Fable, E.

Marinoni, A.

Sauter, O.

Weisen, H.

Zabolotsky, A.

Infoscience - École polytechnique fédérale de Lausanne

Density behavior during electron internal transport barriers in TCV fullynon inductive dischargesE. Fable, O. Sauter, A. Zabolotsky, H. Weisen and A. MarinoniEcole Polytechnique Fédérale de Lausanne (EPFL)Centre de Recherches en Physique des Plasmas (CRPP)Association EURATOM - Confédération SuisseCH - 1015 Lausanne, SwitzerlandAbstractExperimental results of particle transport during electron internal transport barriers(eITBs) achieved in the TCV device (Tokamak à Configuration Variable), in fully noninductive discharges, are shown. A database of steady state density profiles is constructedand studied in detail. An interesting relation between density and temperature scale lengthsis observed. In the fully non inductive eITB scenario, the density profile is observed todevelop a localized gradient, correlated via an almost constant factor with the electron tem-perature scale length up to the barrier foot. Sensitiveness of this behavior to the degree ofthe local confinement improvement is demonstrated by the study of the density response ineITBs with applied inductive current perturbations. The results shown have an impact alsoon the predictions about impurity transport in eITBs, and generally for fully non inductiveoperations in future machines.IntroductionParticle transport observed in Ohmic L-mode and with central electron cyclotron heating hasbeen investigated in several works ([1],[2] and references therein). Understanding the behaviorof density profiles in eITB scenarios with fully non inductive current density, is an important is-sue both experimentally and theoretically. In TCV, thanks to the powerful and versatile electroncyclotron heating system (up to 2.7 MW at 82.7 GHz in X2 mode) high performance eITBsare routinely produced [3]. The fully non inductive discharges presented here are characterizedby the total replacement of the Ohmic current with off-axis electron cyclotron current drive(ECCD) and a high fraction of bootstrap current produced thanks to the improvement in theglobal energy confinement. The resultant current density profile is hollow and peaks off-axis.Experimental observations: overviewThe database under consideration covers the following parameter range: Ip ∼ 70− 100kA,PEC = 0.9−2.3MW , ρEC ∼ 0.3−0.7, q95 ∼ 8−17 and < ne >∼ 0.2−1.1[1019m−3]; in theseplasmas collisionality is low, i.e. νe f f < 0.1, where νe f f ≡ νei/(εcs/a), cs is the ion soundvelocity and a is the plasma minor radius. If not explicitly shown, estimated error bars onthe gradients are about 20%, while for profiles they are 5− 10%. Radial dependent quantitiesare averaged in 0.2 < ρ < 0.6 where ρ is the normalized poloidal flux coordinate. Fig. (1a)shows that the normalized electron temperature and density gradients are proportional with aratio approaching ∼ 0.5 as the eITB becomes stronger; the strength of the eITB is definedas simultaneous high R/LTe ≡ R0<|∇Te|>Te and high HRLW ≡τexpEτRLWE, where τEXPE is the energyconfinement time and it is compared with the Rebut-Lallia-Watkins scaling [4]. In fig. (1b) wecompare the ratio of the logarithmic gradients, 1/ηe ≡ LTe/Lne , with the Ohmic phase.9 10 11 12 13 14 15 16456789R/LTeR/LneHRLW < 33 < HRLW < 3.5HRLW > 3.54 6 8 10 12 14 1600.20.40.60.81R/LTe1/ηeOhmicHRLW < 33 < HRLW < 3.5HRLW > 3.5Figure 1: a) RLne vsRLTefor different values of HRLW , the magenta line has a slope of 0.5; b) 1/ηe ≡ ∂ lnne∂ lnTevs RLTefor different HRLW , Ohmic mode values span a much wider range (magenta circles).0 0.2 0.4 0.6 0.8 111.522.53ρψne/ne(0.85)averaged on t = [1.3 1.7]s#29863 #29866 0 0.2 0.4 0.6 0.8 100.511.52ρψ#29948 ; t=1.1−1.9sne [1019 m−3] Te/5 [keV] black dashed: (fit to Te)0.47 black solid: (fit to Te) cyan: (1/q)0.7 Figure 2: a) Thomson scattering raw data and fit for #29863 and #29866 with the same initial conditionsbut without (blue) and with strong (red) eITB respectively. Density profiles are normalized to the valueat ρ = 0.85; b) Thomson scattering data for electron density and temperature taken from a z-scan duringa steady state eITB of 800 ms.Analysis of profilesIn fig. (2a) we compare raw data and fit for profiles averaged over the steady state phase.Both are normalized to the value of the density at ρ = 0.85. The two discharges have the sameinitial conditions in the Ohmic phase, but have an additional small positive or negative Vloop inthe ECH/ECCD phase, resulting in a strong (red, jOH < 0) or no (blue, jOH > 0) eITB case.Fig. (2b) compares electron temperature and density profiles data taken from a steady stateeITB of ∼ 1s. Between 1.1s and 1.9s, a fine z scan (3.3cm/s) and a simultaneous ECH/ECCDsweeping has been applied, resulting in an increased spatial resolution for the Thomson pointswithout perturbing the eITB. The dashed black line shows the curve ne ∝ T γe , where the best fit isobtained with γ = 0.47. The edge density normalized gradient has been found to be proportionalto the edge q profile normalized gradient, indicating mainly a curvature pinch effect [5] asexpected in Ohmic L-mode.0 0.5 1 1.5 200.511.52TCV #29863time [s]ne / ne(0.7) 0.35  0.6 0.850.975ECH+ECCD 0 0.5 1 1.5 200.511.522.5TCV #29866time [s]ne / ne(0.7) 0.35  0.6 0.850.975ECH+ECCD