Behn, Roland

Duval, Basil

Induni, Gaël

Medvedev, S. Yu.

Pitzschke, Andreas

Porte, Laurie

Sauter, Olivier

Team, TCV

Infoscience - École polytechnique fédérale de Lausanne

Dynamics of pedestal profiles in ELMy H-mode plasmas in TCVat different collisionalitiesA. Pitzschke1, R. Behn1, B.P. Duval1, G. Induni1, S. Yu. Medvedev2, L. Porte1, O. Sauter1,and TCV team1 École Polytechnique Fédérale de Lausanne (EPFL),Centre de Recherches en Physique des Plasmas, Assoc. EURATOM-Confédération Suisse,CH-1015 Lausanne, Switzerland2 Keldysh Institute of Applied Mathematics, Russian Academy of Sciences,Moscow, Russian FederationThe dynamics of the profiles of electron temperature, density and pressure during H-modephases in the TCV tokamak has been investigated for cycles with ELMs of type I and typeIII, characterized by different regimes of collisionality (ν? = 10−14Ze f fR0neT−2e ) of the plasmanear the edge. Quasi-stationary H-mode phases with ELMs of type-III at ν? = 0.75 were alreadyobtained in TCV with ohmic heating only, whereas additional power from electron cyclotronheating (ECH) was necessary to reach a regime at lower collisionality (ν? = 0.40) with largeELMs of type I . The main plasma parameters like current, line-averaged density and shap-ing were identical for both series of shots analysed in this study (Ip : 370kA,BT : 1.43T,q95 :2.3,ne ·L : 3.2− 3.6 · 1019m−2,κ : 1.7,δ : 0.45) and a plasma configuration with single-nullX- point and ion (B×∇B)- drift in the favourable direction was chosen. For ECH at the 3rdharmonic, 2 gyrotrons operating at 118GHz injected a total power of 1MW launched in verticaldirection from the top of the TCV vessel. The profiles were measured using the Thomson scat-tering system on TCV, equipped with channels of high spatial resolution to resolve the expectedgradients in the so-called pedestal region. With an effective repetition rate of 60Hz set by theNd:YAG lasers, this diagnostic does not seem suitable to follow the time evolution during ELMcycles with a period in the range of 5 to 25ms. However, in cases with extended and quasi-stationary H-mode phases and for series of reproducible shots, the method of random samplingand coherent averaging can be used to reconstruct a typical ELM cycle with an effective timeresolution on the ms scale. For this analysis, the profiles measured on a vertical chord were pro-jected onto flux surfaces and then onto radial coordinates in the equatorial plane. After fitting byan analytical function (tanh) the profiles were characterised by a set of parameters to describetheir time evolution [1]. Other diagnostics, like ECE and Lithium ion beams, have been usedto measure edge profiles [2]. A major advantage of using Thomson scattering relies in the factthat electron temperature and density are obtained simultaneously and at the same locations.Therefore, this technique has also been applied on other machines [3].Profile evolution during the ELM cycleThe results for the case of type-III ELMs with ν? = 0.75 at the pedestal are shown in fig. 1.Although the analysis is based on many ELM events within a series of shots, due to goodreproducibility within the selected time windows, the major tendencies are clearly revealed. Acomplication arises from the fact that in these shots the central value of the safety factor wasclose to one and sawtooth activity is observed also during the ELMy H-mode phases. Althoughthe sawtooth period is shorter (by a factor of 2 to 3) than the ELM period, some signs of acorrelation between sawtooth and ELM events are seen (frame vi of fig. 1). During the pre-ELMphase, no significant variation of the profile parameters is observed, apart from a sudden rise inthe gradients during an interval of 1 to 2ms before the ELM event (identified as the maximumof the spike in the Dα emission). This seems to be correlated with the maximum probability fora sawtooth event, which may play the role of an ELM trigger. At the time of the ELM, a rapid38th EPS Conference on Plasma Physics (2011) P1.077collapse of the pedestal heights for temperature and density profiles