K. Sawada

M. Goto

Goto,  M.

Sawada,  K.

NIFS-Repository (National Institute for Fusion Science)

The population distribution over the excited levelsof neutral helium can be obtained with the collisional-radiative (CR) model for a given set of the electron tem-perature Te and the electron density ne. The CR modelfor neutral helium has been extended so as to include thereabsorption effect in the resonance lines1).If the plasma considered is in the ionizing phase andthe quasi-steady-state (QSS) approximation is applied tothe two metastable states as well as the ground state, anyexcited level population is expressed asN(p) = R1(p)N(11S)ne+∑nRn1P(p)I(n1P)N(11S), (1)where N(11S) is the ground state population, I(n1P)is the photoexcitation rate coefficient for the transitionfrom the ground state to n1P, and R1(p) and Rn1P(p) arecalled the population coefficients and functions of Te andne. The first term in Eq. (1) is the conventional ionizingplasma component and the remaining terms are due tothe reabsorption effect. Here, 31P is only considered asthe upper level of the photoexcitation process and all theother photoexcitation processes in Eq. (1) are omitted.We can calculate with this extended CR-model thepopulation distribution in absolute units when Te, ne,I(31P), and N(11S) are known. Here, we attempt tosolve a reverse problem, namely, those parameters aredetermined so that a merit functionf(Te, ne, N(11S), I(31P))=∑p(N exp(p)−N(p)N exp(p))2(2)takes the minimum, where N exp(p) is the measured pop-ulation of level p and the summation is conducted overthe all excited levels for which the population is mea-sured.The observation of emission lines in the visiblewavelength range is made for a discharge shown in Fig. 1,where the line-averaged electron density ne is increasedwith the steady helium gas-puff, while the central elec-tron temperature Te0 is lowered correspondingly.Nine emission lines of neutral helium are identifiedin the observed spectra and used for determining Te andne. The results are shown with the filled squares (Te) andcircles (ne) in Fig. 1. It is known that the line emissionof neutral helium is localized at the plasma edge, so thatthe derived Te and ne are regarded as those at the plasmaedge though the measurement is line-integrated.3210T e0 (keV) 1050 (1019 m-3)Te0He puﬀECH110100T e (eV)101810191020ne  ( m-3)neTe4.74.6R (m)2.52.01.51.00.50t (s)Tenewithwith1.00.80.60.40.20Wp(103  MJ) 403020100PNBI  (MW)WpPNBIRax = 3.6 mFig. 1: Temporal development of the discharge for thepresent analysis. The deduced Te and ne and the emis-sion locations from those parameters are also shown. Thefilled and open symbols show the results with and with-out the reabsorption effect, respectively.For examining the significance of the reabsorptioneffect in the present measurement, we have made a simi-lar analysis in which no photoexcitation process is takeninto account. The open symbols show the results and itis clear that the derived ne is systematically larger thanthat with the reabsorption effect, while the difference inTe is small.It is not possible to judge at this moment whichresult gives the actual parameters. We attempt to deter-mine the emission locations by comparing the derived Teand ne values with their spatial profiles measured by theThomson scattering method. It is found that the loca-tions determined from Te and ne by the CR model withthe reabsorption effect show good agreement with eachother, while those by the model without the reabsorptioneffect gives significant discrepancy. These results suggestthat the present model describes the actual populationdistribution better than the conventional model and hasarrived at the state available for a practical use in theactual Te and ne measurements.1) K. Sawada et al.: Plasma and Fusion Res. 5 (2010)001.121§5. Significant Reabsorption Effect in Formation of Helium Level PopulationsGoto, M., Sawada, K. (Shinshu Univ.)

Significant Reabsorption Effect in Formation of Helium Level Populations

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