Finite ion orbit width effect on the neoclassical tearing mode threshold in a tokamak plasma

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 rho(theta i) are considered, expanding the ion response solution in terms of Delta = w/r << 1, where r is the minor radius. In this limit, the ion distribution can be represented as a function of toroidal canonical momentum, p(phi). 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 O(rho(theta i)). The distribution is then flattened across the drift island, rather than the magnetic island. For small islands w similar to rho(theta i), the pressure gradient is maintained across the magnetic island, suppressing the bootstrap current drive for the neoclassical tearing mode (NTM) growth. As w -> 0, 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.