Bakhareva, O.

Belopolsky, V.

Burhenn, R.

Kuteev, B.

Ledl, L.

Sergeev, V.

Timokhin, V.

English

MPG.PuRe

Studies of C-pellet ablation cloud structure on Wendelstein7-AS stellaratorV.Yu. Sergeev, V.M. Timokhin, O.A. Bakhareva, B.V. Kuteev, V.A. Belopolsky,*R. Burhenn, *L. Ledl and the WVII-AS teamState Technical University, Politekhnicheskaya 29, St. Petersburg, 195251 Russia*Max Planck Institut fur Plasmaphysik, EURATOM Ass., Boltzmannstrasse 2, D-85748Garching,  GermanyIntroduction. Predictions of the particle deposition profiles produced by ablating pellets intokamaks and stellarators are fundamentally interesting for such applications of pellet injec-tion as fuelling, diagnostics and burn control [1], [2]. Studies of the pellet ablation cloudstructure using photographic technique could clarify the mechanisms of the basic physicalprocesses of the pellet-plasma interaction. In this paper, we present results of the ablationcloud structure observations in carbon pellet experiments at W7-AS and compare them withthose obtained elsewhere.Experimental set-up. Carbon pellets of 0.35-0.45 mm in size and with 150-400 m/sec ve-locities were injected in the plasma core direction at ~15o below the equatorial plane. Cloudintensities passing through an interference filter with λ = 723 nm and ∆λFWHM = 9.3 nm(724 nm CII line) were observed using three different schemes: (1) one CCD camera imagewith a 10 ms exposure time that is much longer than the pellet ablation time ~ 0.5-1 ms; (2) asequence of snapshots from a fast CCD camera (exposure time 2 µs, repetition time 20-30 µs); (3) two instant snapshots, one by each of two synchronized fast CCD cameras (expo-sure time 5 µs). Viewing directions of measurements were as follows: from the bottom, in di-rection almost transverse to a pellet flight path for schemes (1-3), and, in addition from therear side of a pellet track for scheme (3). Scheme (3) allowed us to obtain instant pictures ofthe cloud radiation intensity distribution from two directions simultaneously. The pellet tra-jectory has been fitted to the magnetic flux map with fairly good accuracy (~ 1 cm). Details ofthe experimental set-up are described in Ref. [3].Radial cloud asymmetry. The typical example of the ablation CII cloud photos is shown inFigs. 1c-e for the W7-AS #49965 shot (R0 = 2.05 m, a = 0.17 m, B = 2.55 T, ι = 0.35,Te(0) = 1.6 keV, ne(0) = 5.2⋅1019 m-3, PECRH = 900 kW, dp = 0.395 mm, vp = 250 m/s. InFig. 1e, an ‘integrated’ photo, obtained using scheme (1), is shown. The magnetic field linedirection on the image plane is roughly vertical, and the direction of pellet velocity is shownCoordinate along the pellet path (cm)-12 -11 -10 -9 -8 -7 -6 -5 -4 -3 -2 -1 0 1 2dN/dt (1021 at/sec)051015snap-shotvessel centreseparatrixTransverse coordinate (cm)-1,0 -0,5 0,0 0,5 1,0Intensity (a.u.)0,00,51,0Real profile Mirrored profileTransverse coordinate (cm)-1,0 -0,5 0,0 0,5 1,0Intensity (a.u.)0,00,51,0Real profile Mirrored profilea) b)c)e)f)1 cm1 cm 1 cmvp = 250 m/secd)topbottomperiphery centre,periphery centrevpFig. 1. Images of the pellet ablation cloud #49965 and intensity profiles: (a) and (b) – inten-sity profiles in transverse direction, (c) and (d)–snapshots from behind and from below, (e) –integrated photo from below, (f) – pellet ablation rate profile deduced from photo (e).by the arrow. At each moment, the cloud is assumed to be elongated in the magnetic field di-rection and the pellet is being situated in the centre of such cloud [3]. The ablated materialspreads along the local magnetic surface, and the instant total cloud light emission is sup-rotatedby 93.3oclockwiseposed to be proportional to the pellet ablation rate [3] that allowed us to evaluate the ablationrate profile shown in Fig. 1f. The moment of the two snap-shot photos made using scheme (3)is denoted by the vertical dotted line in Figs. 1f and 1e. The instant photo shown in Fig. 1dwas made from the same direction as the integrated photo. The instant photo in Fig. 1c wasmade almost from the rear side of the pellet path. The magnetic field line direction in Figs. 1c,1d is vertical. The thin dotted lines in Figs. 1c,d denote the direction perpendicular to themagnetic field direction. Solid lines in Figs. 1a and 1b show cloud radiation intensity profilesalong these lines correspondingly.The cloud in Fig. 1c seems symmetric, while in Fig. 1d a cloud asymmetry is visible.Dots define positions where the integrals of the cloud intensities along a horizontal line areequal on the left and right side. They form an “averaged line” which is almost straight inFig. 