Andrew, P.

Contributors to the EFDA-JET Work Programme

Dux, R.

Giroud, C.

Morgan, P.

O'Mullane, M.

Tresset, G.

Whiteford, A. D.

Zastrow, K.-D.

English

Zastrow, K.

Whiteford, A.

MPG.PuRe

Neon transport in JET plasmas close to ITB formation with monotonicand reversed q-profiles.C. Giroud1, K-D. Zastrow2, P. Andrew2, R. Dux3, P. Morgan2, M O’Mullane4, G. Tresset1,A. D. Whiteford 4 and the contributors to the EDFA-JET workprogramme*1Association EURATOM-CEA sur la fusion , CEA Cadarache, F13108, St Paul lez Durance, France.2 EURATOM/UKAEA Fusion Association, Culham Science Centre, Abingdon OX14 3DB, U.K.3Max Planck Institut f r Plasmaphysik, EURATOM Association, Garching, Germany.4 Department of Physics and Applied Physics, University of Strathclyde, Glasgow, United Kingdom* See appendix of J. Pamela, Proc. IAEA Conf. (Sorrento 2000)1. IntroductionRecently, optimised shear research at JET has concentrated on two different types ofscenario: one with LHCD preheat resulting in a negative magnetic shear and the other withan ohmic preheat leading to a weak positive magnetic shear.  The ITB formation andcharacteristics of these two types of discharge have been studied by systematic variation ofinput power and torque [1]. In this paper, we study the impact on impurity transport ofscenarios for weak positive and negative magnetic shear ITB plasmas.2. Experimental techniqueIn this series of discharges, small quantities of neon (less than 0.1%) have been injectedduring the whole duration of the main heating phase. Ne was injected by edge gas fuellinglocated on the machine midplane. The valve opening had to be modulated for the gasfuelling system to  deliver such a small amount on average. Neon edge lines were notdetected by VUV spectroscopy. Neon densities have been measured by charge-exchangerecombination spectroscopy (NeX, 5248.5 ).Impurity transport coefficients were deduced from a least-squares fit of neon densitiesmodelled by a 11/2D transport model code (SANCO) to the measured densities. The neoninflux to the plasma is not measured but can be simulated from the pressure drop in thereservoir and knowledge of the fuelling system geometry.  The Ne gas is assumed to diffusewith a flux proportional to grad(n2), n being the density of gas in the fuelling system.Another issue in the determination of transport coefficients is the least-squares fit program.The latter limits the definition of D and V/D profiles to a (changeable) grid of only 4 radialpositions connected by straight lines. The least-squares fit program has a constant D andV/D for  defined time windows. A smooth evolution of D and V/D from a time window tothe next one is not possible at present.Figure 2:  (from top to bottom) Simulated  neon gas influx to theplasma used in SANCO. ITB Criterion: contour plots of  ρ*T ,[2],showing the strength and position of the barrier:  the darker thecolour, the higher ρ*T , the stronger the ITB.  Defined time slicesfor χ2 fit: the coloured rectangles indicate the time slices used  inthe χ2 fit. Time evolution of NeX density measured by CXRS inthe different CCD tracks (in pink) compared to SANCO modelleddensity (in black). Ψp is the normalised poloidal fluxFigure 3:  (from top to bottom) Profiles of  thediffusion coefficient (D) and V/D against √Ψpn usedin the χ2 fit, V being the convection coefficient andΨpn  the normalised poloidal flux . Typical measuredand modelled NeX densities against √Ψpn during thedifferent time slices. Tracks plotted without error barswere not considered in the χ2 fit.                                                            #51573Figure 1:  Time evolution of the pulse #51573Figure 5:  As in figure 2 for #51598 Figure 6:  As in figure 3 for #51598Figure 4:  Time evolution of the pulse #51598                                                             #515983. Determination of impurity transport coefficientsThe transport coefficients are determined for pulses #51573 and #51598. These dischargescorrespond to a negative and weak positive magnetic shear ITB plasma, respectively, closeto ITB formation at 17MW input power with a dominant normal beam heating. Figures 2, 3,5 and 6 show the comparison of the experimental data with the SANCO modelled densities.Justification for the fit #51573: The delayed appearance of neon in tracks 8, 9 and 10compared to the rest of the tracks shows a strongly reduced diffusion coefficient in theplasma core. D is determined at ~44.5s and kept fixed for the rest of the discharge.  Apositive barrier for V/D is required at √Ψpn~0.45 to reproduce the steep gradient betweentracks 6,7 and 8, and a negative convection is needed for √Ψpn  ≤ 0.45 to explain the  timedependence of the density in tracks 8,9 and 10.  A close look at the time dependence of  Nedensity in each track gives information of the ITB time behaviour. The sequence of events,seen in figure 2, can be explained by an expanding positive  (outward convection )V/Dregion, as shown in figure 3.Justification for the fit #51598: No delay in the appearance of Ne between tracks isobserved. The diffusion is therefore kept constant in space, determined at ~ 44.2s and keptfixed in time. Here again, a positive barrier for V/D is required at √Ψpn~0.4 to reproduce thesteep gradient between tracks 6,7 and 8, but for this pulse a positive convection is required at√Ψpn  ≤ 0.4 to reproduce the density time dependence in tracks 8,9 and 10. Here again, anexpanding V/D positive (outward convection) region can explain the density time evolutionin each track.4. ConclusionWe show clear evidence of the dependence on impurity transport of the chosen magneticshear scenario. Neon accumulates in reversed magnetic shear ITB plasmas, due to inwardconvection, but not in weak positive shear ITB plasmas, where convection is outwards.Similar conclusions were drawn from the study of the equivalent discharges at 15 MW inputpower (#51594, #51595). This result is in agreement with the impurity behaviour in highperformance discharges [3]. The implications of this work are two-fold.  Firstly, there is littlehope in controlling the impurity content in negative magnetic shear ITB plasmas. Secondly,there will be no impurity accumulation in weak positive magnetic shear ITB plasmas if theoutward convection can be maintained on a longer time scale.5. Reference[1] C.Challis, et al., at this conference. [3] R. Dux, et al, at this conference[2] G. Tresset, et al, at this conference.

Neon Transport in JET Plasmas Close to ITB Formation with Monotonic and Reversed q-Profiles

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Neon Transport in JET Plasmas Close to ITB Formation with Monotonic and Reversed q-Profiles

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