Improvement of light collection subsystems in the KSTAR Thomson scattering diagnostic system

A Thomson scattering diagnostic system comprises four subsystems: a laser, a light collection system, a spectroscope, and a digitizing system. The Korea Superconducting Tokamak Advanced Research (KSTAR) Thomson scattering system has all of these, but because of the superconducting tokamak, a cassette system (called a port plug in ITER) is required to diagnose the plasma. This cassette system is of length 1.8 m and is mounted on the cryostat. For this reason, when plasma disruption occurs, vibration impact is transferred to the collection lens in the cassette as presently designed, causing the problem of Thomson scattering signal measurement. To solve this problem, an anti-vibration lens support system was installed, and the collection lenses (core and edge lenses) were redesigned to upgrade their performance while also reducing their weight. Simulation of this new lens design has confirmed that it reduces vignetting and improves the modulation transfer function compared with previous lenses [1]. In the new core lens design, the f-number more than doubled from 2.26 (old) to 5.9 (new) and the mass was reduced from 26 to 18 kg. In the new edge lens design, the f-number increased to 2.27 times that of the old lens, and the mass was reduced from 23 to 12 kg. The improved light collection optic systems were used in the KSTAR campaign in 2018 and were able to measure Thomson signals without any vibration effect. Herein we explain the newly designed lenses and briefly describe the collection lens support system.