In the realm of fusion research, the control of plasma density, temperature, and heating is crucial for enhancing reactor performance. Effective confinement of plasma particles and heat, especially maintaining high density and temperature at the core where fusion occurs is essential.
In the Large Helical Device (LHD), challenges persist as the electron density profile often remains flat or even depressed at the center, complicating effort to sustain high central density.
The LHD is equipped with five neutral beam (NB) injectors for plasma heating. Injectors NB#1 to NB#3, deliver beams tangentially to the magnetic field, while NB#4 and NB#5 inject beams perpendicularly. Despite variations in the power ratio between tangential and perpendicular injections, the ion temperature profile remained unchanged. However, Figure 2 illustrates the existence of both peaked (red and green) and flat (blue) electron density profiles.
Adjustments in the tangential to perpendicular energetic ion ratio alter the velocity distribution from isotropic to anisotropic. We explored how density profile depends on the state of these energetic ions by analyzing the ratio of the stored energies in the perpendicular and parallel components, designated as En⊥/En|| from the injected beam power of NB#1 – NB#5.
Modifying the anisotropy within a range from En⊥/En|| = 0.3 to 0.8, showed that En⊥/En|| < 0.4 led to a flat electron density profile, while En⊥/En|| > 0.4 resulted in centrally peaked electron density profiles. Subsequently, the density profile of carbon ions was examined by externally injecting carbon and observing the ion behavior. The profile was centrally depressed in the conventional experimental range of En⊥/En|| < 0.4, yet peaked in the new experimental range where En⊥/En|| > 0.4.
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These findings suggest that the plasma inflow/outflow rates change spontaneously with the presence of energetic ions. Further investigations into the effects of energetic ions were conducted using simulation calculations. Initially, we analyzed the electric field in the radial direction at the plasma core, which simulated -5 kV/m, consistent with measurements from the heavy ion beam probe (HIBP depicted in Figure 1). Although an electric field of this strength is unlikely to influence particle flow significantly, further analysis of particle inflow and outflow due to turbulence was conducted. The results suggest that turbulence may influence both peaked and flat density profiles.
IMAGE CREDIT: National Institute for Fusion Science
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