Balet, B.

Clement, S.

de Esch, B.

de Haas, J.

Fishpool, G.

Haas, G.

Horton, L.

Lingertat, J.

Loarte, A.

Lowry, C.

Monk, R.

Nave, M.

Saibene, G.

Sartori, R.

Thomsen, K.

English

MPG.PuRe

49Steady State Humodes at high plasma density in JETo Saibene, B Balet, s Clement, B de Each, (3 Fishpool, J c M de Haas, o Haas “3', L I)Horton, I Lingertat, A Loarte, C G Lowry, R I) Monk, M F Nave, R Sartori and K ThomsenJET Joint Undertaking, Abingdon 0X14 313A, Oxfordshire, UKm MP1 for Plasma Physics, nassvas, Garching, Gennany.1. IntroductionOne of the reference I'I‘ER operating scenarios ['1] is a high confinement regime with H93 =03to 1, Zara S 1.3 and plasma density at or above the Greenwald density limit (GDL) in steadystate. However, as reported already by JET [2] and other Tokamaks [3], the plasma steadystate density of ELMy I-I modes saturates at values below the GDL, due to a progressive lossof confinement {I-I mode density limit). Dedicated experiments have been carried out in theJET Mark ll (Midi) divertor campaign to study the confinement of Type IELMy Hamodes athigh density, under steady state conditions (ta-e Ta)-The basic type of plasma studied is a 2.5MAJ‘25T discharge (q95=3.4) with 12 MW of NeutralBeam injected power, where D2 gas fuelling scans were performed. The experiment studiedthe effects of the magnetic configuration and divertor plasma geometry, including low. andhigh triangularity discharges (a = 0.2 and 0.3), and variation of the position of the separatrix(horizontal or vertical plates of the divertor).Confinement data have been compared to the TI'ERH93-P ELMmfree scaling for ELMydischarges (IELMFUBS tELMfm). The plasma stored energy is corrected for the fast ioncontributions, unless otherwise stated, and fast ion orbit losses are taken into account. For allthe pulses analysed, the fast energy fraction is small and varies between 15% at low densities(refilxll)lg mg) to approximately 5% for densities above Txlflm m'a.' WWW” ' ' ' ' ; 2. General results_I.- D.I.Fig. 1 shows the time traces for a series ofdischarges with high triangularity, where the gas.k I sung input was varied from pulse to pulse from zero1 ' to 2.8x1022s"1. ELMs are produced during the' ' F whole heating phase, with a frequencyincreasing with fuelling from 12 to 46Hz. Theenergy and particle confinement deteriorate forthe higher fuelling rates, and the line averageddensity stays below the GDL. Similar results areobserved in all configurations.Elfinr:Has,em[mas-11[law]GDD“maritime]inhibit-acomto:P-Enhibfl-‘m;III|1IIIIIIIIIIIIIIIllllrlnlllugllislIII-I; In gflflfiffill, the [0:31 fraction. 0f Tfldifltfid pUWBT13 19 EU 21 22 23 24 25 25time [s]Figure 1: Typical D2 gas scan for on high bawafln 2065q (1'10 335) 311d 506093triangularity configuration. NB: pGWEl" iflpifl, gas (maximum fUBlliflg) IIICI'BaSn gas fuellingrater, H93, density (cl: Greenwold dentin!) and 2,35,: 'varies with the plasma configuration, and isdoes not change the fraction of bulk radiation50(10-30%), while the X-point radiation increases steadily. Divertor MARFEs are not observed.The impurity content is low, with Zeff usually around 1.3—1.4 with gas fuelling.I HEB-Mklla+ Has-Mkllap1.o—II IIos— ;fut. t .t . a .IE 40+ +' 9%fi.?" + " v‘I +Ias-os. . . . . . . . - .a 1 s s ao nous":inFigure 2: H93 before (Micfla) and afier(Mailapl the closure oftlte lay-pass leaks.Each point is a plasma discharge.One effect of increasing the gas fuelling rate (anddensity) is an increase of the ELM frequency. Wefind that the total particle less per ELM is a weakfunction of density audios gas fuelling therefore asthe fuelling is increased. the maximum achievabledensity saturates and then decreases. The high ELMfrequency and loss of confinement result in a netloss of