Description
Control of particle transport is a prerequisite for sustaining burning plasmas, necessitating a precise balance between fuel confinement and helium ash removal. This study investigates the transport bifurcation phenomena in the Large Helical Device (LHD), demonstrating that the velocity-space anisotropy of energetic ions serves as a deterministic control knob to regulate the directionality of radial particle transport.
Initially, bifurcation of electron density profiles into peaked or flattened steady states was observed following carbon pellet injection. Gyrokinetic analysis suggests that Ion Temperature Gradient (ITG) turbulence remains active in these states, implying a complex interaction between turbulence and transport fluxes. To isolate the regulatory role of energetic ions from pellet fueling effects, systematic experiments were conducted by varying the heating power ratio of energetic neutral beams ($P_\perp/P_\parallel$) from 0 (tangential) to $\infty$ (perpendicular) without pellet injection.
A clear transition in the transport regime was observed: electron density profiles became hollow in tangential-dominant regimes ($P_\perp/P_\parallel \approx 0-1.2$) and shifted to strongly peaked profiles in perpendicular-dominant regimes ($P_\perp/P_\parallel \to \infty$). Carbon impurity profiles exhibited a synchronized response. Crucially, this bifurcation is interpreted as a transition in the radial electric field structure. The velocity-space anisotropy modifies the non-ambipolar flux of energetic ions, which reconfigures the ambipolar condition and overrides the intrinsic neoclassical transport governed by bulk temperatures. These findings establish that energetic-ion phase-space physics can decouple particle transport from bulk thermal regimes, offering a novel and robust methodology for optimizing confinement and impurity control in future fusion reactors.