Abstract
A new drift kinetic theory for the response of ions to small magnetic islands in toroidal plasma is presented. Islands whose width w is comparable to the ion poloidal Larmor radius are considered, expanding the ion response solution in terms of , where r is the minor radius. In this limit, the ion distribution can be represented as a function of toroidal canonical momentum, . With effects of grad-B and curvature drifts taken into account, the ion distribution function is a constant on a ‘drift island’ structure, which is identical to the magnetic island but radially shifted by . The distribution is then flattened across the drift island, rather than the magnetic island. For small islands , the pressure gradient is maintained across the magnetic island, suppressing the bootstrap current drive for the neoclassical tearing mode (NTM) growth. As , the ions are largely unperturbed. However, the electrons respond to the electrostatic potential required for quasi-neutrality and this provides a stabilizing contribution to the NTM evolution. This gives a new physical understanding of the NTM threshold mechanism, with implications for the design of NTM control systems for future tokamaks such as ITER.
Highlights
A neoclassical tearing mode (NTM) is an example of a resis tive magnetohydrodynamic (MHD) instability present in a tokamak plasma
This paper focuses on the effect of finite ion orbit width on the bootstrap current contribution to the magnetic island evolution, by considering a stationary island relative to the plasma
We have presented a new drift kinetic theory for the response of ions to small, stationary magnetic island perturbations in a tokamak plasma, as well as the implications for the NTM threshold physics
Summary
A neoclassical tearing mode (NTM) is an example of a resis tive magnetohydrodynamic (MHD) instability present in a tokamak plasma. 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, by considering a stationary island relative to the plasma. Earlier works considered the limit w ∼ ρbi by employing a particleincell (PIC) simulation to solve the drift kinetic equation [27], or approached the problem analytically by focusing on the contribution from the passing particles only [28] They found that an ion density gradient is supported across the island, but did not address the consequences this has for quasineutrality and the electron response.
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