C. Suzuki

I. Murakami

M. Goto

S. Masuzaki

S. Morita

T. Morisaki

National Institute for Fusion Science (NIFS-Repository)

Impurity gas seeding is one of the promising scenar-ios to mitigate divertor heat loads through the enhancedradiation power in edge plasmas. In this context, impu-rity gas seeding experiments have been extensively per-formed in LHD to achieve the divertor detachment in thelast few years1). In addition to nitrogen, neon, and argongasses, a krypton gas seeding experiment has recentlybeen launched in LHD to explore the possibility of moreradiation enhancement from high Z impurity ions. Thevacuum ultraviolet (VUV) spectroscopy would be help-ful to determine which charge states emit dominantly,where they emit the radiation, and how much they con-tribute to the total radiated power. In this study, wehave carried out VUV spectroscopy in the Ne seedingexperiments in LHD to observe temporal evolutions ofline intensities of Ne IV–VIII simultaneously.VUV spectra are observed by a 2 m Schwob-Fraenkel grazing incidence spectrometer2). The spec-trometer can record spectra in two different spectralbands at the same time. In this study, Ne VI–VIII (n=2-3) lines in 8–12.5 nm and Ne III–VII (n=2-2) lines in 45–60 nm can be monitored simultaneously using a gratingwith 133.6 g/mm groove density. The integration time ofthe detector was set at 50 ms, which is enough to followthe temporal evolutions. The line of sight of the spec-trometer was fixed at the position passing through theplasma center in this experiment. Spectral lines used forthe analyses in this study are listed in Table I.Figure 1 shows waveforms of various parameters ina typical Ne seeded discharge, together with temporalevolutions of the intensities of Ne IV–VIII lines listedin Table I. After the Ne gas was puffed during 3.7–3.8s with a flow rate of about 5 Pa m3/s, the total radi-ated power (Prad) increased three times or more, thenslightly and slowly decreased until the end of the dis-Table I: Spectral lines from Ne ions used for the analysesin this study.Ion Transition Wavelengtha Ne VIII 1s22s 2S - 1s23p 2P◦ 8.81 nmb Ne VIII 1s22p 2P◦ - 1s23d 2D 9.83 nmc Ne VIII 1s22p 2P◦ - 1s23s 2S 10.3 nmd Ne VII 2s2p 3P◦ - 2s3d 3D 10.6 nme Ne VI 2s22p 2P◦ - 2s23d 2D 12.3 nmf Ne IV 2s22p3 2D◦ - 2s2p4 2D 47.0 nmg Ne V 2s22p2 3P - 2s2p3 3P◦ 48.3 nmh Ne IV 2s22p3 4S◦ - 2s2p4 4P 54.4 nmi Ne V 2s22p2 3P - 2s2p3 3D◦ 57.2 nmPNBINe puffWp<ne>PradIisabcdefighNe VI–VIII(n=2-3)Ne IV–V(n=2-2)(a)(b)(c)(d)(e)Fig. 1: Waveforms of parameters in a typical Ne seededdischarge together with line intensities of Ne IV–VIIIlisted in table 1 as a–i. Temporal evolutions of (a) to-tal neutral beam heating power (PNBI) and flow rate ofNe gas puff, (b) plasma stored energy (Wp) and totalradiated power (Prad), (c) line-averaged electron density(< ne >) and ion saturation current (Iis) measured byone of the divertor probes, intensities of spectral linesfrom (d) Ne VI–VIII, and (e) Ne IV–V.charge without serious degradation of the stored energy(Wp). The ion saturation current of a divertor probe(Iis) clearly dropped after the Ne gas puff as shown inFig. 1 (c). As is clearly seen in Fig. 1 (d) and (e),temporal evolutions of the line intensities are completelydifferent between higher charged states (Ne VI-VIII; a-e) and lower charged states (Ne IV–V; f -i). Also, thetemporal behaviors of the lower charged states are quitedifferent from that of the increment of Prad. These obser-vations imply that the contributions of Ne IV–V to thetotal radiated power is less than those of Ne VI–VIII.1) Masuzaki, S. et al.: J. Nucl. Mater. 438 (2013) S133.2) Schwob, J. L., Wouters, A. W. and Suckewer, S.: Rev.Sci. Instrum. 58 (1987) 1601.22§17. Vacuum-ultraviolet Spectroscopy in Detached Plasmas with Neon Gas Seeding in LHDSuzuki, C., Murakami, I., Morita, S., Masuzaki, S., Morisaki, T., Goto, M.

Vacuum-ultraviolet Spectroscopy in Detached Plasmas with Neon Gas Seeding in LHD

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