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AbstractSpontaneous dynamics of magnetic islands depending on various plasma parameters in the Large Helical Device (LHD) are described. The structure of the magnetic island undergoes deformation during a discharge. There are two states of magnetic island; they are the growth and self-healing of the magnetic island. The self-healing occurs in the higher-beta and lower-collisionality plasmas. The magnetic island, on the other hand, grows in the lower-beta and higher-collisionality region. The self-healing phenomenon is realized by the disappearance of the local flattening of the Teprofile and the perturbed magnetic field structure compensating the externally imposed perturbation field to produce a seed magnetic island.

AbstractThis paper reviews progress in the study of pressure-driven interchange stability in the Large Helical Device (LHD) for 10 years. When the plasma approaches the boundary of ideal interchange mode, a strong magnetohydrodynamic (MHD) mode appears, leading to a distortion of pressure profile, although no major disruption is caused. The experiments for investigating magnetic shear effects in the magnetic hill configuration indicate that the reduction of magnetic shear leads to a minor collapse due to an excitation of low-order MHD mode. In the high-beta regime of more than 4%, MHD modes excited in the periphery with magnetic hill are observed to dominate, and it was found that the amplitude depends on the magnetic Reynolds number as well as the pressure gradient, which is qualitatively consistent with the prediction of resistive interchange mode. Also, experiments and theory for finding parameter dependence of the onset of the mode indicate that the onset is related to both the magnetic Reynolds number and the stability index of resistive interchange mode.

AbstractIn the vacuum of the Large Helical Device (LHD), we can change the plasma volume, the aspect ratio, the ellipticity, the rotational transform, and the height of the magnetic hill through the control of the vertical and the qudrupole components of the magnetic field and the helical coil pitch parameter. The two effects of the finite beta on the magnetohydrodynamic (MHD) configuration, the magnetic surface torus outward shift and the invasion of the stochastic region into the plasma core, are discussed. The former is qualitatively the same as that by the external vertical field control. According to the comparison between a theoretical prediction in the finite beta and the vacuum field calculation in the vertical field control, the latter effect is strongly affected by the nonaxisymmetric component of the equilibrium current. A theoretical prediction suggests that an MHD equilibrium beta limit different from the conventional one exists due to the lack of the equilibrium force balance in the stochastic region. The key parameters to improve the accuracy of the identification of the MHD equilibrium configuration are shown to be the identification of the toroidal current profile, the anisotropic pressure effect, and the identification of the plasma boundary shape.

AbstractThe magnetic configuration of the Large Helical Device (LHD) changes with the increment in beta. To distinguish between the beta effect and the configuration effect on the gradual degradation of the global confinement property in the high-beta LHD plasmas, the local transport characteristics are studied by considering the change in the major radius of the magnetic flux surface with the beta value. A model transport coefficient that has the same nondimensional parameter dependence as the international stellarator scaling 2004 (ISS04) is introduced and used as the reference. The dependence of the local transport characteristics in high-beta plasmas on the major radial position of a geometric center of the magnetic flux surface is compared with that in low-beta plasmas. The dependence of the local transport in the peripheral region is correlated more with beta itself than the magnetic configuration effect, whereas the core transport appears to be correlated more with the configuration effect. The comparison of the experimental transport coefficients and the calculation results shows that the resistive pressure gradient-driven turbulence can be considered as one of the causes of this degradation.

AbstractProgress in the integrated development of the helical system in the Large Helical Device (LHD) is described in this paper. Understanding of net current–free plasmas has been deepened in the extended operational regime. Geometrical optimization based on neoclassical theory has revealed that good confinement, equivalent to the tokamak H-mode, can be obtained in the collisionless regime. This approach has also demonstrated that anomalous transport is reduced simultaneously, which poses a working hypothesis that optimization of neoclassical transport suppresses turbulent anomalous transport as well. With regard to the magnetohydrodynamic instability, LHD has discovered that interchange instability is benign in the magnetic hill. These two findings have produced a synergistic effect on the enhancement of confinement and plasma β. Remarkable proof of the advantage of helical systems can be seen in very high density operation, which is not accessible in tokamaks. Abundant integrated knowledge about three-dimensional physics has been extracted from these achievements. This progress is important in the assessment of the potential of a helical fusion reactor and makes a significant complementary contribution to tokamaks as well.

AbstractThis paper describes recent progress in magnetohydrodynamics (MHD) study of high-beta plasmas of the Large Helical Device. Control of the plasma aspect ratio (Ap) in the range of 6.3 to 8.3 was done in order to optimize the configuration for high-beta plasma production and to investigate the MHD characteristics. The experiments brought a maximum average beta of 4.3% at the Ap= 6.6 configuration. MHD activities in the periphery are dominantly observed in such a high-beta region, and their amplitudes increase with decreasing magnetic Reynolds number (S) and have clear dependence on the Sparameter. When the plasma aspect ratio is increased, minor collapse due to the m/n= 1/1 mode without rotation occurs. It is enhanced further by the plasma current reducing magnetic shear and degrades the beta value by >50%. The results are expected to give important information on the operation regime and the future design of the helical fusion reactor and to contribute to experimental knowledge of ideal and resistive instability.

AbstractRemarkable progress to access the reactor-relevant regime has been made in a recent experiment in the Large Helical Device. Optimizing the rotational transform, the average beta value of 4.3%, which is the highest record among helical devices, was achieved. The high-performance plasma with a fusion triple product up to ~2.2 × 1019m−3·keV·s was sustained for >7 s by repetitive hydrogen pellet injection. With regard to steady-state operation, which is one of the key issues to realize a fusion reactor, discharges for >30 min were successfully sustained by ion cyclotron range of frequency heating with the aid of the magnetic axis swing technique to reduce the heat load to the plasma-facing component. In the discharge, the total input energy to the plasma reached 1.3 GJ, which also established a new record.