Overview of LHD Plasma Diagnostics

The Large Helical Device (LHD) is the world's largest heliotron-type device with l = 2, m = 10 continuous superconducting helical coils and three pairs of superconducting poloidal coils. The major and minor radii of the plasma are 3.6 to 3.9 and 0.6 to 0.65 m, respectively. A plasma with an elliptic cross section confined in the helical magnetic field rotates poloidally along the magnetic axis and has no axial symmetry. For the installation of various kinds of diagnostic instruments, large-sized ports are equipped. The diameter of the largest horizontal ports is 2410 mm, which enables us to easily access the full plasma cross section with multichannel viewing chords aligned parallel to one another. For the precise measurement of plasma quantities in a three-dimensional helical plasma, an extensive set of diagnostics has been developed with national and international collaborators and is routinely operated in LHD. The diagnostic system now consists of [approximately]50 measuring instruments and includes many challenging diagnostics that have been developed and operated for the study of LHD plasma confinement. These are classified as profile diagnostics, fluctuation diagnostics, and advanced diagnostics, some of which are selected for introduction in this article. In addition, diagnostics for the divertor and for energetic particles are discussed, along with topics that are somewhat unique to helical devices such as diagnosing three-dimensional phenomena and flux surface mapping. This large number of diagnostics in LHD rely on a data acquisition system that has broken world records for the amount of information accumulated in one shot. Finally, looking to the near future, countermeasures have been taken to protect diagnostics from the neutrons and gamma fluxes anticipated during deuterium-deuterium experiments, such as placing much of the diagnostic instrumentation behind a 2-m-thick concrete biological shield encompassing the LHD test cell.