eITB onset Figure 3: a) Time traces at different radial positions of the density profile divided by the value of thedensity at ρ = 0.7, for a case without an eITB; b) case with a strong eITB. The latter exhibits a cleardensity peaking while the first has central density decreasing.Study of time tracesIn fig. (3), time traces at different radial location of the density profile are shown for onedischarge with no barrier (3a), and for one with strong barrier (3b). The values are divided bythe value of the density at ρ = 0.7, such that we are observing a measure of the local densitypeaking factor. It is worth noting that, while in the L-mode no-eITB case the density profileflattens after ECH injection and does not rise again, in the case with strong eITB, after a firstphase of flattening, the peaking increases at t ∼ 1s, when the eITB forms, following the increasein electron temperature gradient. Due to the absence of particle source this demonstrates theusual outward convection in non-eITB cases and the presence of a dominant inward pinch ineITBs.Inductive current perturbation experimentsOhmic current perturbation experiments performed in TCV show that it is possible to controlthe barrier performance with a fine scan in loop voltage [6]. The Ohmic transformer is activatedsuch as to produce a small amount of inductive current to modify the degree of magnetic shearreversal inside the plasma core. The behavior of density respect to the inductive perturbationsis shown in fig. (4). In fig. (4a) we see that the presence of an Ohmic component can easilycompromise the barrier performance also resulting in a worse particle confinement via a strongflattening effect at positive Vloop. The time traces in fig. (4b) show the enhancement of the eITBwith a negative (counter) Ohmic perturbation. The density response is dictated by the electrontemperature behavior. On the other hand, if we assume neoclassical diffusivity levels insidethe barrier (up to the foot), yet a small inductive perturbation should result in a non negligibleWare pinch effect, contributing with an outward convection in the case with negative currentperturbation, but not seen in the cases studied here.ConclusionsAn overview of experimental results on density behavior during eITBs obtained in fully noninductive discharges in TCV has been presented. In this scenario the density profile becomesstrongly coupled to the electron temperature profile almost up to the foot of the barrier. As theeITB becomes stronger, the density naturally evolves toward a profile which replicates temper-ature such as to have the ratio of the two scale lengths 1/ηe ∼ 0.5. We mention here that this−0.05 0 0.05 0.1 0.1500.10.20.30.40.50.60.7Vloop [V]1/η eVloop scan−R∇Te/Te < 1010 < −R∇Te/Te < 15−R∇Te/Te > 150.5 1 1.5 205101520time [s]−R∇Te/Te−R∇ne/ne1/ηe*10Vloop*10 [V]EC power [MW]Figure 4: a) 1/ηe for a Vloop perturbation scan on non-inductive eITBs; b) Comparison of time traces fortwo cases with co (dashed, #29598 with Vloop ∼ 40mV ) and counter (solid, #29592 with Vloop ∼−40mV )Ohmic perturbation. With jOH(ρ = 0)> 0 (Vloop > 0), q becomes less reversed, while it becomes morereversed with jOH(ρ = 0)< 0 (Vloop < 0).ratio is also predicted by neoclassical theory in the form of an inward thermodiffusion effect(the value has only a weak dependence on the trapped particle fraction and on Ze f f ). This re-sult is supported by the analysis of the static database. The density response observed duringexperiments assessing the performance of the eITB with inductive current perturbations furtherdemonstrates the result. Theoretical modelling of microinstabilities and global transport mightthrow some light on the physics of the transition region where the eITB is forming, especiallywith regard to the role of the magnetic shear in changing the properties of particle and heat trans-port and the relations between them. Preliminary results indicate a transition from anomalousdriven to neoclassical driven thermodiffusion pinch although it is not still clear which level ofdiffusivity we have inside the eITB. To this purpose, dedicated experiments with deuterium andimpurity injection are planned. The role of the Ware pinch in the presence of inductive currentperturbations is observed to be negligible but further assessment is required.AcknowledgementsUseful discussions with Drs. C. Angioni and A. Bottino are acknowledged. We acknowledgealso Dr. R. Behn for providing good Thomson scattering data. This work was supported in partby the Swiss National Science Foundation.References[1] A. Zabolotsky et al., Plasma Phys. Contr. Fus. 45, 735 (2003)[2] C. Angioni et al., Nucl. Fus. 44, 827 (2004)[3] S. Coda et al., Phys. Plasmas 12, 056124 (2005).[4] P. H. Rebut et al., 1989 Proc. 12th Int. Conf. of Plasma Physics and Controlled FusionResearch (Nice, 1988) Vol 2 (Vienna: IAEA) pag. 191[5] D. R. Baker and M. N. Rosenbluth, Phys. Plasmas 5, 2936 (1998)[6] O. Sauter et al., Phys. Rev. Lett. 94, 105002 (2005)

Density behavior during electron internal transport barriers in TCV fully non inductive discharges

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Density behavior during electron internal transport barriers in TCV fully non inductive discharges

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