In three-dimensional configurations, the confinement region is surrounded by the stochastic magnetic field lines
related to magnetic islands or separatrix, leading to the fact that the plasma–vacuum boundary is not so definite
compared with tokamaks that the various modulations of the plasma–vacuum boundary will be induced around the
stochastic region by synergetic effects between a transport around the stochastic region and a large Shafranov shift of
the whole plasma, in especially high-β operations. To examine such modulation effects of the plasma boundary on
MHDinstabilities, high-β plasmas allowing a large Shafranov shift or a large Pfirsch–Schl¨uter current are considered
in the inward-shifted LHD configurations with the vacuum magnetic axis Rax of 3.6 m, for which previous theoretical
analyses based on fixedMHDequilibria indicate that pressure-driven modes are significantly more unstable compared
with experimental observations. The concept of the averaged flux surfaces allowing a movement of the equilibrium
plasma into the stochastic region is introduced, which induces a boundary modulation and, at the same time, reduces
the discrepancy on MHD equilibria between the experimentally obtained and theoretically considered. As a result,
it is shown that the boundary modulation, namely, the whole plasma outward-shift due to a large Pfirsch–Schl¨uter
current has significant stabilizing effects on ideal MHD instabilities, leading to partially resolving the discrepancy
on MHD stability between experimental results and theoretical analyses. |