B. "Pegourie

G. Motojima

M." Goto

R. Sakamoto

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.Pellet injection, by its capacity to directly depositthe fuel inside the last closed flux surface, is expected tobe a highly efficient fueling method for magnetic fusionreactors. However, parameters of present day devices aretoo different to those expected in a reactor for a simpleextrapolation to be reliable. It follows that an accuratecharacterization of the ablation physics is essential forconstraining the pellet ablation-deposition models. Inparticular, it is crucial to understand the over-ablationassociated to the high-energy ions and electrons gener-ated by auxiliary heating.To investigate the behavior of hydrogen pellet abla-tion, a novel method of high-speed imaging spectroscopyhas been used in the Large Helical Device (LHD) foridentifying the internal distribution of the electron den-sity and temperature of the plasma cloud surroundingthe pellet. This spectroscopic system consists in a five-branch fiberscope and a fast camera, with each objectivelens equipped with a different narrow-band optical filterfor the hydrogen Balmer lines and the background con-tinuum radiation [1].For interpreting the measurements, a 3D model ofthe cloudlet emission was built, under the assumptionof full LTE, as established in [2]. The temperature anddensity distributions are assumed to be spherically sym-metric close to the pellet - at the center of the cloudlet -and to display a cylindrical symmetry in its partly ion-ized outer part. Values can be either imposed a priori, ortaken from a HPI2 pellet ablation-deposition code sim-ulation [3]. The cloudlet spectrum - taking into accountradiative transfer effects - is calculated in the 300− 700300 400 500 600 700100101102wavelength   [nm]normalized spectrum   [a.u.]  MeasurementSimulationFig. 1: Measured and simulated spectra.µm wavelength range, as well as emission maps corre-sponding to the different filters of the imaging system de-scribed above. Physical phenomena taken into accountare line emission (Hα, Hβ , Hγ), radiative attachment, ra-diative recombination and the two bremsstrahlung com-ponents (that intrinsic to the cloudlet and that due tothe slowing down of the plasma electrons impinging thecloud).A preliminary simulation result is displayed in Fig-ures 1 and 2, where the measured and calculated spec-trum and light emission profiles (in the parallel andcross-field directions) are compared for the cloudlet. Thecorresponding density and temperature distributions in-side the cloudlet are displayed in Figure 3.1) G. Motojima et al., Rev. Sci. Instrum. 83 (2012) 093506.2) M. Goto et al., Plasma Phys. Control. Fusion 49 (2007) 1163.3) B. Pe´gourie´ et al., Plasma Phys. Control. Fusion 47 (2005) 17.−0.2 0 0.200.51.01.5plasmoid cross−section [m]normalized intensity [a.u.]  Sim. [Hβ]Sim. [Continuum]Exp. [Hβ]Exp. [Continuum]−0.2 0 0.200.51.01.5plasmoid length [m]normalized intensity [a.u.]  Sim. [Hβ]Sim. [Continuum]Exp. [Hβ]Exp. [Continuum]Fig. 2: Measured and simulated radiation profiles inthe cross-field (left) and parallel directions (right).Fig. 3: Cloudlet density and temperature distributionsin the cross-field (left) and parallel directions (right).29§24. Determination of Space Resolved Characteristics of Pellet Ablation CloudPégourié, B. (IRFM/CEA Cadarache),  Motojima, G., Sakamoto, R., Goto, M.

Determination of Space Resolved Characteristics of Pellet Ablation Cloud

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Determination of Space Resolved Characteristics of Pellet Ablation Cloud

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