C.F. Dong

E.H. Wang

H.M. Zhang

M." Goto

S. Morita

T. "Oishi

X.L. Huang

National Institute for Fusion Science (NIFS-Repository)

  Two-dimensional (2-D) distribution of carbon has been measured to study the impurity transport in stochastic magnetic field layer of LHD1). When the 2-D distribution is observed, the impurity behavior can be separately studied between the inboard and outboard edge X-points.  The 2-D distribution of CIV observed at different magnetic axis positions is shown in Figs.1 (a) to (c) for Rax=3.60m, 3.70m and 3.75m, respectively.  Symbols of axes, Z and Y, indicate vertical and horizontal positions of LHD plasma.  Then, Y=0 means toroidal position at horizontally elongated plasma cross section (ϕ=18°).  In Rax=3.60m the CIV emission from inboard edge X-point is clearly stronger than that from outboard edge X-point.  When the magnetic axis position is shifted outwardly, the impurity behavior near X-points drastically changes.  In Rax=3.7m the CIV intensity seems to be identical between inboard and outboard X-points, and in Rax=3.75m the CIV intensity distribution along X-point is inversed reflecting the outboard edge X-point trajectory.  This intensity change seems to reflect the plasma volume in the vicinity of edge X-point.  2-D CIV distribution is simulated with 3-D edge plasma transport code, EMC3- EIRENE code. The result is plotted in Figs.2 (a) and (b) for Rax=3.60m and 3.75m, respectively.  The value of ϕ=18° in horizontal axis of figures is a toroidal angle normalized by toroidal helical pitch number of M-10, which corresponds to the position of Y=0 mm in Fig.1.  The X-point trajectory can be clearly seen in the 2-D distribution of CIV. The CIV emission in Rax=3.75m is strong at the outboard edge X-point, while the CIV emission in Rax=3.60m seems to be enhanced at both the inboard and outboard X-points.  The simulation at Rax=3.75m in Fig.2 (b) shows a good agreement with the measurement at Rax=3.75m in Fig.1 (c), although the result at Rax=3.60m tends to be different between the measurement and the simulation.At present we have no clear comments on the discrepancy between the measurement and the simulation for the Rax=3.6m case due to the limited data base.  Nevertheless, it should be pointed out at least that theneutral particles are particularly localized at the inboard side of ϕ=18° toroidal position due to the geometricallyclosed space.  The localized neutrals can change the local ionization source for density buildup and resultantly also change the density gradient along the magnetic field line. As a result, the edge impurity transport largely affected by the neutrals at the inboard side of torus in ϕ=18° may modify the carbon density distribution so that the CIV emission distribution can be enhanced along the inboard X-point.  The 3-D simulation with locally enhanced neutrals is being carried out to explain the present result.Fig.1 Measured two-dimensional distribution of CIV intensity at (a) Rax=3.60, (b) 3.70 and (c) 3.75m configurations. ᴾ ᴾ The toroidal trace of inboard and outboard edge X-points is indicated with dashed lines in figures (a) and (c), respectively.Fig.2 Simulated two-dimensional distribution of CIV emissivity at (a) Rax=3.60m and Rax=3.75m configurations.  Horizontal axis means normalized toroidal angle.  Diamond dashed line denotes LHD outboard port dimension which mainly defines the image area of the present two-dimensional EUV spectroscopy.1) S.Morita et al., Joint 19th ISH and 16th RFP workshop, 16-20 Sepώ 2013, Padova, Italy (to be published in PPCF).    The effects of thick stochastic magnetic field layer located outside the core plasma of LHD called “ergodic layer” on the impurity transport have recently attracted attention.  A space-resolved vacuum ultraviolet (VUV) spectroscopy using a 3 m normal incidence spectrometer (McPherson model 2253) is utilized to measure impurity emission profiles in the ergodic layer and divertor plasmas of LHD in wavelength range of 300 - 3200 Å.1,2)  The spectrometer is installed on a horizontal diagnostic port (#10-O).  The vertical profile of VUV emissions as a wavelength-dispersed image is projected on a CCD detector by a slit for spatial resolution called “space-resolved slit” mounted between the entrance slit and the grating in the spectrometer.  The optical axis was arranged perpendicular to the toroidal magnetic field in the bottom edge at holizontally-elongated plasma cross section to adjust the observation range to the ergodic layer.  The observation range can be expanded to measure the full plasma profile with wider viewing angle by inserting a mirror unit which consists of a flat mirror and a cylindrical mirror installed between the spectrometer and the torus. Figure 1 shows the observation range of VUV spectroscopy for the full profile measurement on the poloidal cross section with the magnetic axis of Rax = 3.75 m.  Full vertical intensity profile of CIV 1548.20 × 2 Å spectrum measured for a hydrogen discharge with electron density ne ~ 6.0 × 1013 cm-3 is also shown together.  We can find intensity peaks at both top and bottom edge of the intensity profile indicated by the solid allows in Fig. 1. at Z = 471 mm and -451 mm.  They are exactly located along the top and bottom edge of the ergodic layer, which are called “edge O-points”.  It has been experimentally certificated that the CIV emission is located in the outermost region of ergodic layer because the ionization energy of 65 eV for C3+ ions is extremely low compared to the edge temperature of LHD plasmas.  Therefore, long integral paths along those observation chords result in peaks around the LCFS in the intensity profile.  On the other hand, the observation chords at the CIV intensity peaks indicated by the dotted allows at Z = 135 mm and -65 mm are located on the outboard divertor legs.  It is a clear experimental demonstration for a three-dimensional simulation results calculated by EMC-EIRINE3 indicating that C3+ ion density increases around the divertor legs near the edge X-point in high density discharges. Space-resolved measurement can provide vertical profile of ion temperature, Ti.  Figure 2 shows Ti obtained from Dopplar broadening of CIV spectrum with Rax = 3.60 m.  Ti measured using observation chords locating top and bottom edge O-points and inboard X-point is plotted against edge electron density, ne(a99), which is defined as electron density at the effective minor radius enclosing 99 % of the electron stored energy.  It is the first observation that suggests Ti measured in the edge O-point is higher than that measured in the edge X-point.  It also indicates that difference of Ti between O-point and X-point is enhanced in the case with higher Ti and lower ne.  The variety of Ti along the poloidal location includes a result of heat and particle transport characteristics in the ergodic layer and difference of a line integral effect between observation locations.  This should be clarified as a future subject on the transport study in edge plasmas.                     Fig. 1. The observation range of VUV spectroscopy shown on the poloidal cross section of LHD together with full vertical profile of line intensity of CIV 1548.20 × 2 Å spectrum.                               Fig. 2. Dependence of ion temperature Ti obtained from Dopplar broadening of CIV spectrum on edge electron density ne(a99).     1) Oishi, T. et. al.: Plasma and Fusion Research 8 (2013) 2402093.  2) Oishi, T. et. al.: Journal of the Korean Physical Society 64 (2014) 840. 21§16. Ion Temperature Measurement of Carbon Impurities in the Ergodic Layer of LHD Using 3 m Normal Incidence VUV SpectrometerOishi, T., Morita, S., Dong, C.F., Wang, E.H.,  Huang, X.L., Zhang, H.M., Goto, M.

Ion Temperature Measurement of Carbon Impurities in the Ergodic Layer of LHD Using 3 m Normal Incidence VUV Spectrometer

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Ion Temperature Measurement of Carbon Impurities in the Ergodic Layer of LHD Using 3 m Normal Incidence VUV Spectrometer

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