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Description
Core density collapses (CDC) events observed in the Large Helical Device (LHD) are a major concern for the performance of future Stellarator reactors, particularly for quasi-symmetric magnetic field configurations with weak magnetic shear. CDC severely restrict the maximum β of LHD outward shifted magnetic axis configurations, strongly fueled by pellets and heated by neutral beam injector (NBI), leading to a peaked pressure profile and unstable ballooning modes at the plasma edge [1,2]. Nonlinear FAR3d simulations, based on previous CDC modeling studies [3], are performed to analyze the transport of energetic particles (EP) during CDC events. Three different EP populations are considered in the analysis: Passing EP injected by the tangential NBI, helically trapped EP injected by the perpendicular NBI as well as a theoretical reactor relevant case with alpha particles. Passing EP simulations show 50% losses induced by the combination of saturating middle n ballooning modes in the middle plasma region and low n modes at the inner plasma, leading to ballistic EP transport linked to the stochastization of the magnetic field. CDC precursors and low n perturbation during the CDC induce intense EP losses and fluxes. Trapped EP simulation indicates an almost depletion of the EP population, 80% losses, caused by the magnetic field stochastization at the plasma edge during the ballooning modes saturation. CDC precursors induce ballistic transport and large trapped EPs fluxes before the CDC destabilization. FIDA measurements indicates a decrease of the EP content at the plasma core, mainly passing EPs generated by the tangential NBI, once the CDC is triggered. The radiance is reduced to the half after the CDC, consistent with the passing EP losses obtained in the simulation. Consequently, both CDC and precursors, reduce the device plasma confinement and heating performance. The simulation of the theoretical model with alpha particles shows 50% losses, induced by the combination of saturating middle n ballooning modes in the middle plasma region and low n modes at the inner plasma, leading to a ballistic alpha particle transport linked to the stochastization of the magnetic field, similar to the passing EP case. Nevertheless, the largest losses and fluxes are induced by the CDC precursors. That means, CDC can severely deteriorate the performance of future Stellarator reactor limiting their economical viability and must be avoided.
[1] S. Ohdachi et al, Contrib. Plasma Phys. 50, 552 (2010).
[2] S. Ohdachi et al, Nucl. Fusion 57 066042 (2017).
[3] J. Varela et al, Nucl. Fusion, 66, 026017 (2026).