occurs; at the same timethe edge gradient in the density and pressure profiles flatten. Central temperature and densityare not affected by the ELMs; their variation during the cycle can be attributed to correlatedsawtooth activity. Recovery of the profile parameters to their pre-ELM values is accomplishedafter 5 to 8ms, i.e. after about half of the ELM period.The same procedure has been applied to analyze the cases with ELMs of type I under con-ditions of additional ECH and the results are shown in fig. 2. The temperatures at the edgepedestal are significantly higher (500eV instead of 300eV) and Te,ped as well as ∇Te rise mono-tonically during the pre-ELM phase. Again, the ELM event provokes a sudden drop of Te,pedand ∇Te. The evolution of the density profiles is similar, but there is no increase of pedestal2040∇(T e)5101520∆ Te0.20.30.4T e,ped0.911.1T e(0)0.550.60.65<T e>vol−10 −5 0 5 1000.10.2P(ST)t−tELM [ms](i)(ii)(iii)(iv)(v)(vi)(a)24∇(n e)5101520∆ ne2345ne,ped5.566.57ne(0)44.555.5<ne>vol−10 −5 0 5 1000.10.2P(ST)t−tELM [ms](i)(ii)(iii)(iv)(v)(vi)(b)Figure 1: Time evolution of profile parameters during cycles with type-III ELMs(a) temperature, (b) density, (i) max. gradient, (ii) pedestal width, (iii) pedestal height,(iv) centre value, (v) vol.-averaged value, (vi) probability for sawtooth event050100150∇(T e)51015∆ Te0.40.60.8T e,ped1.522.53T e(0)11.5<T e>vol−15 −10 −5 0 5 10 1500.050.1P(ST)t−tELM [ms](iii)(iv)(v)(vi)(i)(ii)(a)05∇(n e)5101520∆ ne34ne,ped45ne(0)345<ne>vol−15 −10 −5 0 5 10 1500.050.1P(ST)t−tELM [ms](ii)(iii)(iv)(v)(vi)(i)(b)Figure 2: Time evolution of profile parameters during cycles with type-I ELMssame identification as in fig. 138th EPS Conference on Plasma Physics (2011) P1.077height nor max. gradient during the pre-ELM phase. Rather, the central ne(0) decreases slightlywhile Te(0) rises, which is interpreted as a consequence of pump-out due to central ECH. Thedecrease in ne,ped and even 〈ne〉vol due to the ELM are indicative of a significant loss of particlesfrom the plasma. The re-build of the profiles after the ELM event seems to be slightly faster forthe density than for the temperature profiles, with time constants varying from about 1/4 to 1/2of the ELM period.−20 −15 −10 −5 0 5 10 15 2000.10.20.30.40.50.60.70.80.9T e [keV]R−RLCFS [mm]  −9.000 ms−1.125 ms+2.000 ms+6.100 ms(a)0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 100.511.522.53T e [keV]ρψ  q~1 q~2−9.000 ms−1.125 ms+2.000 ms+6.100 ms(b)−20 −15 −10 −5 0 5 10 15 2000.511.522.533.54ne [1019 m−3 ]R−RLCFS [mm]  −9.000 ms−1.125 ms+2.000 ms+6.100 ms(c)0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 11.522.533.544.55ne [1019 m−3 ]ρψ  q~1 q~2−9.000 ms−1.125 ms+2.000 ms+6.100 ms(d)Figure 3: Profiles of Te and ne at selected timesduring a cycle with type-I ELMsAdditional insight into the timeevolution is gained from the rep-resentations of the fitted pro-files shown in fig. 3, both forthe pedestal region and the fullnormalized radius. In case ofthe large type-I ELMs obtainedby ECH, the collapse of thetemperature affects the full pro-file up to the centre, which ex-plains the rather high relative en-ergy loss per ELM of 36% forthe electron population (20% ac-cording to global measurementsby a diamagnetic loop). Anal-ysis of the soft X-ray emissiv-ity have shown that there is nomeasurable delay between thecrash seen near the edge andin the centre, which excludesthe implication of diffusive pro-cesses. Further investigations and simulations are necessary to clarify whether there isan interaction of MHD modes located near the edge and in the core, respectively.