G. Motojima

H. Yamada

J.S. Mishra

R. Sakamoto

National Institute for Fusion Science (NIFS-Repository)

Stereoscopic observation of pellet ab-latant in LHD.J. S. Mishra (Sokendai), R. Sakamoto, G. Motojima,H. YamadaApplying stereoscopic technique1)three dimen-sional motion of an ablating pellet in presence of tan-gential NBI beam is discussed.Stereoscopic images are taken for the ablating pel-lets, injected from the repetitive pellet injector2)to tan-gentially NBI heated plasma [Clock wise (CW) - 3.7 MWand Counter clockwise (CCW) - 2.8 MW ] in LHD. Theimages taken by fast camera are of spatial resolution ≈90 K pixels and temporal resolution of 50 µs with 2 µsexposure time. Using projection matrices and the imagepoints obtained from the pair of images, the three di-mensional pellet position is reconstructed. A schematicof the stereo observation with NBI injection in LHD isshown in Fig.1.Observation of the Hα and the camera data shows apeak in signal outside Last Closed Flux surface (LCFS)only for a CW NBI injected plasma. This observationindicates the heating of pellet by the CW NBI, as itcomes under the direct inﬂuence of beam path beforeentering into the plasma.Deviation of pellet trajectory from its injected di-rection [radial] has been seen in presence of both NBIconditions. An example of deﬂection of pellet trajectoryand the deﬂection velocity along radial (VR), vertical(VZ) and toroidal (Vφ) direction for CW NBI injectedpellet along the toroidal plane is shown in Fig. 2. Thetoroidal deﬂection observed in both NBI cases is ≈ 20cm but poloidal deﬂection observed in case of CW NBIonly, is ≈ 10 cm. Toroidal deﬂection velocity lies in therange 400 m/s - 800 m/s for CW NBI and 200 m/s - 500m/s in case of CCW NBI. Toroidal acceleration for CWNBI is (0.7 - 1.5)×106 ms−2 and (0.4 - 0.8)×106 ms−2in other case. The deﬂection observed is probably dueto the asymmetric ablation on both side of pellet by fastions generated from tangential NBI and subsequently for-mation of rocket eﬀect in the toroidal direction. Consid-ering one-dimensional heat ﬂux term in the equation ofmotion along the toroidal direction, pellet accelerationcalculated in the range (0.15 -1.5 )×106 ms−2, which isin close agreement with the observed acceleration in bothcases.The trajectory deﬂection starts at a normalized mi-nor radius ρ ≈ 0.85 for CW NBI and ρ ≈ 0.75 for CCWNBI. This observation can be explained in terms of co-injection and counter-injection. As the toroidal magneticﬁeld is in the CW direction, the CW NBI is co-injectionand CCW NBI is counter-injection. The drift orbit offast ions shifts outward for co-injection and inward forcounter-injection. Hence the observation qualitativelyagrees with the fast ion deposition proﬁle in LHD. In caseof CW NBI injection, pellet speed slowing down alonginjection direction has been seen but there is an acceler-ation of pellet at the last phase of motion for CCW NBIinjected pellet. The reason behind this is still unclearand to be investigated. The penetration depth observedin this context (ρ ≈ 0.75) is much less than calculated(ρ ≈ 0.63) value in case of CW NBI and it is more ( ρ ≈0.65) than calculated value (ρ ≈ 0.7) for CCW injection.This result is obvious by considering the radial speedVR, within pellet life time. Additionally in case of CWNBI, pellet deﬂecting to a squeezing plasma cross sectionwhere as the deﬂection for CCW NBI is to an expandingcross section, which aﬀects the pellet penetration depthin terms of normalized minor radius ρ596Observationpoint  10.01.0Observationpoint  2 XZ2.03.04.5 4.0YPellet   directionCW NBICCW NBI15oDistance [m]Fig. 1: Schematic of the stereo observation with NBIconﬁguration in LHDCCWCW-400 -200 0 200 40020002200240026002800Z [mm]Y [mm]2000 2200 2400 2600 2800-400-2000200400X [mm]Z [mm]3.7 MW(a) (b)0.0 0.4 0.8 1.2-500-2500250500VZ Vφ VRV  [m/s]T [ms](c)Fig. 2: Trajectory deﬂection along (a) toroidal plane(b) poloidal plane and (c) ablating pellet speed in ra-dial (VR), vertical (VZ) and toroidal (Vφ) direction inpresence of CCW NBI.1) R. Sakamoto et al, Rev. Sci. Inst. 76, 103502 (2005)2) H. Yamada et al, Fusion Eng. and Design 69 (2003)36§12. Stereoscopic Observation of Pellet Ablatant in LHDMishra, J.S. (Grad. Univ. Advanced Studies), Sakamoto, R., Motojima, G., Yamada, H.

Stereoscopic Observation of Pellet Ablatant in LHD

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