MHD Simulation of Current Drive by Repetitive Plasmoid Injection in Helicity-Driven Spheromaks.

Abstract

The dynamics of spheromak plasmas in coaxial helicity injection (CHI) systems has been investigated using three-dimensional magnetohydrodynamic (MHD) numerical simulations. It was found that toroidal current is driven by repetitive asymmetric plasmoid injection, which is related to the n = 1 oscillations. In addition, we propose that multiple pulse operation of the helicity injection is effective for improving confinement because it reduces the n = 1 fluctuations. Keywords:coaxial helicity injection, spheromaks, repetitive plasmoid injection, MHD, simulation author's e-mail: skagei@elct.eng.himeji-tech.ac.jp Coaxial helicity injection (CHI) has demonstrated the ability to form and sustain spheromak and spherical tokamak (ST) plasmas on several devices [1,2]. In these experiments, magnetic field fluctuations with toroidal mode number n = 1 are observed during sustainment, and these fluctuations are considered responsible for the current drive. However, the detailed physical mechanism understanding this phenomenon is not yet well understood. Sovinec et al. demonstrated numerical simulations of helicity-driven spheromaks [3], but the detailed dynamics of toroidal current generation in the gun-driven-system was not clearly revealed. In order to reveal this, 3-D magnetohydrodynamic (MHD) numerical simulations for spheromak plasmas were executed.The simulation region consists of two cylinders, each with a center post: one is a gun region (0.175 ≤ r ≤ 0.65 and 0 ≤ z ≤ 0.5), the other a confinement region (0.15 ≤ r ≤ 1.0 and 0.5 ≤ z ≤ 2.0), as shown in Fig. 1. Grid sizes (N r × N θ × N z ) are (39 × 64 × 40) and (69 × 64 × 121) in the direction of the gun and the confinement regions, respectively. Bias magnetic flux penetrates electrodes at the inner and outer boundaries of the gun region. Boundaries of the confinement region are assumed to be perfectly conducting walls. The initial spheromak configuration is given by numerically solving ∇ × B = λ B (λ is the force-free parameter) under these boundary conditions. The governing equations are one-fluid MHD equations, as follows:In this simulation, the mass density is spatially and temporally constant for simplicity. All physical quantities are normalized by initial mass density ρ 0 , typical Alfvén speed V A , and maximum length of the cylinder radius L 0 . The conductivity κ , the resistivity η, and the viscosity ν are fixed to 1 × 10 -3 , 2 × 10 -4 , and 1 × 10 -3 in the normalized units (γ -1) -1 k n 0 L 0 V A , µ 0 L 0 V A , and ρ 0 L 0 V A , respectively. The simulation starts with the application of a toroidally symmetric radial electric field (E inj ) across a gap between two electrodes.The parameters used in this simulation are λ = 4.95

Magnetohydrodynamic Simulation of Kink Instability and Plasma Flow during Sustainment of a Coaxial Gun Spheromak

“…Experiments must be compared to numerical simulations. We investigate the fundamental physics of the MHD relaxation process and current drive using three-dimensional (3D) nonlinear MHD simulations [8,9]. In this paper, we focus on the helical kink instability and subsequent plasma flow driven by magnetic reconnection during sustainment of a coaxial gun spheromak.…”