Within the range of plasma shapes and plasma currents investigated, the electron confinement time, Tau_E increases with density, elongation and negative triangularity (-0.4<delta<+0.4), similar to Ohmic heating (in these low density discharges). In addition, TauEe increases with q_a up to q_a~5 after which it decreases. There is little dependence of TauEe on the heating location provided it is inside the q= I surface. As the heating location is moved outside the q=l surface, TauEe decreases. This may be the explanation of the observed decrease in TauEe at high q_a. The power-induced degradation exponent found is generally as expected: alpaha_P = -0.5

Behn, Roland

Coda, Stefano

Dutch, Michael

Duval, Basil P.

Esipchuk, Y.V.

Furno, Ivo

Goodman, Timothy

Henderson, Mark

Hofmann, Ferdinand

Hogge, Jean-Philippe

Lister, Jonathan Bryan

Llobet, Xavier

Martin, Yves

Martynov, Andrei A.

Moret, Jean-Marc

Nieswand, Christoph

Pietrzyk, Adam

Pochelon, Antoine

Reimerdes, Holger

Rommers, Jeroen Hubert

Sauter, Olivier

Tonetti, Gilbert

Tran, Minh Quang

van Toledo, Wiebo

Weisen, Henri

Infoscience - École polytechnique fédérale de Lausanne

2nd Europhysics Topical Conf. on RF Heating of Fusion Devices,Brussels, January 1998, Vol. 22A, 253Preliminary Confinement Studies during ECRH in TCVA. Pochelon, Z.A. Pietrzyk, T.P. Goodman, M. Henderson, H. Reimerdes, M.Q. Tran,R. Behn, S. Coda, M.J. Dutch, B.P. Duval, I. Furno, F. Hofmann, J-P. Hogge, J.B. Lister,X. Llobet, Y. Martin, J-M. Moret, Ch. Nieswand, J. Rommers, O. Sauter, W. van Toledo,G. Tonetti, H. WeisenY.V. Esipchuk†, A.A. Martynov†Centre de Recherches en Physique des Plasmas, Ecole Polytechnique Fédérale de LausanneAssociation EURATOM-Confédération Suisse, CH-1015 Lausanne, Switzerland† Russian Research Centre Kurchatov, 12182 Moscow, RFIntroduction - Following extensive Ohmic hea-ting confinement studies on various plasma shapes[Moret 97], ECRH heated experiments werestarted on small, low elongation plasmas in theTCV tokamak with positive and negativetriangularities. For these studies, an EC power ofup to 1 MW at 82.7 GHz (second harmonic X2)was injected, representing for such discharges upto 14 times the Ohmic power during EC [Pochelon97]. When complete, the ECW system willprovide 3 MW second harmonic and 1.5 MW thirdharmonic at pulse lengths of 2 seconds [Goodman97]. For TCV (R = 0.89 m, a = 0.25 m,Ip < 1.2MA), the nominal field of B = 1.44 T andthe frequency of 82.7 GHz place the resonanceslightly on the high-field side (HFS) of themagnetic axis (∆ρ=-0.16 to -0.2). The effects ofdifferent heating localisations on confinement andMHD were studied using 1) the mobility of thelauncher mirrors (in particular for poloidal angle θsweeps), 2) the large vertical room for displacingthe plasma in the TCV vessel and 3) the radialdisplacement of the cyclotron resonance withmagnetic field. Initial studies of heating andconfinement have con-centrated on close-to-circular plasmas to study power absorption in alarge variety of beam-plasma geometries and tocompare with earlier results of other machines - anecessary first step before investigating morestrongly shaped plasmas.Confinement - To study basic confinementdependencies (B, q, ne, P), second harmonic ECpower was launched on a small target plasmaplaced in front of the launcher with quasi-horizontal launch in order to minimise refractioneffects at high density (θ=18o, z=38, κ=1.31,00.20.40.60.811.2-8 -6 -4 -2 0 2 4 6 8∆Te/TeRres -Rinv [ cm ]a)-0.7-0.6-0.5-0.4-0.3-0.2-8 -6 -4 -2 0 2 4 6 8∆τEe/ τEeRres -Rinv [ cm ]b)11.522.533.544.50.78 0.8 0.82 0.84sawtooth period [ ms ]resonance position [ m ]inversionc)Fig. 1 Radial deposition scan relative to q=1surface by varying B:a) Relative Teo increase, 2<q<6b) Relative global τEe degradationc) Sawtooth period, q=1 position indicated(κ=1.32, q=5, neo=2 1019[m-3], PEC=360kW)2nd Europhysics Topical Conf. on RF Heating of Fusion Devices,Brussels, January 1998, Vol. 22A, 253δ=0.15). Scan domains were: 1.32<B<1.45T,representing a 20% plasma radius displacement of the resonance around q=1;1.5×1019m-3 < neo < over-dense (X2 cut-off density is 4.25×1019m-3, TORAY guaranteedfull absorption up to 4.0×1019m-3); 2.2<q<6; PEC≤1MW. In the q-scan, ratios of Ptot/POHfrom 3 to 14 were obtained during ECRH for 1MW power injection.The effects of localisation are very important during ECRH. As an example, in a B-field scan at moderate power (0.5MW), the