−0.500.5(R0−RLCFS ) / ∆−0.500.5(R0−RLCFS ) / ∆−15 −10 −5 0 5 10 15−0.500.5(R0−RLCFS ) / ∆t−tELM [ms]Tenepe(i)(ii)(iii)Figure 4: Displacement of edge barrier with respectto separatrix for type-I ELMsFrom this analysis, we also found indica-tions for a periodic displacement of the re-gion of maximum gradient relative to the sep-aratrix. The effect is more pronounced in thecase of type-I ELMs and may be described asan inward displacement during the pre-ELMphase, followed by fast motion in the oppo-site direction after the ELM event (see fig. 4,displacement normalized to pedestal width,scale relative to separatrix location RLCFS).The experimental data presented here pro-vide strong constraints on transport modelsdescribing the ELM collapse. Preliminary re-sults from a 1 1/2-D transport code (ASTRA)have shown that a simple increase of the localheat diffusivity is not sufficient to reproduce this complex behaviour.Stability limits according to ideal MHDOn the basis of the measured pressure profiles, the stability of the plasma edge against bal-looning and coupled kink-ballooning modes was investigated using KINX [4]. Since during38th EPS Conference on Plasma Physics (2011) P1.077this study, the CXRS diagnostic on TCV could not yet provide ion temperature and densityprofiles with sufficient temporal and spatial resolution, the total pressure profiles were obtainedby scaling of the electron component. However, the assumption of Te = Ti at the pedestal shoul-der has been confirmed by time-averaged CXRS measurements. As opposed to cases presentedearlier for type-III ELMs [1], the resistivities at ν? = 0.75 and in particular at ν? = 0.40 aresufficiently low to apply ideal MHD. Fig. 5 shows stability diagrams in the parameter space ofnormalized pressure gradient and normalized parallel current density near the edge. The edgecurrent density is dominated by the bootstrap current component, which was calculated fromthe density and temperature profiles using [5]. The diagrams reveal the stability limits set bythe pressure-driven high-n ballooning modes (solid red line) and the kink-ballooning modesat intermediate and low n, which become important with increasing edge current density. Thepoints marked by square symbols and labelled A, B, C, and D refer to specific conditions duringthe ELM cycle (A : time-average over the ELM cycle, B : -5ms before ELM, C : -1.5ms beforeELM, D : +1.5ms after ELM). In the case of ELMs at higher ν?, the operational point C is close0 0.5 1 1.5 2 2.5 3 3.5 400.511.5J ///<J>α13581015203040bootstrapreversed shearA BCD(a)0 0.5 1 1.5 2 2.5 3 3.5 400.511.5J ///<J>α13581015203040bootstrapreversed shearACDB(b)0 0.5 1 1.5 2 2.5 3 3.5 400.511.5J ///<J>α13581015203040bootstrapreversed shearADBC(c)Figure 5: Stability diagram for the plasma edge from ideal MHD calculationsReferring to : (a) type-III ELMs, (b) and (c) type-I ELMs.Stability limits : (red) high-n ballooning modes, (green, blue) kink-ballooning modesto the limit for high-n ballooning modes (Fig.5(a)). The excursions in J−α parameter spaceduring an ELM cycle remain small, leading to a shorter ELM period. Proximity to a stabilitylimit seems to be a necessary condition, but a clear threshold effect could not be identified. Forthe type-I ELMs in EC-heated TCV plasmas, the analysis needs to take into account the dis-placement of the gradient region with respect to the separatrix. Fig. 5(b) refers to a location ofthe maxima for p′ and bootstrap current at ρ0 = 0.99 whereas Fig. 5(c) has been obtained usingρ0 = 0.98, i.e. an inward shift in agreement with the experimental data (point C). In this case,access to the 2nd stability region permits higher max. pressure gradients and the stability limitsare set by medium-n kink-ballooning modes. These modes have wider radial extent than thoseresponsible for the type-III ELMs, which partially explains the higher energy and particle losses.This work was supported in part by the Swiss National Science Foundation.References[1] Behn R et al 2007 Plasma Phys. Control. Fusion 49 1289[2] Wolfrum E et al 2009 Plasma Phys. Control. Fusion 51 124057[3] Beurskens M N A et al 2009 Nucl. Fusion 49 125006[4] Degtyarev L et al 1997 Comput. Phys. Comm. 103 10[5] Sauter O et al 1999 Phys. of Plasmas 6 283438th EPS Conference on Plasma Physics (2011) P1.077

Dynamics of pedestal profiles in ELMy H-mode plasmas in TCV at different collisionalities

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Dynamics of pedestal profiles in ELMy H-mode plasmas in TCV at different collisionalities

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