22C09
Temperatures of Electron, Ion and Gas in Low Temperature Recombining
Plasmas Measured Using Laser Thomson Scattering and Stark Spectroscopy
レーザートムソン散乱法およびシュタルク分光法を用いた低温再結合プラズ
マの電子温度・イオン温度・ガス温度	 
Shinichiro Kado
門	  信一郎
School of Engineering, The University of Tokyo, Bunkyo-ku, Tokyo 113-8656, Japan東京大学大学院工学系研究科 〒113-8656	  東京都文京区本郷7-3-1
Development of the laser Thomson scattering system in the MAP-II steady-state linear divertor/edge
plasma simulator intended for the study of low-temperature recombining plasmas (also referred to as thedetached plasma), having electron temperature below 0.1 eV, is reviewed. In addition, a method todetermine the neutral temperature, electron density, and ion temperature in the recombining plasmas fromthe spectral line shapes with Doppler and Stark broadenings of three different transitions, 21S–31P (501.567nm), 21S–71P (335.455 nm), and 23P–73D (370.500 nm) was proposed. The results from both diagnostics
were compared at the recombining front of the helium plasma.

1. Introduction
Recombining plasma in the divertor/edge region
in magnetically confined fusion-relevant plasma
can be a sink of particle and heat fluxes due to theneutralization of the plasma before reaching thedivertor plate (this phenomenon being referred to as"detachment"). Characteristics of the recombining
plasma, such as electron-ion recombination (EIR)
consistingof
radiativeand
three-bodyrecombination, or molecular assisted/activatedrecombination (MAR) induced by hydrogen orhydrocarbons are determined by the atomic andmolecular processes that are strongly dependent on
the electron temperature. Moreover, in therecombining plasmas, because there is no externalenergy input, such as discharge electric field oradditional heating, ions and neutral atoms are also
expected to influence and to be influenced by theelectron temperature because of their highcollisionality.
In spite of its importance, however, conventional
diagnostics methods are difficult to be applied to
the recombining plasma.
The electron current of the single probe exhibits
values anomalously lower than those expected fromthe ion saturation current (known as an anomalous
current-voltage
characteristic)[1,2].
The
spectroscopic measurement, such as atomic
Boltzmann plot method for the electron temperature
in a partial local thermal equilibrium (pLTE) - oneof the typical properties of the volumetricrecombining plasmas - always reflects the brightest
point. In addition, there is a transition layer between
the ionizing and EIR regions where the excitation

rates are so low that the line emissivity is small.
This paper thus reviews the development of theLaser
Thomson
Scattering(LTS),andDoppler-Stark Spectroscopy of He I lines at
MAP-II divertor simulator at the University ofTokyo[3] as shown in Fig. 1. The former wasdesigned specifically for the measurement ofelectron temperature and density in the recombining
plasma, while the latter was applied for themeasurement of both ion and atomic temperatures,
when applied to the brightest point of the He EIR
plasmas.
2. Laser Thomson Scattering for the plasma
below 0.1 eV
LTS is regarded as being a reliable electron
temperature measurement. However, since the 1 eVelectron temperature corresponds to about 1.5 nm in
the Doppler half-width at 1/e maximum at 532 nm
for an orthogonal viewing chord, it is difficult forcommercially available narrow-band interference
filters (which typically have a pass band broaderthan about 1 nm) to resolve the Doppler profiles.
For this reason, a double monochromator equippedwith a physical notch filter, called a Rayleigh block,at the intermediate slit have been adopted.
First the author and his collaborators (A.
Okamoto et. al) developed a conventional double
monochromator,
homo-tandem
type,
with
dispersion addition, using four commercial
photographic lenses (Nikon, f = 135 mm, F/2.8).
This system could measure the electron temperature
down to 0.13 eV covering the MAR plasma[4].
However, it was still insufficient for the EIR region.

