Turbulence Response in the High T_i Discharge of the LHD

Abstract

A high ion temperature (T_i) was achieved using a combination of perpendicular and parallel injected neutral beams in the Large Helical Device (LHD). Microturbulence spatial profiles in a high-T_i discharge were measured by two-dimensional phase contrast imaging (2D-PCI) through almost the entire vertical central chord. The 2D-PCI microturbulence spectral ranges covered wavenumbers (k) of 0.1-1 mm⁻¹ and frequencies (f) of 20-500 kHz. The ion thermal conductivity (χ_i) increased in the entire region with increasing T_i. However, the difference between the experimental and neoclassical values of χ_i became smaller at ρ < 0.5, where ρ is the normalized position, in the high-T_i phase. Increasing fluctuation was not observed at this location, suggesting improved ion energy transport in this region. On the other hand, at ρ > 0.5, χ_i deviated from the neoclassical value due to enhancement of the experimental χ_i and reduction in the neoclassical χ_i by a positive radial electric field. Increasing turbulence was observed at ρ = 0.6-0.8, with fluctuations likely propagated to the ion diamagnetic direction in the plasma frame, suggesting that the observed turbulence degrades the ion energy transport at this location in the high-T_i phase.