Balbin, R.

Bleuel, J.

Fiksel, G.

Hidalgo, C.

Milligen, B.

Niedermeyer, H.

Pedrosa, M.

Sanchez, E.

English

MPG.PuRe

1665Sheared Poloidal Flows and Turbulence in the EdgePlasma Region of Stellarator and Tokamak DevicesR. Balbfn. C. Hidalgo, E. Sa’nchez, MA. Pedrosa, B. van MiligenEurarom-Ciemat Association, Madrid, SpainJ. Bleuel, H. NiederrneyerEurarom-IPPAssocimimz, Garching, GennanyG, FikselDepartment ofPhysics, University of Wisconsin, USI. IntroductionIt is known that a poloidal shear flow in a magnetically confined plasma has ingeneral a stabilizing effect“. Recent calculations have shown that the dependence of thephase angle between density (fig) and electric field fluctuations ([339) on flow shear may bean important aspect of the E, X B shear flow suppressiou3. Simultaneously it is knownthat poloidal acceleration will occur if there are radial gradients in the turbulent Reynoldsstress“. The aim of the presented experiments is to investigate the effect of sheared Er x Bflow on turbulent fluxes (coherence and phase angles between density and electric fieldfluctuations) and the interaction between poloidal flows and turbulence. The radialbehaviour of the Reynolds stress (both electrostatic and magnetic) in the proximity of thevelocity shear layer in the TJ-TU torsatron has been investigated.The turbulence (fluctuation levels, cross phase, cross coherence) in the proximity ofthe naturally occum'ng velocity shear layer in the TM tokamak (R = 0.3 in, E = 0.1 m, fie= (l — 3) x 1019 m3. 3,: 1 T, I}, z 30 kA), in the TJ-lU torsatron (PECt: 200 kW, t(0) =0.23, R z 0.6 m, a = 0.1 In, fig 2 0.5 X 1019 1113,1312 0.67 T) and in the W7-AS stellarator(P551241: (200 - 400) kW, t(0) = (0.24 ~ 034), R = 2 m, 5 z 0.17 m, fie = 2 x 1019 m'3, Br= (1.25 - 2.56) T) have been studied.11. Turbulence and sheared poloidal flowsData analysis has focused in the evaluation of the coherence and phase anglebetween E9 and tie at the plasma edge of the W7~AS stellarator. Fig, 1a shows the radialprofiles of mean values of ion saturation current (1“,) and floating potential (dfl).measured with a fast reciprocating Langmuir probe and referred to the velocity shear layerposition. The fluctuating poloidal electric field (EB) is calculated as the difference of two¢fl separated 4 mm in the poloidal direction and the density fluctuations are taken to beproportional to the 1m assuming negligible temperature fluctuations. Fig. lb shows the1666a) 40 . . . . . . 0.08K :‘N‘n- ‘o . ' <__'ut«0.069 - E, - EHV: -40- ¢ -0-04 A9 , O _ a-so- i -0.02’9 . can“ 'b) 1 . . . . . .Au 06.59 " _ 4 <€3— _ gymsooga 410%3 0.6 F ° ‘ A8 ‘45 . aE ' Q . 53D A1:9 0.2 g .4104 aC) ‘ o'ccis—E) ' ' '6,3 0,6 ' o C( (is) co ‘g _ v C( 1—¢2) Devan5 O 4 _W' 9 .E 'WW' °'. '‘ CE 0-2 ‘m M 'd) I . . . . . 0Q . mfl— @-ea 06. ° .(‘*””°"‘ '1 51° ' m a"E: _ fife ‘2 inD v.1: a8 ozr :5;- -30.9 l 1.1 1.2 F. L»ShutFigure 1: Radial profiles of plasmaparameters measured with a fast reciprocatingLangmuir probe at the plasma edge of W7-ASduring a typical shot with 1.26 T and 180 kWECR heatingpoloidal coherence of the rim measuredwith two tips placed 4 mm apart in thepoloidal direction as well as phasevelocity of fluctuations (vph) deducedfrom the tbfl measurements. Asexpected, the poloidal coherence (fig1.b) and rms crofiswcorrelationC(¢1—¢2)= Cilia”) , ~be:iGian)=(ntr)¢2<r+ri)/cr(¢1)a(¢z)(fig. 1.e) between floating potentialsignals decrease at the velocity shearlayer location where vph z 0. It isinteresting to note that the rms value ofcross correlation Chair) (fig. 1.c)shows a minimum at the radial locationr/as = 1.05 that cannot be explained asa Doppler shift effect because in theradial region 1< r/as S 1.15 the VP}, israther constant within the error bars.Note that ism and 1739 are measured atexactly the same poloidal position. Fig.1d shows the coherence between 1m,and fig. and the cross phase betweenthese magnitudes. A decorrelation and aphase change between fie and is areobserved related to the maximumchange in Er, whereas the level offluctuations does not significantlychange. The spectrally resolved analysisshows that these changes occur mainlyfor frequencies above 25 kHz.Statistical analysis of theprobability distribution function (PDF)of the 17m, signal has been done and a clear departure from gaussian behavior is obserVedin the SOL. In the plasma edge the PDF tends to show a more gaussian behavior, inconsistency with previous experiments in the TH tokamak and the TJ-IU torsatrons.a) 4 fl"3 2 .C10 A:a o.