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Improvement of Aperture Configuration to Reduce the Stray Light for Thomson Scattering Measurement Using a Peripheral Beam Profile Monitor

Yuya KAWAMATA, Akira EJIRI, Kyohei MATSUZAKI, Yuichi TAKASE, Naoto TSUJII, Takumi ONCHI¹⁾ and Yoshihiko NAGASHIMA¹⁾

The University of Tokyo, Kashiwa 277-8561, Japan

1)

Kyushu University, Kasuga 816-8580, Japan

(Received 16 January 2019 / Accepted 11 March 2019 / Published 11 April 2019)

Abstract

The stray light is a major problem in Thomson scattering (TS) measurements. The main cause of stray light is unnecessary divergence of the incident laser beam and an aperture is a standard component to reduce it. In order to improve the aperture configuration (including size, number and position in the laser injection tube), a peripheral beam profile monitor, consisting of a screen with a through hole for the laser beam and a CMOS camera, was developed. Instead of the actual laser injection tube a mock-up tube was used to measure the peripheral beam profiles under various aperture configurations. The configuration with four 15mm diameter apertures was chosen and installed on the TS system for the TST-2 spherical tokamak. The stray light was reduced to about 4% compared to the smaller diameter injection tube with no apertures. As a result, it became possible to make TS measurements in the electron density range above 1.0 × 10¹⁷ m⁻³.

Copyright (c) 2019 The Japan Society of Plasma Science and Nuclear Fusion Research

Keywords

Thomson scattering, stray light, diagnostic, TST-2, spherical tokamak

DOI: 10.1585/pfr.14.1402072

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