Intermittent bursts induced by neoclassical double tearing modes reconnection via multihelicity simulation

In the long-pulse steady-state operation of future advanced tokamaks, intermittent bursts induced by magnetohydrodynamic (MHD) instabilities present a critical challenge to plasma confinement. Therefore, the nonlinear dynamics of intermittent bursts induced by neoclassical double tearing modes in reversed magnetic shear plasmas are systematically investigated using a reduced MHD model in a cylindrical coordinate system. Comparisons between single-helicity (SH) and multihelicity (MH) simulation results reveal that the burst amplitude and average energy of the dominant

mode in the MH case are lower than those in the SH case. This phenomenon is mainly attributed to the redistribution of perturbation energy during the MH nonlinear coupling process: a portion of the magnetic free energy is transferred to the subdominant

m divided by n equals 5 divided by 2 m / n = 5 / 2 $m/n=5/2$

mode and higher harmonics. Furthermore, an increased bootstrap current fraction modulates the intermittent bursting characteristics by reducing the ohmic current proportion, thereby weakening the

q q $q$

-profile restoration capability. Consequently, when the bootstrap current fraction (

f Subscript b f b $f_{b}$

) reaches a certain threshold (

f Subscript b Baseline greater than or equals 0.2 f b ≥ 0.2 $f_{b}\geq 0.2$

), the burst amplitude of the dominant mode tends to saturate, and the burst period begins to decrease gradually. Particularly at a high bootstrap current fraction (

f Subscript b Baseline greater than or equals 0.4 f b ≥ 0.4 $f_{b}\geq 0.4$

), constrained by the suppressed overall energy level, the MH system maintains only a weak relaxation process, exhibiting greater reductions in burst amplitude and period than the SH case.