Reciprocal linear dispersion and instrumental
width at 492 nm were 0.86 pm/pixel and 8.2 pm(9.5 pixels) (FWHM), respectively.
Highly excited states (principal quantum numbern ≥ 6) of helium atoms are dominated by therecombining component, except the 1P states thatare optically coupled with the ground state.
Therefore, the authors have proposed a method todetermine the neutral temperature, electron density,and ion temperature in the recombining plasmas
from the spectral line shapes with Doppler andStark broadenings of three different transitions, 21S-31P (501.567 nm), 21S -71P (335.455 nm), and 23P-73D (370.500 nm) [7].

Fig.1. Schematic diagrams of the source chamber in the
upstream of MAP-II divertor simulator and opticalarrangement of LTS and high-resolution OES.

The author and his student (F. Scotti) thenupgraded the double monochromator to a bright(using F/2 chromatic lenses) hetero-tandem type,composed of two non-symmetric stages - f = 200mm for the first one to increase the dispersion and f= 100 mm for the second one to conserve the total
dispersion [5].
A frequency-doubled Nd:YAG laser beam (532
nm in wavelength, a 10 Hz repetition rate, 7 ns
pulse duration, and typically 400 mJ of energy perpulse, 8 mm in diameter) was used as the probe
beam and an image-intensified charge-coupleddevice (ICCD) detector (Hamamatsu C4078+C4742,1344 × 1024 pixels) was adopted for the gating andaccumulating the scattered component.
3. Doppler-Stark Spectroscopy for EIR plasmas
The authors and his students (K. Suzuki et al.)have observed that the Doppler broadening of theHe I lines gives different atomic temperaturedepending on the quantum states and that as theelectron density increases, the difference becomes
more remarkable even though the Stark broadening
is taken into consideration. For example, 1D and 3Dseries temperatures increase with the principal
quantum number but with different rates.
A high-resolution visible spectrometer, 1 m in
focal length and 2400 lines/mm in groove
frequency, with image expander optics [6] attachedwith a cooled CCD detector with electron multipliertechnology (Andor DU971N of 1600 × 400 pixels
and 16µm × 16µm in pixel size) is applied for theline-broadening measurement for the He-EIR
plasmas in the upstream chamber of MAP-II.

4. Results and Conclusions
The author had expected that in the recombining
plasmas, the thermal equilibrium is likely to beachieved among electron, ion and atom. Indeed, theprevious work [5] has revealed that the measured
electron temperature using both LTS and theRydberg series spectra yields the similar values
about 0.05 - 0.07 eV.
As the measurement position approached therecombination front, both ion and atomic
temperatures deduced from the Doppler component
became close to the electron temperature by LTS,indicating the achievement of the thermal
equilibrium between electrons, ions, and neutral
atoms[7].
Acknowledgments
The author acknowledges A. Okamoto, F. Scotti
K. Suzuki and the other MAP-II group members fortheir contributions presented here. This work wassupported partially by NIFS collaborative research
program (NIFS04KOAB009, NIFS10KLMH002),
and by the Ministry of Education, Science, Sports
and Culture, Grant-in-Aid, for Young Scientists (B),16740311, 2004 and for Scientific Research (C),21540507, 2009.
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[2] S. Kado, H. Kobayashi, T. Oishi and S. Tanaka: J.
Nucl. Mater. 313-316 (2003)754.
[3] S. Kado, Y. Iida, S. Kajita et al.: J. Plasma Fusion
Res. 81 (2005) 810.
[4] A. Okamoto, S. Kado, S. Kajita and S. Tanaka: Rev.
Sci. Instrum. 76 (2005) 116106.
[5] F. Scotti and S. Kado: J. Nucl. Matter. 390 - 391,(2009) 303.
[6] S.Kado and T.Shikama: J. Plasma Fusion Res. 79,(2003) 841.
[7] S. Kado, K. Suzuki, Y. Iida and A. Muraki: Journalof Nuclear Materials 415 (2011)1174.