Synergetic effects of diamagnetic flow and EB shear flow on neoclassical tearing modes (NTMs) are numerically investigated by using a two-fluid four-field model. Such seed islands are driven by the boundary perturbation rather than through core seed islands. Firstly, in the single-helicity simulations, the initial equilibrium includes the 3/2 mode with negative Δ′. The boundary perturbation can induce a driven-reconnection on the 3/2 rational surface with island width as , where is the boundary perturbation amplitude. When the 3/2 island widths exceed a critical value (equivalently seed island), the 3/2 NTM is triggered with a much larger island width. It is found that when diamagnetic flow is greater than a threshold , the 3/2 mode cannot enter the NTM phase, but still keeps in the driven-reconnection saturation state. A strong positive scaling is obtained. Further increasing diamagnetic flow accelerates the early island growth due to the excitation of drift tearing mode, but reduces the saturation of the driven-reconnection. The EB shear flow can effectively enhance or offset the suppressing effect of the diamagnetic flow depending on their co- or counter-direction. Secondly, in the multiple-helicity simulations, an extra 2/1 mode with positive Δ′ is added in the system. The 3/2 mode enters quickly the nonlinear NTM saturation due to the boundary drive, but the early growth of 2/1 mode is slow. When the 2/1 mode reaches saturation, it can suppress the 3/2 NTM to a lower level saturation. It is found that the diamagnetic flow can keep the 3/2 mode in the driven-reconnection saturation, but raise the 2/1 mode saturation. The increase of 2/1 islands can trigger the 3/2 NTM with larger 3/2 islands on the 3/2 rational surfaces. The simultaneous growth of both 2/1 and 3/2 islands generates a wider flattened region of the pressure, significantly reducing the drive of bootstrap current for the 3/2 NTM. As a result, the 3/2 NTM returns to a very low driven-reconnection level or is completely stabilized. Moreover, the synergetic effects of diamagnetic flow and EB shear flow with fixed and unfixed profiles are also studied, and the Poincaré plots of magnetic topology evolution are illustrated to clearly understand the interaction between multiple-helicity islands.
Seed island formation by forced magnetic reconnection