1c and trends to the plasma center in Fig. 1d. Furthermore, one can see that the profile inFig. 1a is symmetric in the transverse direction, while the profile in Fig. 1b is asymmetric – itsright wing decays slower than the left wing. To make this difference more evident, a part ofthe left wing was mirrored and plotted by dotted line in Fig. 1a,b. The cloud symmetry in thevertical direction and asymmetry in the radial direction are typical and well reproduced in anumber of shots.The asymmetry of the snap-shot clouds in the radial direction discussed above may beexplained by the radial drift of the cloud plasma towards the plasma centre. The direction ofthis drift differs from results of similar experiments on tokamaks, where the hydrogen cloudplasma drifts in the opposite direction (i.e. outwards the plasma core) have been observed [4],[5]. The tilt angle of the ‘average line’ and the assumption of sonic speed of the cloud plasmawith temperature Tcl ≈ 1 eV allows us to estimate a drift velocity of ~1 km/sec which is oneorder of magnitude less than those evaluated in [4], [5].Cloud toroidal decay length. The cloud decay length λ in the magnetic field direction hasbeen evaluated using photos obtained in CII lines. For this purpose, the wings of the cloudintensity profiles along the magnetic field have been fitted with an exponential function.Measured λ profiles for the W7-AS shot #43580 (ne(0) = 6.2⋅1013 cm-3, Te(0) = 1.4 keV,PECRH = 410 kW, dp = 0.4 mm, vp ≈ 300 m/s) are shown in Fig. 2 by the solid line (scheme(1)) and triangles (scheme (2)). The longitudinal CII cloud decay length values have an orderof several millimeters. The CII ionization length as the sum of ionization lengths for neutralsand the first carbon ions was calculated using the ionization cross-sections of Ref. [6] and theTe, Ne profiles of ambient plasma. The cloud velocity was taken as the sonic speed with cloudplasma temperature Tcl ≈ 1 eV thatcould be derived from simulationresults of carbon pellet cloud pa-rameters in W7-AS plasma [7]. Thecalculated CII ionization length pro-file is shown in Fig. 2 by the dottedline. The ionization length valuesagree rather well with the cloud de-cay length values. In the quasi-3Dpellet ablation code [7], only ioniza-tion by cold cloud electrons is con-sidered in the pellet cloud. Thedashed line in Fig. 2 shows evaluation of the CII ionization length by cold cloud electronswith maximum carbon pellet cloud parameters Tcl = 3 eV and ncl = 3⋅1016 cm-3 simulated inRef. [7]. This ionization length due to cold cloud electrons is substantially greater then thedecay length λ  measured in experiment and those calculated for ambient plasma electrons. Itsuggests that an influence of the cold (Tcl ≤ 3 eV) cloud electrons on the ablatant ionizationmight be negligible. However, we should note that the role of cold electrons in the cloud ioni-zation essentially depends on the Tcl values. For instance, for Tcl =4-5 eV and ncl = 3⋅1016 cm-3the CII ionization length by cold cloud electrons becomes comparable or even less then decaylength values measured in experiment.Summary. The structure of carbon pellet ablation CII clouds in W7-AS experiments has beenstudied. The cloud longitudinal decay length values (of order of several millimeters) generallyagree with those calculated assuming the ionization of carbon cloud ablatant by hot plasmaelectrons. Snapshots of the CII clouds reveal a radial cloud asymmetry that is likely due to aradial cloud drift in plasma center direction with velocities of about 1 km/sec.References[1] Milora S.L. et al. Nucl. Fusion 35 (1995) 657.[2] B.V. Kuteev. Rus. J. Technical Physics 44 (1999) 1058.[3] L. Ledl. IPP Report III-257 February 1999.[4] J. de Kloe et al. Phys. Rev. Lett. 82 (1999) 2685[5] Mueller H.W. et al. Phys. Rev. Lett. 83(1999) 2199.[6] K.L. Bell, et.al., J. Phys. Chem. Ref. Data 12 (1983) 891.[7] L.L. Lengyel  et al. Nuclear Fusion 39 (1999) 791.Coordinate along the pellet path (cm)-16 -14 -12 -10 -8 -6 -4Ionization and decay lengths (cm)0246ionization length by hot electronsionization length by cold electrons decay length from ’integrated’ photodecay length from snapshot photoseparatrixFig.2. Cloud decay length and ionization length pro-files for W7-AS shot #43580.

Studies of C-Pellet Ablation Cloud Structure on Wendelstein 7-AS Stellarator

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Studies of C-Pellet Ablation Cloud Structure on Wendelstein 7-AS Stellarator

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