Takashi Kanki

The internal magnetic field structures and current density profiles in the Helicity Injected Spherical Torus plasma driven by coaxial helicity injection

In the Helicity Injected Spherical Torus device [Nagata et al., Proceedings of the 17th International Atomic Energy Agency Fusion Energy Conference, Yokohama, 1998 (International Atomic Energy Agency, Vienna, 1998) CD-ROM, EXP4/10], internal magnetic field and current density structures of spherical torus (ST) plasmas sustained by coaxial helicity injection (CHI) have been revealed via intensive internal magnetic measurements. The internal magnetic configuration of the ST plasma formed by CHI is in good agreement with the results of numerical equilibrium fitting calculations. The generation of closed poloidal flux of ST has been verified by varying the external toroidal field strength in the same device. Interestingly, the transition of the current profile from hollow to peaked has been observed during the sustainment phase, which could be explained by inductive effects of currents on open field lines winding the central conductor.

Partially relaxed magnetohydrodynamic equilibria with bias-flux leakage obtained in a helicity-driven spheromak

In the flux amplification compact torus experiment, the bias flux penetrates the wall of the flux conserver because it has been switched on a suitably long time prior to the production of a seed spheromak. The magnetohydrodynamic equilibrium configurations with a non-constant λ(≡µ 0 j · B / | B | 2 ) profile of a helicity-driven spheromak incorporating the situation of the bias-flux leakage out of the flux conserver are numerically determined by using the novel combination of the finite difference and the boundary element method. Here µ 0 , j and B are the permeability of vacuum, the current density and the magnetic field, respectively. The results of computations show that the effects of the bias-flux leakage cause a slight rise of the entire safety factor q in the case of I C / I P =0.4 ( I C and I P denote the total current of coil and plasma, respectively) because of the decrease in poloidal field over all space. Then, they also give rise to a 22.0% decrease in the magnetic flux inside the separatrix.

Magnetohydrodynamic simulation of dynamical behavior of a field-reversed configuration during magnetic mirror reflection

A two-dimensional magnetohydrodynamic (MHD) simulation of the reflection dynamics of a field-reversed configuration (FRC) plasma is performed by numerically modeling a confinement region of the FRC Injection Experiment [H. Himura et al., Phys. Plasmas. 2, 191 (1995)] machine. The FRC plasma is reflected by a downstream magnetic mirror field at the end of the confinement region without severe destruction of the closed magnetic flux surfaces even when injected at supersonic velocity into the magnetic mirror region, showing the robustness of the FRC against external perturbations. By examining the details of FRC motion, it is also predicted for any translation velocities that the FRC might eventually settle down in the confinement region and approach a MHD equilibrium condition. Interestingly, it is observed that the formation of a discontinuous wave front is caused by a shock when the FRC at supersonic velocity is reflected by the magnetic mirror.

Computation of two-fluid flowing equilibrium of helicity-injected spherical torus plasma

A two-fluid flowing equilibrium of a helicity-injected spherical torus (HI-ST) plasma in the more realistic region, including a flux conserver and a coaxial helicity source, is numerically determined by using the combination of the finite difference and the boundary element methods. It is found from the numerical results that magnetic configurations change from the high-q HI-ST (safety factor q>1) with paramagnetic toroidal field and low-beta (volume average beta value, langbetarangap2%) through the helicity-injected spheromak and reversed-field pinch to the ultralow-q HI-ST (0<q<1) with diamagnetic toroidal field and high-beta(<beta>ap18%) as the external toroidal field at the inner edge regions decreases and reverses the sign. The effects of the ion flow on the MHD equilibrium configurations are discussed

Numerical studies of reflection process on a field-reversed configuration plasma

A two-dimensional magnetohydrodynamic (MHD) simulation of a reflection on a field-reversed configuration (FRC) plasma is performed in the parameter range of the FRC injection experiment (FIX). The full set of MHD equations are solved on a rezoned Lagrangian mesh which employs an adaptive algorithm to concentrate the grid in regions of sharp plasma pressure gradients. It is shown from the simulation results that the FRC plasma is reflected by downstream magnetic mirror field at the end of the confinement region in the FIX machine without destroying the closed magnetic flux surfaces. The effects of this field on FRC plasma are discussed.

Two-fluid Flowing Equilibria of Helicity-driven Spherical Torus Plasmas

Two-fluid flowing equilibria of a helicity-driven spherical torus (HD-ST) are numerically investigated by using the combination of the finite difference and the boundary element methods. It is shown from the numerical results that electron fluids near the central conductor are tied to an external toroidal field and ion fluids are not. The magnetic configurations change from the high-q HD-ST (q>1) with paramagnetic toroidal field and low-β (volume average β value, ≈2 %) through the helicity-driven spheromak and reversed-field pinch to the ultra low-q HD-ST (0 ≈18 %) as the vacuum toroidal field at the inner edge regions decreases and reverses sign. The effects of the toroidal ion flow on the magnetic configurations are discussed.

MHD Simulation of Reflection Dynamics of Field‐Reversed Configuration Plasma

A two‐dimensional magnetohydrodynamic simulation of a reflection dynamics on a field‐reversed configuration (FRC) plasma is carried out by numerically modeling the confinement region of the FRC Injection Experiment (FIX) machine. It is shown from the simulation results that the FRC plasma is reflected by the downstream magnetic mirror field without severe destruction of the closed magnetic flux surfaces even when injected with supersonic velocity into the magnetic mirror region, showing the robustness of the FRC against external perturbations. The effects of the magnetic mirror field and the translation velocity on the FRC plasma are discussed.