Fusion Science and Technology / Volume 58 / Number 1 / July 2010 / Pages 160-175
Chapter 4. MHD / Special Issue on Large Helical Device (LHD)
In 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.