temperature response depends strongly on thelocation of power deposition relative to the inversion surface obtained from soft X-raytomography (often assumed equal to the q=1 surface). The central electron temperature Teo ismaximum for a power deposition close to the q=1 surface (Fig. 1a); although some low q~2.5discharges may show some increase for very central heating. The global electron confinementtime τEe (Fig 1b) is relatively independent of resonance position when heating inside q=1 anddrops from about a/10 outside the inversion surface, meaning that efficient heating requiresdeposition roughly inside q=1. Data in Fig. 1 a,b) cover a large range of q values: 2.3<q<6.7.Some low-q high-power shots may indicate a τEe maximum (as with Teo) for q~1 deposition.It is however not yet completely clear whether deposition at the magnetic axis is less efficientthan heating at the q=1 surface. Nevertheless, the sawtooth period is maximum for q~1deposition (Fig. 1c), which yields a dependence similar to the central electron temperatureresponse [Pietrzyk 98].As a function of the safety factor q, theconfinement time τEe increases with q,maximising at q~5 and dropping beyond(Fig. 2). In the high q shots of this scan(PEC=500kW, 0.77<Rres<0.84 m), the powerwas deposited significantly outside the q=1surface when increasing q. The drop at highq can probably be explained by analogy tothe drop observed in the radial B-field scan(Fig. 1b) when heating outside q=1. Figure 2also contains the corresponding ohmic-confinement-time q dependence, which satu-rates at q>5. Even though confinementexhibits the Neo-Alcator q scaling as inohmic plasmas [Weisen 97, Moret 97],confinement improvement with q is not ashigh as for ohmic heating and confinementpower degradation appears stronger at highq. It is likely that higher additional powerwill further degrade the favourable scalingwith q. In the present heating experiments,however, this beneficial ohmic scalingfeature still holds for ECRH power ratiosPECRH / POH > 9.The dependence of τEe as a function ofthe density on axis is shown in Fig. 3. Lowelongation plasmas (κ=1.16 and 1.32) withq≈5, located in front of the launcher to mini-mise refraction effects at high density, werechosen, for this scan. Heating just below345678910112.533.544.555.562 3 4 5 6 7OH only ECRH res 0.77-0.84 mτ Ee [ ms ] τEe [ ms ]qaOH-onlyECRH  Fig. 2    τEe versus q, for different deposition radii               in a Bϕ scan in ECRH, and ohmic plasmas.024680 1 1019 2 1019 3 1019τ Ee [ ms ]ne(0)  [ m-3 ]κ=1.16κ=1.32Fig. 3   τ Ee as a function of neo for twoelongations (κ=1.16, 1.32; δ=0.07, 0.16, q=4.7, 5.2)2nd Europhysics Topical Conf. on RF Heating of Fusion Devices,Brussels, January 1998, Vol. 22A, 253cut-off density appeared operationally difficult due to the ECRH-induced density pump-out,which explains the absence of data close to cut-off. The confinement time scales nearlylinearly with density, as in previous low density EC experiments [e.g. Alikaev 85]. The twoelongations covered indicate the favourable effect of elongation.4681012140 2 105 4 105 6 105 8 105 1 106 1.2 1063 10192 1019τ Ee [ ms ]total power [ W ]τE e=cPαpαP=-0.49αP=-0.41Fig. 4  τEe power degradation for two densities, q=5Power-induced confinementdegrad-ation was studied with 1MW ofadditional power. Heating results withPE C >500kW contain an ECCDcomponent since one of the launchers wasplaced with a non-zero toroidal angle.This should, however, have a limitedeffect on confinement, since powerdegradation in ECCD was observed to besimilar to ECRH [Esipchuk 95]. Thepower-induced degradation exponent αP,(τEe~PαP), measured for q=5 at neo=2 and3×1019m-3 (see Fig. 4), and at q=2.5 for adensity close to 2×1019m-3, is as expected:αP ~ -0.5, or possibly somewhat lower.Note that this confinement degradation may have been influenced by the effect of slightly off-axis heating on profiles and MHD-activity. In particular, the sawtooth period is lengthened fora deposition close to q=1, and sawtooth activity is observed to disappear at the higher.Summarising, this yields density and power exponents comparable to the ones obtainedin T-10 electron cyclotron heating experiments: αn ~ 1, αP ~ -0.5 [see Alikaev 85].The study of confinement with triangularity was done with medium q=3.5 at 500kW.The power was deposited at the q=1 surface. Both ECRH and ohmic cases are shown inFig 5a. As with ohmic heating, confinement improves with increasing negative triangularityduring ECRH. Note that the improvement of confinement with ECRH is less than with purelyohmic heating, as shown in Fig. 5b.24681012-0.5 0 0.5shot13017 - 13024ECRH 500kWOHτ Ee [ ms ]δa Fig. 5a) τEe as a function of δ, ohmic and ECRH data.    (PEC=500 kW, κ=1.32, q=3.5, neo=2 1019 [m-3]).-0.6-0.5-0.4-0.3-0.2-0.5 -0.25 0 0.25 0.5∆τEe/τEeδab) Relative τEe power degradation as a function of δ    (same data as in Fig. 5a).2nd Europhysics Topical Conf. on RF Heating of Fusion Devices,Brussels, January 1998, Vol. 22A, 253Power coupling to the plasma - ECRH heated discharges have been simulated using theASTRA transport code. The T-10 transport model [Dnestrovskij 93] in the ASTRA code wasused without any change of transport model or coefficients and is found to reproduce thecentral temperature evolution of ohmic TCV discharges. In the ohmic regime, the measuredTCV temperature profiles Te(r) are also well described (accuracy ±10%) by the T-10 model(i.e. similar electron transport and confinement laws can be used). To simulate correctly themeasured central temperature during ECRH, it was necessary to use an absorption efficiencyfactor k~0.65 (where Pab = k Pinput or more correctly, Pab = k PabTORAY), as in other ECRHexperiments [Alikaev 85]. The T-10 model then correctly describes not only values andprofiles of Te(r) and ne(r) but also the temperature responses for different power depositionprofiles. Simulated discharges had sawteeth, q=3.1, κ=1.26, and oblique beam incidenceangles on the resonance: 15o<θ<45o. For a non-sawtoothing discharge with high q~6, κ=1.32,neo=2×1019m-3 in quasi-horizontal launch (θ=18o), where Teo=3keV was obtained with0.5MW, a higher k-factor, close to unity was appropriate.The derivative of the stored energy deduced from the diamagnetic probe yields powercoupling efficiencies similar to those from transport calculations, i.e. ranging approximatelyfrom 65 to 85%. Note that when transport calculations give good coupling, so do thediamagnetic probe measurements. The observation that the k-factor changes with theexperimental configuration, while the calculated absorption from ray tracing remains constant(100%) indicates a violation of the assumptions made either in TORAY or ASTRA codes, aswell as the diamagnetic loop method of determining the absorbed power.Conclusions - Heating effects, MHD-activity and confinement depend strongly on thelocalisation of the EC power deposition. Sawteeth are stabilised for power deposition on q=1and mode activity is destabilised for power deposition slightly inside the q=1 surface withcorresponding effects on electron temperature: dramatic changes in central temperatures occurwhen heating is close to q=1 or towards the magnetic axis.Within the range of plasma shapes and plasma currents investigated, the electronconfinement time, τEe, increases with density, elongation and negative triangularity(0.4>δ>-0.4), similar to Ohmic heating. In addition, τEe increases with qa up to qa~5 afterwhich it decreases. There is little dependence of τEe on the heating location provided it isinside the q=1 surface. As the heating location is moved outside the q=1 surface, τEedecreases. This may be the explanation of the observed decrease in τEe at high qa.The power-induced degradation exponent (τEe~PαP) is generally as expected: αP~ -0.5.However, there are some indications that the power degradation may be dependent on qa andδa. In particular the power degradation may be stronger at negative triangularity.This work was partially supported by the Fonds National Suisse pour la RechercheScientifique.ReferencesV.V. Alikaev et al., Proc. 10th Int. Conf. on Plasma Phys. Cont. Nucl. Fus., I, 419 (1985).Y.N. Dnestrovskij, D.P. Kostomarov, WO "Nauka", Moscow, 1993/ Springer Verlag 1986Y. Esipchuk et al., J. Moscow Phys. Soc. 1, 119 (1991) and ITER-IL-PH-4-9-5-10 (1991).Y. Esipchuk, Plasma Phys. and Contr. Fus. 37, Suppl. 11A, A267 (1995).T.P Goodman et al., Proc. EC-10 Conf., presented by M.Q. Tran, Ameland 1997.J.-M. Moret et al., Phys. Rev. Lett. 79, 2057 (1997).Z.A. Pietrzyk et al., this Conf.A. Pochelon et al., 24th EPS Conf. on contr. Fus. and Plasma Phys., 21A, PartII, 537(1997).H. Weisen et al., Nucl. Fus. 37, 1741 (1997).

Preliminary Confinement Studies during ECRH in TCV

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Preliminary Confinement Studies during ECRH in TCV

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