Particle Orbit Analysis in the Finite Beta Plasma of the Large Helical Device using Real Coordinates

Abstract

High-energy particles in a finite beta plasma of the Large Helical Device (LHD) are numerically traced in a real coordinate system. We investigate particle orbits by changing the beta value and/or the magnetic field strength. No significant difference is found in the particle orbit classifications between the vacuum magnetic field and the finite beta plasma cases. The deviation of a banana orbit from the flux surfaces strongly depends on the beta value, although the deviation of the orbit of a passing particle is independent of the beta value. In addition, the deviation of the orbit of the passing particle, rather than that of the banana-orbit particles, depends on the magnetic field strength. We also examine the effect of re-entering particles, which repeatedly pass in and out of the last closed flux surface, in the finite beta plasma of the LHD. It is found that the number of re-entering particles in the finite beta plasma is larger than that in the vacuum magnetic field. As a result, the role of reentering particles in the finite beta plasma of the LHD is more important than that in the vacuum magnetic field, and the effect of the charge-exchange reaction on particle confinement in the finite beta plasma is large.

Monte-Carlo Study Based on Real Coordinates for Perpendicularly Injected High-Energy Ions in the LHD High-Beta Plasma

“…When χ of ions change from 0.5π to 0.4π or 0.6π because of the collisions, the ions become chaotic-orbit particles. For the conditions used, most chaotic-orbit particles also become the re-entering particles [8]. Therefore, F LCFS (χ) is underestimated near 0.4π or 0.6π.…”

“…This code has already been applied to the ions produced by the tangential-NBs in LHD, and it was shown that the re-entering particles play an important role in the analyzing the distribution function of ions author's e-mail: seki.ryohsuke@lhd.nifs.ac.jp a) present address: National Institute for Fusion Science, Toki 509-5292, Japan produced by the tangential-NBs for B ax = 3 T [3]. On the other hand, it has also been shown that most of the protons produced by the perpendicular-NB become the re-entering particles in high-beta plasmas [8]. Thus, the re-entering particles may play a more important role in the distribution function in the high-beta plasma.…”

“…1 (b) of Ref. [8], in which the magnetic axis and the center of the LCFS are at 3.90 m and 3.71 m, respectively. For simplicity, it is assumed that the temperature and density of the background plasma are constant throughout the region inside the VV wall.…”

“…1 (a), there are remarkable differences between F VV (ρ) and F(ρ) without orbit following. The ions produced by the perpendicular-NB tend to be helically trapped banana-orbit particles in LHD [8]. In the high beta plasma of LHD, the banana-orbit particle moves across the flux surfaces, and it deviates significantly from the flux surface on which its initial point is set.…”

Evaluation of Efficiency of Power Transferred from ICRF Fast Ions to Bulk Plasma Based on Orbit Following in Real Coordinates in LHD

“…Moreover, in helical devices such as the LHD, the shape of the resonance layer is more complicated than that of a tokamak. The fast ions heated by the ICRF wave become a "banana orbit particle (helical trapped particle)" [2] which forms a closed drift surface or a "chaotic orbit particle" [2] which does not form the closed drift surface depending upon the shape of the resonance layer. The shape of the resonance layer may affect the transferred power efficiency from minority fast ions to bulk plasma.…”