response to the island perturbation
This is a rather remarkable result, as it means that the effect of the current perturbation is to heal the island and represents new threshold physics that cannot be explained by a reduced bootstrap drive alone
Summary
One of the challenges that future tokamaks will face is the effect of neoclassical tearing modes (NTMs), which are characterized by the evolution of a magnetic island chain. A new theory is required to accurately determine the relative sizes of the ∆bs, ∆pol and ∆′ contributions, including their dependence on the curvature and finite particle orbit width effects. This will allow us to quantitatively predict the threshold width for ITER. This paper focuses on the effect of finite ion orbit width on the bootstrap current contribution to the magnetic island evolution, extending the existing drift kinetic theory [6] to describe the ion response to the island perturbation. In the limit of a very small island, the electrons provide the stabilizing contribution; not explained by the standard bootstrap theory
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