K. Ichiguchi

Large Helical Device (LHD) program

The largest superconducting fusion machine, Large Helical Device (LHD), is now under construction in Japan and will begin operation in 1997. Design and construction of related R&D programs are now being carried out. The major radius of this machine is 3.9 m and the magnetic field on the plasma center is 3 T. The NbTi superconducting conductors are used in both helical coils and poloidal coils to produce this field. This will be upgraded in the second phase a using superfluid coil cooling technique. A negative ion source is being successfully developed for the NBI heating of LHD. This paper describes the present status and progress in its experimental planning and theoretical analysis on LHD, and the design and construction of LHD torus, heating, and diagnostics equipments.

MHD characteristics in the high beta regime of the Large Helical Device

Note: Proc. 18th IAEA Fusion Energy Conference, Sorrento, Italy, 4-10 October 2000, IAEA-CN-77 (EXP3/12), p. 157 (2000) Reference CRPP-CONF-2000-073 Record created on 2008-05-13, modified on 2017-05-12

Multi-scale MHD analysis incorporating pressure transport equation for beta-increasing LHD plasma

A multi-scale MHD numerical scheme is developed for analysis of nonlinear evolution of a beta-increasing plasma. The scheme is based on iterative calculations of nonlinear dynamics based on the reduced MHD (RMHD) equations and three-dimensional static equilibrium. The equation for average pressure in the RMHD equations plays the role of a transport equation that involves a heat source term and background pressure diffusion terms. The heat source term is controlled so that the beta value should be increased at a constant rate. The scheme is applied to a Large Helical Device (LHD) plasma up to average beta of 1.05%, which is unstable against linear ideal interchange modes while beta values much higher than the stability limit are obtained in the experiments. The result with the multi-scale scheme indicates that many local flat regions are generated in the background pressure profile in the nonlinear evolution of the interchange modes. This structure of the pressure profile suppresses disruptive phenomena because it reduces the driving force of the modes at higher beta value. Such self-organization in the pressure profile is considered to be the stabilizing mechanism in the plasma.

External kink modes in a Large Helical Device (LHD) equilibrium with self-consistent bootstrap current

Magnetohydrodynamic (MHD) stability studies of low-mode-number free-boundary kink modes in a finite-β Large Helical Device (LHD) [A. Ilyoshi, et al. Fusion Technology 17, 169 (1990)] equilibrium series with self-consistent bootstrap current find some serious free-boundary modes. They indicate that care must be taken in the design of high-β experiments. Since the LHD configuration is flexible, with the possibility of introducing or modifying dipole and quadrapole fields, unbalancing the currents in the helical coils, operating with a high-temperature divertor, and changing the collisionality regime by working with different values of temperature and density, paths to high-β operation should exist. Comparison of the experimental results with these theoretical predictions for the studied equilibrium sequence will provide understanding of the MHD stability properties of LHD.

Internal disruptions in Heliotron E

In Heliotron E [K. Uo, Nucl. Fusion 25, 1243 (1985)] shifted-in vacuum magnetic field configuration, the q profile varies from just above 2 at the magnetic axis to 0.4 at the plasma edge. For low-β plasmas, resistive interchange modes are the dominant low-n instabilities at the plasma core. They saturate at low fluctuation levels. Above a threshold value, the ideal m/n=2/1 modes become unstable. They can be resonant or nonresonant modes depending on the value of q(0). In either case, their nonlinear evolution leads to a sudden crash of the pressure within the r/a=0.3 radius (sawtooth oscillation). When beta is increased further, the q=2 surface moves out of the plasma, and the ideal m/n=2/1 modes are effectively stabilized when q(0)<1.85. As a result, the sawtooth oscillations are suppressed.

Three-dimensional MHD analysis of heliotron plasma with RMP

The interaction between pressure driven modes and magnetic islands generated by a resonant magnetic perturbation (RMP) in the large helical device (LHD) is numerically analyzed. In this analysis, three-dimensional treatment is essential in the equilibrium and dynamics calculations, because the equilibrium pressure profile is deformed by the RMP. The deformation changes the linear mode structure from the interchange type to the ballooning-like type that is localized around the X-point of the island in the equilibrium magnetic field including the RMP. This mode causes a pressure collapse in the nonlinear evolution, which spreads from the X-point to the core. Therefore, the spatial phase of the collapse is fixed to the island geometry. The fixed phase agrees with the LHD experimental results with a natural error field.

Interaction between static magnetic islands and interchange modes in a straight heliotron plasma with high resistivity

Fundamental mechanism of the nonlinear interaction between static magnetic islands generated by an external field and a resistive interchange mode is investigated in a straight heliotron plasma with high resistivity by using a numerical method based on the reduced magnetohydrodynamics equations. The behavior of the magnetic islands is examined at the steady state after the nonlinear saturation of the interchange mode. The width and the phase of the magnetic islands are changed by the mode evolution. These changes are almost determined by the linear combination of the two perturbed poloidal magnetic fluxes, the flux imposed externally and the flux attributed to the interchange mode, in spite of the fact that the changes result from the nonlinear process. It is also obtained that the amount of the local change of the pressure at the resonant surface in the saturation state depends on the phase of the static magnetic islands.

Collisionality dependence of Mercier stability in LHD equilibria with bootstrap currents

The Mercier stability of the plasmas carrying bootstrap currents with different plasma collisionality is studied in the Large Helical Device (LHD). In the LHD configuration, the direction of the bootstrap current depends on the collisionality of the plasma through the change in the sign of the geometrical factor. When the beta value is raised by increasing the density of the plasma with a fixed low temperature, the plasma becomes more collisional and the collisionality approaches the plateau regime. In this case, the bootstrap current can flow in the direction so as to decrease the rotational transform. Then, the large Shafranov shift enhances the magnetic well and the magnetic shear, and therefore, the Mercier stability is improved. On the other hand, when the beta value is raised by increasing the temperature of the plasma with a fixed low density, the plasma collisionality becomes reduced to enter the collisionality regime. In this case, the bootstrap current flows so that the rotational transform should be increased, which is unfavourable for the Mercier stability.

Numerical magnetohydrodynamic analysis of Large Helical Device plasmas with magnetic axis swing

A partial collapse observed in magnetic axis swing experiments in the Large Helical Device (LHD) is analyzed with a nonlinear magnetohydrodynamic (MHD) simulation. Real time control of the background field in the operation is incorporated by means of a multi-scale numerical scheme in the simulation. The simulation result indicates that the changing background field accelerates the growth of an infernal-like mode and causes the partial collapse.

Multi-scale approach to the solution of nonlinear MHD evolution of heliotron plasma

A multi-scale nonlinear magnetohydrodynamic (MHD) evolution scheme is developed for the numerical analysis of a heliotron plasma as beta increases. In this scheme, the fast time scale dynamics is given by the nonlinear MHD code based on the reduced MHD equations and the slow dynamics is carried out with three-dimensional static equilibrium code. The time evolution is calculated iteratively with a linear interpolation technique for the equilibrium quantities. This scheme is applied to the analysis of the Large Helical Device plasma. Self-organization of the pressure profile induced by the interchange mode is obtained.