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AbstractHeliotron reactors have several features suitable for a fusion power plant, such as no need for current drive, no plasma current disruptions, suitability for steady-state operation, and a wide space between helical coils useful for maintenance of in-vessel components. According to recent reactor studies based on the experimental results in the Large Helical Device (LHD), the plasma major radius of a heliotron reactor is set to 14 to 16 m in order to install shielding and breeding blankets with total thickness of 1 m. The central toroidal field for the self-ignition is 5 to 6 T under the assumption that the confinement enhancement factor is 1.2 to 1.4 with respect to the LHD. The stored magnetic energy is estimated to be 120 to 150 GJ. Both the major radius and the magnetic energy are three times larger than those of ITER. Its large helical windings, however, can be realized by steady extension from the ITER technology, because cable-in-conduit conductors similar to those for ITER toroidal field coils can be adopted. Improvement of plasma confinement is essential to reduce the number of magnet systems. A roadmap to a heliotron DEMO is discussed.

AbstractEnergetic ion-driven magnetohydrodynamic instabilities such as Alfvén eigenmodes (AEs), energetic particle modes (EPMs), and their impacts on energetic ion confinement are being studied on the Large Helical Device (LHD). The magnetic configuration of this device is three dimensional and has negative magnetic shear over a whole radial region in the low-beta regime. Two types of toroidicity-induced Alfvén eigenmodes (TAEs) are typically observed in LHD plasmas that are heated by tangential neutral beam injection: One is localized in the plasma core region near a central TAE gap and the other is a global TAE having a radially extended eigenfunction. Core-localized TAEs with even and odd radial mode parities are often observed. The global TAE is usually observed in medium- to high-beta plasmas where broad regions with low magnetic shear are present. Helicity-induced Alfvén eigenmodes (HAEs), which exist in gaps unique to three-dimensional plasmas that have both toroidal and poloidal mode couplings, were detected for the first time. Recently, reversed magnetic shear Alfvén eigenmodes (RSAEs) having characteristic frequency sweeping were discovered in reversed magnetic shear (RS) plasmas produced by intense counter-neutral beam current drive. In the RS plasma, the geodesic acoustic mode (GAM) excited by energetic ions, which is a global-type mode different from localized GAM excited by drift waves, was also detected for the first time in a helical plasma. Nonlinear couplings between RSAE and GAM modes and also between two TAEs were observed. Bursts of TAEs and EPMs often enhance radial transport and loss of energetic ions at low toroidal magnetic field (<0.75 T).

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.

AbstractEffects of the bumpy field component on the bootstrap current in the low-collisionality regime are discussed for L= 1 helical plasmas. It is usually considered that the bumpy field component or toroidal mirror ratio suppresses the bootstrap current in helical plasmas since it is a symmetry-breaking component. In the Heliotron J device, however, it has been observed that the direction of the bootstrap current can be changed according to the magnitude of the bumpy field component. In this study, we calculate the bootstrap current in the low-collisionality regime using a model magnetic field spectrum and discuss the condition under which the reversal of the bootstrap current direction occurs.