AbstractIn the National Institute for Fusion Science, a large helical device (LHD) is under construction, with the objectives of understanding the physical phenomena and behaviors of a magnetically confined high temperature plasma, and of obtaining basic data which can be extrapolated to a reactor-relevant plasma in a helical system. Superconducting coils with pole number l = 2. pitch number m = 10 and major radius R = 3.9 m produce a steady state helical magnetic field for confinement, together with poloidal coils. The magnetic field strength on the axis will be 3.0 T in the phase I experiment and 4.0 T in the phase II experiment. The plasma major radius of the LHD is 3.75 m and the averaged plasma radius is 0.6 m. The plasma volume will be about 30 m3. A plasma will be produced and heated by electron cyclotron heating, and heated with neutral beam injection and ion cyclotron resonance frequency heating. Three-dimensional diagnostics should be provided for helical systems, because the elliptic plasma cross-section rotates poloidally along the magnetic axis, and has no axisymmetry in comparison with tokamaks. It is also planned to produce a steady state plasma in the LHD. Therefore, the plasma diagnostics for the LHD have various features that are different from those in conventional tokamaks. The potential distribution in the plasma is an important quantity to be measured in a hellical system, because it is related to the plasma confinement. Proper functioning of the diverter is vitally important in a long-pulse machine. Divertor diagnostics are being examined with special precautions. Among various diagnostic developments carried out so far, a multi-point Thomson scattering system with high time and space resolution, a multi-channel far-IR laser interferometer and heavy ion beam probing will be explained in detail, after presenting details of the design principle, general description and a future plan for plasma diagnostics in the LHD.
Advances in techniques for diagnosing fusion plasmas