Design and performance of the Thomson scattering diagnostic on LHD

This article describes the design and performance of a multi-point (200) high repetition rate (4×50 Hz) Thomson scattering diagnostic installed on the Large Helical Device. A unique feature of this system is its oblique back scattering configuration, which enables us to observe the entire plasma region along a major radius on the midplane under a severely restricted port constraint. High throughput collection optics using a mosaic mirror of 1.5 m×1.8 m area yield high quality data even with 0.5 J pulse energy delivered from 50 Hz repetition rate Nd: yttrium–aluminum–garnet lasers. High repetition and high spatial resolution (2–4 cm) of the system enable us to study island evolution in the plasma.