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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 HASEGAWA¹⁾, Junichi HIRATSUKA, Yoshihiko NAGASHIMA¹⁾, Keishun NAKAMURA, Kazumichi NARIHARA²⁾, Yuichi TAKASE, Hiroshi TOJO³⁾, Naoto TSUJII, Ichihiro YAMADA²⁾, Takashi YAMAGUCHI and the TST-2 Team

The University of Tokyo, Kashiwa 277-8561, Japan

1)

Kyushu University, Kasuga 816-8580, Japan

2)

National Institute for Fusion Science, Toki 509-5292, Japan

3)

Japan Atomic Energy Agency, Naka 311-0193, Japan

(Received 8 December 2013 / Accepted 13 December 2013 / Published 31 January 2014)

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.

Copyright (c) 2014 The Japan Society of Plasma Science and Nuclear Fusion Research

Keywords

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

DOI: 10.1585/pfr.9.1202005

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References

• [1] M. Yu. Kantor et al., Plasma Phys. Control. Fusion 51, 055002 (2009).
• [2] R. Yasuhara et al., Rev. Sci. Instrum. 83, 10E326 (2012).

This paper may be cited as follows:

Hiro TOGASHI, Akira EJIRI, Makoto HASEGAWA, Junichi HIRATSUKA, Yoshihiko NAGASHIMA, Keishun NAKAMURA, Kazumichi NARIHARA, Yuichi TAKASE, Hiroshi TOJO, Naoto TSUJII, Ichihiro YAMADA, Takashi YAMAGUCHI and the TST-2 Team, Plasma Fusion Res. 9, 1202005 (2014).