B. Pegourie

G. Motojima

H." Yamada

I. Yamada

R. "Sakamoto

R. Yasuhara

National Institute for Fusion Science (NIFS-Repository)

Pellet injection timing synchronized Thomson scat-tering measurements have been carried out to under-stand the elementary processes of the pellet fueling.Extremely asymmetric electron density profiles betweenthe inboard-side and outboard-side are observed by theThomson scattering measurement during pellet ablation.The density peaks in the asymmetric density profile con-sider to be due to plasmoids which across the laser of theThomson scattering measurements.Pellet fueling consist of the two processes, namely,solid hydrogen ablation and homogenization of the ab-lated materials. The pellet ablation process can be in-vestigated by observing strongly emitted visible lightfrom the ablated materials. At the same time, obser-vation of the homogenization process is very restrictedbecause the completely ionized ablated material, namely,plasmoid doesn’t emit strong light. Thomson scatteringmeasurement, which is installed at a section apart fromFirstexpansionchamberSecondexpansionchamberFinalexpansionchamberLHDD(2)D(3)DLCFSDTS t=0tLGtD+tTSLGU= 2LGD= 3LG1LG2LG3Fig. 1: Pellet injector and multiple light-gate systemfor realtime estimation of pellet position.6543210ne   [1020 m-3]4.54.03.53.02.5R   [m] 3.733580 s 3.746636 s 3.746836 s 3.766912 sAblation Light   [a.u.]3.74723.74683.74643.7460Time   [s]3.746636 s 3.746836 s43210Te   [keV]4.54.03.53.02.5R   [m] 3.733580 s 3.746636 s 3.746836 s 3.766912 s(a) (b) (c)Fig. 2 (a) Ablation light signal and laser timing of Thomson scattering measurement , (b) electron temperature and(c) electron density profiles.the pellet injection position, is employed to observe theinvisible plasmoid. Since the pellet plasmoid is homog-enized within the time scale of several 10 µs, a laserof the Thomson scattering measurement is fired in syn-chronization with the pellet ablation position which isestimated by the multiple right-gate signal (Fig. 1 andEq. (1)) in real-time using a delay pulser with FPGA;Field Programable Gate Array.tD =DLCFS +DTS −D(LGU)D(LGD)−D(LGU) tLG − tTS (1)Pellet ablation light emission signal and laser firing tim-ing of the Thomson scattering measurement are indi-cated in Fig. 2 (a), and electron temperature and elec-tron density profiles are shown in (b) and (c), respec-tively. Strongly asymmetric density profiles with sharppeaks only at the outboard side are observed during ab-lation as shown by symbols (© and ), at the sametime electron density profile become homogenized bothinboard and outboard side. It is satisfactory to con-sider the asymmetric sharp peaks as an expanding pel-let plasmoids. Since the density propagates with ionsound velocity while the temperature propagates withelectron thermal velocity which is about 50 times fasterthan the ion sound velocity, the plasmoid density is nothomogenized in a flux surface at the timing of the Thom-son scattering measurements. The plasmoid density iscompared with the time-dependent simulations by thepellet ablation/deposition code HPI2. The measuredpeak density agrees with the calculated plasmoid den-sity. Since both the pellet injection trajectory and laserof the Thomson scattering measurement are located onthe mid-plane with a section distance (∼ 36◦) each other,the pellet plasmoid that is expected to extend along afield line from the pellet ablation position cannot crossthe laser. This fact indicates that the pellet plasmoiddoes not simply extend along the field line. Possible ex-planation is that the cross-field plasmoid transport dueto the ∇B induced drift is exist in the plasmoid ho-mogenization process. To support this assumption, thecross-field dynamics of the plasmoid is investigating tak-ing into account the three-dimensional effects.28§23. Direct Observation of the Pellet Plasmoid Homogenization Process in LHDSakamoto, R., Yasuhara, R., Motojima, G.,  Pégourié, B., Yamada, I., Yamada, H.

Direct Observation of the Pellet Plasmoid Homogenization Process in LHD

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