Experimental Group LHD

H. Funaba

K. Sato

K.V. Khlopenkov

N. Tamura

S. Kato

S. Muto

S. Sudo

V.Yu. Sergeev

National Institute for Fusion Science (NIFS-Repository)

§53. Impurity Transport Study by Means of Tracer-Encapsu lated Sol id Pellet Injection on LHD Tamura, N., Sudo, S., Khlopenkov, K.V., Kato, S., Sergeev, V.Yu., Muto, S., Sato, K., Funaba, H., LHD Experimental Group In order to realize a practical fusion reactor, impurity transport is still one of the important issues to be clarified. In recent Large Helical Device (LHD) experiments, Tracer-Encapsulated Solid PELlet (TESPEL) [1] injection has been implemented for the study of impurity particle transport [2]. TESPEL consists of polystyrene (-CH (CJIs) CHz-) as an outer shell (typically ~ 0.7 mmq,) and tracer particles as an inner core (typically ~ 0.2 mm size). In LHD, metallic impurity accumulation has been found only in hydrogen discharges with a line-averaged electron density ne_bar around 2 x 1019 m-3 [3]. In order to estimate the transport properties of this phenomenon quantitatively, the transport analysis using the TESPEL injectiori has been done. The behavior of the emission lines from the highly ionized Ti tracer impurity, Ti Ka. (EH~1ike ~ 4.7 keY) and Ti XIX (A. = 16.959 nm), have been observed by an X-ray pulse height analyzer (PHA) and a vacuum ultra violet (VUV) spectrometer, respectively. AB seen in Fig. 1, the decay time ofTi Ka measured by PHA20 increases gradually as the value of ne_bar increases from OJ x 1019 m-3 to 1.9 x 1019 m-3• There are points above 3.0 x 1019 m-3, which have a considerably longer decay time. This is consistent qualitatively with the experimental results by observations of the behavior of an intrinsic impurity [3]. In order to estimate transport coefficients at the higher electron . density, the impurity transport code, MIST has been used. The trial and error analysis with MIST indicates that the temporal evolution of the emissions of Ti Ka and Ti XIX is in fairy good agreement with the case of diffusion coefficient D = 600 cm2/sec and convective velocity at the edge V(a) = -76 cm/sec (the inward) (see Fig. 2). From the point of view of global behavior, the Ti impurity transport with neJx:r = 3.5 x 1019 m-3 can be explained with the value of D = (300 ~ 900) cm2/sec and V(a) = -(19 ~ 114) cm/sec. In this case, the inward convection should be taken into account In order to examine the experimentally deduced convective velocity, the neoclassical convective velocity is calculated In case of ne_bar = 3.5 x 1019 m-3, the neoclassical convective velocity at p = 0.7, (52.7 ± 46.6) cmIsec, has the opposite sign (the outward), compared with the experimentally deduced one at p = 0.7, -5303 cm/sec. Therefore, the estimated convective velocity cannot be explained in case of neJm = 3.5 x 1019 m-3 solely by the effects of the pure neoclassical impurity transport and the effect of some kind, which originates the inward flux, should be taken into account additionally. Moreover, by taking advantage that TESPEL can produce very localized particle source, the local deposition of tracer particles inside a magnetic island, which is expanded by Local Island Divertor coils, has been accomplished In that discharge, the effects of the magnetic island on the impurity particle transport have been observed. i= 2 l-e + Q) § 1 - • >- J. • • ~ Q) 0 'C 0 2 3 5 line-averaged electron density (x 10 19m.J) Fig. 1. The dependence of the decay time ofTi Ka emission on the line-averaged electron density in the case of balanced NBI heated plasmas. 1.2 ~"""""'"I""'"T"...,.......,... ............. ...,.......,...,............,.-y~L:::.;H::;D...::;sh..:.;o;.;.:.;t#2;:..;.,7.:;.;89,.:...,1 0.8 0.6 ~ 0.4 ClI ; 0.2 .ijj 2 o~~~******~~~~~~~ .5 4 '0 Q) ~ 3 E g 2 o 1~~~1~.5~~~2~~~2.5~~~3~~~3.5 time (sec) Fig. 2. Comparison of normalized temporal evolution of (a) Ti Ka. emission (open circles) measured by the PHA20 and (b) Ti XIX emission ( open diamonds) done by the VlN spectrometer with those calculated by MIST with several sets of D and Vat the value ofne_bar = 3.5 x 1019 m-3• References: 1) Sudo, S.: 1. Plasma Fusion Res. 69, (1993) 1349 2) Tamura, N. et al.: 1. Plasma Fusion Res. SERIES 4, (200 I) 442 3) Nakamura, Y: Proc. 28 th EPS Conf Controlled Fusion Plasma Phys. 25A, (200) 1481 (Funchal, Portugal) 57 

Impurity Transport Study by Means of Tracer-Encapsulated Solid Pellet Injection on LHD

Impurity Transport Study by Means of Tracer-Encapsulated Solid Pellet Injection on LHD

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