-7- . . . l ,-1.74 -l.'i v1.66time (ms)0)Es.2v3a:.§ooos , '0.0 0 2.00ampim de t s igmacondlio nd average [rt a shear ; 1.04)4.44 714 -236time. (ms)430condltlo rid average (rt a shear . 0.93)Figure 2: Normalized Isa: raw data and its conditional average at tworadial positions at the SOL (a) and the plasma edge (b) in the TJ—I tokamak. Atthe SOL burst of turbulence growing faster than decaying are clearly observed.The conditional average plot abciss (c and (1) display the amplitude of thepulses and ordinate the time delay with respect to the position of themaximamflninimam of the pulse. The raw data has been normalized removingthe DC component and making the rms equal 1.In the TJ-i tokamak the structure of the is“, fluctuations is strongly intermittent inthe proximity of the velocity shear layer. That might suggest a competition between the100 tililiiilIltlllll)iillllllAl|lllIAEV —r/a=l.125——-r/a=1.08:100 ""'r/a20.99......-.1-/a z: 0.93PDF10- a?urnrq1 H""i""l"""'i""'6 200 400 600Pulse AreaFig 3. PDF of the is“, pulse timeintegral at three radial positions in W7~iAS showing an exponential decay athigh amplitudesdriving and damping mechanisms ofturbulence. A systematic analysis of thedegree of asymmetry in the shape of thetemporal pulses using a conditionalaveraging method has been done. Fig. 2shows the raw data at two radial positions,the SOL and the plasma edge, and theresulting statistical average over differentamplitudes of the pulses, The asymmetry(in the sense that low frequency pulseshave faster growth than decay timeconstants) is significant in the SOL Whilethe asymmetry disappears inside of thevelocity shear layer. Similar features have1668been found in the TJ—IU torsatron and the W7—AS steilarator.The better statistics of data from W7—AS have allowed the study of the PDF of pulsetime integrals (areas). The decay of the PDF is aproxirnately exponential outside thevelocity shear layer and at the plasma edge as can be seen in Fig. 3.II]. Reynolds stress measurements in the edge region of the TJ-IU torsatrona) 3 .a . : «-g 2- O -1v 0.A- um“ ' O oLU“ 1-0 000 g “V .. e g .O z Ib) 1105 t Y ..i’5 ' o 0"a ° -8°°3 0 , " aE 03%9':> _ . _4105 - - . =~20 O 20r — 35m (mm)Fig. 4 Radial profiles afReynoldsstress and velocity propagation offluctuations measured a! the TJ~IUrorsarron using a radialwpoloidal army ofLangmuir probes.The radial profile of the cross-correlation between E]; and E), and 39and Br have been investigated in the TJ-lUtorsatron. The coherence between 39 andEr is quite high (close to 0.8 forfrequencies below 70 kHz). The magneticcomponent of the Reynolds tensor,proportional to <E9Br >, was measuredand found to be negligible due the lowfluctuatiOn amplitude and the fact that thepoloidai and the radial component of thefluctuating magnetic field are shifted byME. Fig. 4 shows the radial profile of(1321739) (Fig. 4a) and W (Fig. 4b)obtained from two floating potentialmeasurements of tips placed 2 mm apart inthe poloidal direction and 6 mm in theradial direction. It can be argued that theradial derivative of (E, EQ> evolves in theproximity of the velocity shear positionFurther analysis of the experimental resultsare in progress to quantify the importance of poloidal flow driven by (electrostatic)Reynolds stress.1T. Chiuen, P.Wl Terry, PH. Diamond and J .E. Sedlak. Phys. Fluids 29, 231 (1986)2 Kit Burreil, EJ. Doyle, P. Gohil et al. Phys. Plasmas 1. l536 (1994)3 PW. Terry, et al. Proc. 16th IAEA (IAEA—CN-64lDP-20) (1996)4 PH. Diamond and YB. Kim. Phys. Fluids BS, 1626 (1991)5 E. Sinchez, C. Hidalgo. M.A. Pedrosa. R. Balbin, I Garcia-Cortes, T. Estrada and B. vanMilligen. Transport, Chaos and Plasma Physics 2. S. Benkadda, F. Doveil and Y. Elskens EdsiWorld Scientific Publishing Co. (1996)

Sheared Poliodal Flows and Turbulence in the Edge Plasma Region of Stellarator and Tokamak Devices

Schittenhelm, M.

Bartiromo, R.

Wagner, F.

http://hdl.handle.net/21.11116/0000-000A-969C-D

Sheared Poliodal Flows and Turbulence in the Edge Plasma Region of Stellarator and Tokamak Devices

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