Drift kinetic response of ions to magnetic island perturbation and effects on NTM threshold

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 rho(bi) similar to 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 rho theta i/w (rho(theta i) is the ion poloidal Larmor radius; rho(bi) similar to epsilon(1/2) rho(theta i)). When w similar to rho(theta i), 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 << rho(theta i), 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.