Nonlinear MHD analysis for LHD plasmas

Abstract

For the large helical device (LHD), the nonlinear evolution of equilibria that are linearly unstable to ideal interchange modes is studied using the reduced MHD equations. At sufficiently low beta, each individual mode saturates without affecting directly the evolution of the other modes. They only couple through the modification of the averaged pressure profile. The change of the averaged pressure profile is limited to the local flattening near the resonant surfaces. At higher beta values and for the same initial pressure profile, a bursting phenomenon in the kinetic energy is observed. This bursting activity is caused by the overlap of multiple modes, which results in a global reduction of the pressure. However, increasing beta and using a pressure profile obtained from the nonlinear evolution at the lower beta suppress this bursting behaviour. This result indicates that the pressure profile can be self-organized so that the LHD plasma could reach a high beta regime through a stable path.

Mercier Stability Improvement in Nonlinear Development of LHD Plasma

“…The basic equation of the NORM code is the reduced MHD equations for stream function Φ, poloidal magnetic flux Ψ , and plasma pressure P [3]. The equations are as follows:…”

“…However, linear ideal interchange modes (Mercier modes) were predicted to be unstable in this configuration [2]. In order to investigate the stabilizing mechanism of the modes, we developed a nonlinear MHD code, NORM, based on the reduced MHD equations [3,4]. In such an investigation, it is crucial to follow the continuous change of the pressure profile with respect to the increase in the beta value.…”

Characteristics of magnetic island formation due to resistive interchange instability in helical plasma

“…6 There are also many simulation studies about the effect of the resistive interchange instability in the nonlinear saturation phase on the plasma confinement property. [7][8][9][10][11] The characteristics of the perturbed current due to the resistive interchange instability and the consequent magnetic island have been reported, although the reports are for the limited range of the b value and the S number. However, it is increasing for the demand of the systematical prediction of the characteristics of the resistive interchange instability, e.g., the magnetic island formation property, to interpret the LHD experiments results.…”

Three-Dimensional Numerical Analysis of Pressure Driven Mode in RMP-Imposed LHD Plasma

“…growth rate and the nonlinear saturation level are reduced in the increase of the island width by utilizing the NORM code [4], which is based on the reduced MHD equations. However, their study is limited to the case that the island and the dominant component of the mode have the same mode numbers of (m, n) = (1, 1) and the phase difference between the island and the mode structure is fixed.…”