29pC04
ヘリウム原子近赤外分光法のダイバータ模擬装置MAP-IIへの適用	 
Near-Infrared Spectroscopy for Atomic Helium Linesfor MAP-II Divertor Simulator
門信一郎, 飯田洋平*, 山野守史, 渡邉智史, 阿部翔太**
KADO Shinichiro, IIDA Yohei*, YAMANO Morifumi, WATANABE Satoshi, ABE Shota**
東大院工 (現所属： *筑波大, **米UCSD)
Univ. Tokyo (present affiliations: *Univ. Tsukuba, **UCSD, USA)
Intensity ratio of atomic helium line (He I) has widely been used to evaluate the electron density andtemperature in fusion-relevant divertor and in divertor-simulating plasma facilities. This method employs acollisional radiative model to determine the quasi-steady state solution of the rate balance equations in termsof population density of the excited states. In addition to the excitation, ionization, recombination or
spontaneous emission processes, radiation trapping – also referred to as reabsorption, self-absorption,
radiative transfer, or radiation transport – has been pointed out to play a crucial role in low-temperature(namely low-degree of ionization) plasmas [1]. In particular, the radiation trapping between ground and 21Pstates causes the strongest effect on the population distribution through redistribution to the other states [2].
Even though, direct determination of the 21P states population is difficult because the emission to the groundstate has the wavelength of 58.4 nm in the vacuum ultraviolet (VUV) regime. Moreover, such resonanttransitions with ground state are optically thick due to the high ground state density, so that the interpretationof the measured spatial distribution of the line intensity may need careful consideration.
In this study, we have developed a near-infrared (NIR) spectrometer system to measure 21S - 21P
optically thin radiative transition of 2058.130 nm to evaluate the spatial distribution of the 21P state (see Fig.
1). A merit in this approach is that the visible spectroscopic technique can apply with some modification.
We modified a Czerny-Turner spectrometer (SOLAR TII inc., MS3504i) 35 cm in focal length, 3.8 in
F-number. The grating with the groove frequency of 300 G/mm allows access to NIR spectral regime. The"flat-field" of the spectrometer is 28 x 10 mm2 in dimension where a two-stage thermoelectrically cooled (~-20 ºC) InGaAs linear imaging sensor (Hamamatsu G9208-256W, 0.03(H) x 0.25(V) mm2 in pixel size, 256ch, 0.05 mm in horizontal pitch between pixels) isinstalled. The sensitive wavelength region is 900 - 2550
nm covering the 23S - 23P (1083 nm) line as well.
Furthermore, the detector has some sensitivity in visible
region, so that the strong visible lines and their higher
order diffraction can be used to calibrate the wavelengthand the reciprocal linear dispersion, which has been
determined to be 9.3 nm/mm around 2058.130 nm.
By applying the pure helium discharge in MAP-IIdivertor simulator [3], remarkable broadening in thespatial profile of the 21P state was observed, suggesting
significant contribution of the radiation trapping. This
result is useful in evaluating the spatial distribution ofradiation trapping [4] and including it in the
self-consistent collisional radiative model calculation.
[1]S. Kado et al., J. Plasma Fusion Res. 86, 631(2010). [inJapanese][2] Y. Iida, S. Kado, A. Muraki and S. Tanaka, Rev. Sci.
Instrum. 81, 10E511 (2010).
[3] S. Kado et al., J. Plasma Fusion Res. 81, 810(2005).
[4] Y. Iida, S. Kado, S. Tanaka, Phys. Plasmas 17, 123301(2010).

Fig. 1 Grotrian diagram of atomic helium
line spectra (He I) with transition
wavelength in nm.