22P131-P
Development of Fiber-Optic Diagnostic on Vacuum Vessel Current
of QUEST (2)QUESTにおける光ファイバーを用いた真空容器電流計測器の開発 (2)Fujii Genki1, Shunji TSUJI-IIO1, Hiroaki Tutui1, Osamu Mitarai2, Kazuo Nakamura3,
Makoto Hasegawa3, Hideki Zushi3 and Tsuyoshi Akiyama4
藤井 元紀1, 飯尾 俊二1, 筒井 広明1, 御手 洗修2, 中村 一男3,長谷川 真3, 図子 秀樹3, 秋山 毅志41) Tokyo Institute of Technology 2-12-1-N1-11, Ookayama, Meguro 152-8550, Japan
2) Tokai University, Kumamoto 862-865,Japn
3) Kyushu University, Kasuga, Fukuoka 816-8580, Japan4) National Institute for Fusion Science, Toki, Gifu 509-5292, Japan
1) 東工大, 2) 東海大, 3) 九大, 4) 核融合研
In spherical tokamak QUEST at Research Institute for Applied Mechanics, Kyushu University, thevacuum-vessel current makes the error magnetic field. It influences the start-up of the current of plasma.
We are planning the current measuring instrument using Faraday rotation in the optical fiber. We adopted
the dual photo elastic modulator polarimetry. The measurement accuracy is required to be not less than
three-digits under ambient temperature variation. By bench testing, we evaluated the accuracy of theoptical system and the temperature dependence of optical fiber sensors. The temperature dependence of theoptical fiber and the measurement accuracy were confirmed to be within tolerance.

1. Introduction
This study is intended to measure thevacuum-vessel currents of spherical tokamak at
Kyushu University, QUEST [1]. Error-magnetic
fields generated by vacuum-vessel currents obstruct
the ramp-up of the plasma current in QUEST since
there is no toroidal gap to avoid to toroidal currentson the vessel parallel to the plasma current.
Measurements of the total current flowing
through the vacuum vessel are required for theconfirmation of the accuracy of the calculated eddy
current with evaluation models. The QUEST isnormally operated with bake-out temperature of thevaccum vessel at about 100°C. Requirements forthis measurement are long-term stability and highsensitivity ao that we adopt polarimetry to avoid
integration drift in the case of commomly used
Rogowski coil. We measure the Faraday rotation
angle along an optical fiber in this study. And, weevaluate the electric current from the faraday
rotation angle. Optical fiber current sensor hasadvantages, which are wide dynamic range, fastresponse, high sensitivity, strong inductive noise,compact, lightweight, low-loss optical transmission
and electrical insulation. The most important issue
at present is the temperature dependence of the fibersensor. This paper provides an overview of thedevice configuration, and the required accuracy,
about the results of bench testing.

2. Fiber Sensor Instrumentation Overview

Fig. 1. Illustration of the measurement setup on QUEST

We will wind a poloidal loop of an opticalfiber around the vacuum vessel as shown in Fig. 1and measure the toroidal current in the same way as
an optical current transformer. The loop must go
through the torus center since there is no room
between the center stack (CS) coil and the vacuum
vessel so that the vacuum-vessel current must be
evaluated under significant background signals up
to 1.73 MA‧turns by the CS–coil currents. The
Ampere-turns of the CS-coil current will beseparately measured with a coil of optical fiber
wound at the coil feeder and it will be numericallysubtracted from the total Ampere-turns measured
with the loop. In order to measure the vacuum
current in accuracy of less than 1 kA, the requiredmeasurement accuracy is not less than three digits.
We measured the temperature distribution along theplanned route of the loop with thermocouples andfound that the integrity of the optical fiber ispreserved since the highest temperature does notexceed 50°C.
3. Optical Test Bench
The Verdet constant of a flint glass fiber isabout six times that of a silica glass fiber and thephoto elastic constant of the former is much smaller
than that of the latter, which enable us to wind theoptical fiver with smaller bending radii. We tested asingle-mode flint glass fiber for 1550 nm with a SLD(super-luminescent diode) of wavelength 1545 nm aslight source. We adopted the dual photo elasticmodulator (HINS Instruments, Inc., PEM-90 I/FS50and II/FS42) polarimetry. The modulator axes of thetwo modulators are at 45 degrees with the polarizer
passing axis at 22.5 degrees with each modulator. The
second harmonic amplitudes of the photodiode output
are measured with two lock-in amplifiers.

dependence of inversely proportional to the square of
the wavelength is taken into account [2,3].

fig.3. Polarization angle dependence ofthe measurement errors.

fig.4. The Verdet constant evaluated as a functionof the room temperature.
fig.2. Optical setup of bench testing

3. Results of bench tests
An example result of bench testing of current
measurements of a solenoid is shown in Fig. 3. The
standard deviation of the measured polarization angle
was confirmed to be well below a required accuracyof the Faraday rotation angle measurement of 0.37
degrees.
The temperature dependence of the Verdet
constant of the flint glass fiber, however, is slightly
stronger than that of silica fibers. Figure 4 shows apreliminary evaluation result of the Verdet constant byvarying the room temperature. The weak dependence
may become a problem to assure the applicability ofAmpere’s theorem to measure the current. The
averaged value agrees with the catalog value when the4. Summary
Standard deviation of measured polarization
angle is within target resolution (  0.37deg.) .
Uniform temperature along the optical fiber isrequired not to be affected by the weak temperature
 constant of the fiber.
dependence of the Verdet
References[1] Kazuaki HANDA, et al.: PFR. Volume 5, S1007(2010)[2] Jianring QIU, et al.: J. Ceram. Soc. Jpn. 106(1998)
228-230.
[3] Paul A .William, et al.: Appl. Opt. 30(1991)