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The tokamak hybrid scenario is a candidate for long-pulse, sawtooth-free operation in ITER. This sawtooth-free state is attributed to its robust, near-flat q≳1 core safety factor profile, maintained by the long-lived MHD-driven current redistribution that hinders the peak current profile. A notable example is the long-lived m/n=1/1 quasi-interchange mode. However, this m/n=1/1 mode leads to a long-lived core toroidal asymmetric state, also known as “helical core” (HC)1. Studies predicted that a large HC could spontaneously form in ITER2. Its non-negligible n=1 toroidal asymmetry may affect the overall plasma stability, as all toroidal modes are linearly coupled3-4 into a single entity. In this study, we assess the MHD stability of ITER-scale HC equilibrium using the kinetic MHD energetic-particle (EP) simulation code. We find that this toroidal asymmetry leads to the destabilization of the moderate-to-short-wavelength pressure-driven and EP-driven MHD modes, which otherwise are stable in the axisymmetric equilibrium. Both modes consist of broad toroidal and poloidal spectra. All toroidal harmonics of the mode balloon in the bad curvature region, but they are aligned such that they constructively interfere along the HC compressed flux region and destructively interfere elsewhere. For the pressure-driven mode, its linear growth rate follows the resistive ballooning mode scaling, which may suggest that it is a cluster of “n” ballooning modes synchronized by HC. For the EP-driven mode, it resides in the Alfvenic-acoustic gap. Unperturbed orbit analysis shows that the toroidal sideband resonances of this mode overlap spatially, which may explain its destabilization in HC equilibrium. The nonlinear simulation predicts that these pressure-driven and EP-driven MHD modes lead to a minor redistribution of the bulk plasma and fusion-born alpha particles. Nonetheless, this quantitative prediction should be interpreted cautiously, as the scale length of the short-wavelength spectra of the modes may be comparable to or smaller than the ion gyroradius, and they are also sensitive to viscous–resistive dissipation imposed in our model.
[1] W.A. Cooper et al 2010 Phys. Review Letters 105.3 035003
[2] A. Wingen et al 2018 Nucl. Fusion 58 036004
[3] Y.I. Kolesnichenko et al 2002 Phys. of Plasmas 9.2 517-528
[4] D. Spong et al 2003 Phys. of Plasmas 10.8: 3217-3224