Researchers find new way to control plasma density in fusion reactors

In the field of fusion research, control of plasma density, temperature, and heating are important for optimizing reactor performance. Good packing of plasma particles and heat, especially maintaining a high density and temperature in the core where fusion occurs, is important.

In the Large Helical Device (LHD), challenges continue as the electron density profile often remains flat or even depressed in the center, complicating efforts to maintain a high center density.

The LHD has five beam nozzles (NB) for plasma heating. Injectors NB#1 to NB#3, rapidly inject radiation into the magnetic field, while NB#4 and NB#5 inject radiation through the aperture. Despite the difference in power ratio between tangential and perpendicular injections, the ion temperature profile remained unchanged.

A change in the ratio of tangential to perpendicular ion dynamics changes the velocity distribution from isotropic to anisotropic. The researchers discovered how the density profile depends on the state of these energetic ions by analyzing the ratio of energy stored in the perpendicular and parallel components, designated as En._⊥/ In_| from the injected beam power of NB#1–NB#5.

Adjust the anisotropy within the range from En_⊥/ In_| = 0.3 to 0.8, showed that En_⊥/ In_| < 0.4 resulted in a flat electron density profile, while En_⊥/ In_| > 0.4 resulted in a higher electron density profile.

Afterwards, the carbon ion density profile was investigated by injecting carbon out and observing the behavior of the ions. The research has been published in the journal Plasma Physics.

Profile depressed in En test series_⊥/ In_| < 0.4, it still peaked in the new test series where En_⊥/ In_| > 0.4.

These results indicate that plasma entry/exit rates change independently with the presence of strong ions. Further investigations into the effects of energetic ions were carried out using simulation calculations.

Initially, the researchers analyzed the electric field in the radial direction at the plasma core, which simulated -5 kV/m, based on measurements from the heavy ion beam probe (HIBP). Although an electric field of these strengths is unlikely to significantly affect particle flow, further analysis of particle inflow and outflow due to turbulence was performed. The results show that turbulence can affect high and flat density profiles.

Importance of research results and future development

This discovery explains that the direction and volume of particles entering and exiting the fusion plasma barrier region can be effectively controlled by using the anisotropic state of the active ions, thus keeping the plasma in an optimal state.

According to the researchers, a new physics mechanism behind this must be elucidated. The researchers say they will further develop their research to contribute to the high performance of fusion reactor plasma, the reduction of fusion reactors, the improvement of energy output, and the control of plasma burning conditions.