Injection and Confinement of a Laser Pulse in an Optical Cavity for Multi-Pass Thomson Scattering Diagnostics in the TST-2 Spherical Tokamak Device

Hiro TOGASHI; Akira EJIRI; Makoto HASEGAWA; Junichi HIRATSUKA; Yoshihiko NAGASHIMA; Keishun NAKAMURA; Kazumichi NARIHARA; Yuichi TAKASE; Hiroshi TOJO; Naoto TSUJII; Ichihiro YAMADA; Takashi YAMAGUCHI; the TST-2 Team

doi:10.1585/pfr.9.1202005

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Injection and Confinement of a Laser Pulse in an Optical Cavity for Multi-Pass Thomson Scattering Diagnostics in the TST-2 Spherical Tokamak Device

Hiro TOGASHI, Akira EJIRI, Makoto HASEGAWA, Junichi HIRATSUKA, Yoshihiko NAGASHIMA, Keishun NAKAMURA, Kazumichi NARIHARA, Yuichi TAKASE, Hiroshi TOJO, Naoto TSUJII, Ichihiro YAMADA, Takashi YAMAGUCHI, the TST-2 Team

Author information

Hiro TOGASHI
The University of Tokyo
Akira EJIRI
The University of Tokyo
Makoto HASEGAWA
Kyushu University
Junichi HIRATSUKA
The University of Tokyo
Yoshihiko NAGASHIMA
Kyushu University
Keishun NAKAMURA
The University of Tokyo
Kazumichi NARIHARA
National Institute for Fusion Science
Yuichi TAKASE
The University of Tokyo
Hiroshi TOJO
Japan Atomic Energy Agency
Naoto TSUJII
The University of Tokyo
Ichihiro YAMADA
National Institute for Fusion Science
Takashi YAMAGUCHI
The University of Tokyo
the TST-2 Team
The University of Tokyo

Keywords: Thomson scattering, multi-pass, TST-2, diagnostics, spherical tokamak device

JOURNAL FREE ACCESS

2014 Volume 9 Pages 1202005

DOI https://doi.org/10.1585/pfr.9.1202005

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Abstract

A multi-pass Thomson scattering (TS) system based on confining laser pulses in an optical cavity was constructed for measuring very low-density plasma in the TST-2 spherical tokamak device. This paper describes the setup of the optical system, injection of the laser pulse into the cavity, and properties of the confined laser pulse. A combination of Pockels cell plus polarizer, which serves as an optical shutter, allows us to inject and then confine intense laser pulses in the cavity. A photodiode signal monitoring the very weak light leaking from the cavity mirrors demonstrated that the laser pulse makes many round trips, with a round-trip efficiency of approximately 0.73. The effective number of round trips (i.e., the signal enhancement factor) is approximately 3.7. For an injection efficiency of approximately 0.69, a cavity-confined laser pulse, applied to Thomson scattering, will yield a scattered signal that is five times larger than that from a single-pass laser pulse.

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