25C08
Advanced Thomson Scattering Diagnostics in a Burning Plasma
燃焼プラズマにおける先進トムソン散乱計測
Eiichi Ystsuka, Takaki Hatae and Yoshinori Kusama
谷塚英一, 波多江仰紀，草間義紀
Japan Atomic Energy Agency
801-1, Mukoyama, Naka 311-0193, Japan
日本原子力研究開発機構 〒311-0193 那珂市向山801-1
Thermal and nuclear loads from a burning plasma cause many novel issues even in a sufficiently developed
diagnostic system such as Thomson scattering system. The degrading of spectral transmissivity due to theneutron and gamma-ray irradiation is one of the most severe problems in obtaining the Thomson scatteringspectra. In this paper, two types of in-situ calibration method for the relative spectral transmissivity andabsolute transmissivity at a wavelength, respectively, of the optical system are discussed. The method tomeasure anisotropic electron temperature by using Thomson scattering system is also discussed.

1. Introduction
Calibration of spectral transmissivity is one of
the most urgent and crucial issues for obtaining aThomson scattering (TS) spectrum resulting fromburning plasmas. An in-situ calibration method ofelectron temperature and relative spectral
transmissivity by injecting an additional laser
whose wavelength is different from that of the main
diagnostic laser is proposed [1].
Two results of electron temperature (Te)measurements obtained by TS and electron
cyclotron emission (ECE) tend to be different
under strong NBI [2]. In such a plasma, detecting
non-Maxwellian electron velocity distributionfunction (EVDF) was tried [3]; however, noevidence of the distortion of EVDF fromMaxwellian for the specific direction wasdetected [3]. While, it was suggested anisotropy
was important in an equilibrium of plasma whenauxiliary heating was high [4].
2. In-situ Calibration of Spectral Transmissivity
We developed a simulation method of theavailability of the calibration method proposed inRef. [1]. It is crucial to sufficiently overlap thespectra of TS resulting from the injection of themain and the additional lasers. For example, as theadditional laser for the edge TS system in ITER, aruby laser is one of the promising lasers from thepoint views of the wavelength and the pulse energy[5]. Figure 1 shows an example of the accuracy incalibrating the relative spectral transmissivity. Fromthe point view of overlapping the two spectra, thewavelength of an additional laser (C) is required to
be located near the main diagnostic laser (0 =1064
nm is assumed in Fig. 1). However, when C is too
close to 0, short wavelength region cannot becalibrated accurately due to small amount ofdetected photons in the case for relatively low Temeasurement.

Fig.1. Comparison of the accuracy in the calibration ofrelative spectral transmissivity as a function of
scattered wavelength and wavelength of theadditional laser (C). Te is assumed to be 2 keV.

Since the intensity of the detected signal in TS is
proportional to both ne and the spectral
transmissivity, a signal except for spectra of TS isnecessary for independent calibration of ne and thespectral transmissivity. We proposed a novel
calibration method of ne by utilizing a backgroundlight at experiments [6]. If we choose anappropriate wavelength band for the backgroundlight measurement, the bremsstrahlung is thedominant component of the background light. Notethat emission due to the transition of electron in
atom and molecule should be avoided. In addition,
when the temperature of ion having ionic charge Z
is comparable or less than Z2Ry, where Ry denotes
the Rydberg energy, the free-bound emission due tothe recombination is significant. When we measure
wavelength of more than 834 nm (1.51 eV=13.6
eV/32), the spectra due to the recombination to 4thor outer shells only (in the case for recombinationof hydrogen atom) are detected, and bremsstrahlungis sufficiently stronger than free-bound emission forsuch wavelength [7]. On the other hand, thermalradiation from the first wall is not negligible for thenear-infrared region in a burning plasma experiment.
In order to avoid the thermal radiation from the firstwall, measured wavelength should sufficiently beshort. As the result, approximately 850-900 nm ofwavelength is appropriate for the application of
in-situ ne calibration by utilizing bremsstrahlung.
Since this calibration method utilize bremsstrahlung, effective ionic charge Zeff can be obtained
simultaneously. Moreover, since bremsstrahlung isradiated from whole plasma, data of backgroundlight in all spatial channels should be analyzed
simultaneously whereas those of TS in each spatialchannel are analyzed independently. Figure 2 shows
an example of simulated accuracy in this calibration
method for an assumed geometry. Both profiles ofne and Zeff can be re-constructed simultaneously
from data of background light.

Fig.2. Re-constructed profile of (a) ne and (b) Zeff. Thick(thin) lines with large (small) closed circles denoteassumed (re-constructed) profiles. 10 types of simulated perturbations due to the shot-noise of detector in
the intensity of detected photon are shown.

3. Local Measurement of Non-Maxwellian
Velocity Distribution Function
Since a TS spectrum reflects an EVDF for thespecific pitch angle of the measurement configuration, we can obtain an EVDF for the specific
two pitch angles by using two lines of sight. If wehave collinear incident beams which are oppositely
propagate each other, we can effectively obtain twolines of sight of one collection optics. For applying
the anisotropic Te measurement, the configurationof optical components should have sufficienttolerance about the directions of incident laser
propagation, line of sight and the magnetic field.
We investigated the tolerance in the configurationof the measurement. Figure 3 shows that a deviationin the direction of the magnetic field rotating
parallel to the scattering plane of approximately a
few tens degrees from the ideal direction isapplicable [8]. Similarly, it is also shown that adeviation in the direction of the magnetic fieldtilting from the scattering plane of approximately a
few tens degrees from the ideal direction isapplicable.

Fig.3. Expected accuracy in (a) T// and (b) T ⊥ as afunction of T⊥ and , where  denotes the angle
between the laser beam and magnetic fieldprojected on the scattering plane. T// and thescattering angle of the first pass  are assumed to be
5 keV and 140 degrees, respectively. Hutchedregion shows =/2±20 degrees or =/2+90±20degrees.

4. Summary
A Thomson scattering spectrum resulting froman additional laser, whose wavelength is different
from the main diagnostic laser, and bremsstrahlung,which is the dominant component of the background light at appropriate wavelength band, areutilized for the calibration of relative spectral transmissivity and absolute transmissivity at a wavelength, respectively. When we have two collinear
beams and the configuration of optical system isappropriate, parallel and perpendicular electron
temperatures can be independently measured. Those
calibration and diagnostic methods will be crucial
and significant, respectively, for burning plasma
diagnostics.
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[3] K. V. Beausang et al: Rev. Sci. Instrum. 82 (2011)
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[4] W. Zwingmann et al: Plasma Phys. Control. Fusion
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[5] E. Yatsuka et al: J. Plasma Fusion Res. SERIES 9(2010) 12.
[6] E. Yatsuka et al: Proc. 2010 Autumn Meeting on Jpn.
Phys. Soc., Osaka, 2010, p. 203 (in Japanese).
[7] P. J. Brussaard and H. C. van de Hulst: Rev. Mod.
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[8] E. Yatsuka et al: to be published in Nucl. Fusion.

*The views and opinions expressed herein do notnecessarily reflect those of the ITER Organization.