Description
Recent experiments in the Large Helical Device (LHD) have identified a striking transport bifurcation driven by energetic-ion phase-space anisotropy. This phenomenon manifests as the emergence of two distinct density states—peaked (#182744) and flattened (#182745)—under nearly identical macroscopic conditions. This study investigates the physical origin of this bifurcation by focusing on the reorganization of the radial electric field ($E_r$) and its consequences for multi-species transport.
Direct measurements using a Heavy Ion Beam Probe (HIBP) reveal systematic differences in the $E_r$ structure between the two states, indicating a fundamental modification of the ambipolar balance. To assess whether such bifurcation can be explained within neoclassical theory, global FORTEC-3D simulations were performed using experimentally constrained profiles of density, temperature, and magnetic equilibrium.
The main results of this analysis are as follows:
• Ambipolar $E_r$ Validation: The simulations successfully reproduce the experimentally observed negative core $E_r$ of approximately $-5$ kV/m. However, discrepancies emerge in the mid-radius region ($r/a \approx 0.4$–$0.6$), where neoclassical particle and heat fluxes exhibit strong sensitivity to small variations in $E_r$. This behavior suggests that while neoclassical effects provide a foundational framework, they are insufficient by themselves to fully account for the observed state separation.
• Impurity-Mediated Particle Balance: Species-resolved flux analysis reveals a key role of impurity transport. In both density states, the $C^{6+}$ impurity flux is strongly inward in the core region. Quasi-neutrality then requires a compensating outward $D^+$ flux, even though the main-ion flux does not undergo a direct sign reversal. This indirect coupling demonstrates that impurity dynamics can decisively modify the bulk-ion particle balance.
• Sensitivity to $E_r$ Structure: Additional sensitivity studies, in which the outer-region $E_r$ was varied from $-40$ kV/m to $+20$ kV/m, show that strongly negative $E_r$ substantially enhances inward $C^{6+}$ transport. In contrast, the $D^+$ flux remains relatively insensitive when $E_r$ is near-zero or positive.
These results support a scenario in which energetic-ion-driven $E_r$ reconfiguration induces a neoclassical transport transition shaped by impurity-mediated particle balance. Remaining discrepancies between simulation and experiment indicate a synergistic interplay between neoclassical and turbulent transport, providing insight into density bifurcations in helical plasmas.