density. The maximum density achieved withgas fuelling saturates below the GDL and isdetermined by the balance between fuelling andconfinement losses (see figs 3 and 4). The rolluoverof nE and H93 corresponds to a transition from TypeIto Type III ELMs.The effect of neutral pressure on the density limit .and the confinement was studied before and after the closure of the bypass leaks around thestructure of the Mid] divertor. The closure resulted in a decrease of the edge neutral pressurein the midplane (30%} while the divertor neutral pressure remained the same. However, theglobal confinement did not change appreciably (fig 2).3. Effect of configuration on density and confinement1.1I vision! 4* hiefetlt V Misha!:- viafaihl <2:- htsretht e. unrealLil-II!"O 0us?— g’55.a ‘ “’3:- 'bare 0m can 8 D DI a. Q31‘- 0U.'.I"- D ‘l D.t)A l {JH.6-3-5 u i no 1 2 s 422 -Iow [to s]..... O0 v 0v 0da- -' 8 til-D an (2, DD03' 0 0 #3 ga o at 3 it 'aoa- OE 1'a“ 3cs-N B a visual c eaten a swano ween-e o hisleiht 1.? harem:VIN '- 'I 1 II I II I u II: 1 E 3 ll22 -‘Ion“ he s 1Figures 3 and 4: Thermal confinement degradation and maximum achieved densityr (expressed as a fraction ofthe Gilli.) for all the magnetic configurations studied fit-v = horizotuai-vertical target, it-ht = high-low A. ife-sfe-hfe = iow-atandard-highflux expansion)Fig 3 illustrates that the energy confinement does not depend on the magnetic configuration.whereas fig 4 shows that triangularity affects the maximum density achievable in steady state.In fact, for the same value of H93. the high triangularity discharges reach up to = 90% of theGDL. while at low triangularity they achieve only 75%.51Most of the discharges analysed are characterised by an initial hollow density profile. This isalso the case for no additional gas fuelling hence we conclude that edge recycling isresponsible for the hollow profiles. The effect of recycling is evidenced by the fact that theinitial density rise is about twice the beam fuelling, also in the absence of additional gasfuelling. Usually, the density profile fills up during the discharge and is flat in steady state. Insome cases however, (medium gas fuelling, high A), the steady state ne profile peaks. Forthese discharges, the GDL is exceeded locally in the central 10% of the plasma volume, whilethe edge density and the overall average density remain below the Greenwald density.' _ 4. TRANSP code analysis.”33599-15 Figure 5 illustrates the results of TRANSP simulationsa asasr - no gas of two discharges, on the horizontal plates and at low A,' with zero and high gas fuelling (2.?xll3t22 s”), withH93(no gas)=0.83 and H93(gas)=0.69. The calculatedprofiles of the transport coefficients show that the lowerconfinement of the fuelled discharge is accounted for byenhanced energy transport in the edge (outermost 20%of the plasma radius), while the central confinement is Inot affected. This result seems to contradict theKotschenreuther—Dorland model [4].2'9 2'4 Malof'idms [:13 3'5 There are indications that the change in transport occursFigure 5: Radial! profiles ofzwcnd Xian: mainly in the ion channel. Additional TRANSP runs“mm” by TRANSP‘ were carried out to test the sensitivity of these results to. the assumptions on particle recycling. Thesesimulations, where the energy of the recycling particlesor the particle confinement time were varied (fixed 1:1:- or’EPWTE), confirm that the changes in the edge transportare relatively insensitive to the assumptions on the edgerecycling in the code.5. ELMs and edge parametersThe pedestal electron density and temperatures havebeen analysed for gas scans at low and hightriangularity. In the edge nJI'E diagram [5,6], theexperimental stability limit for type I ELMs is found toFigure 6: Empiriccistabiiity plotfor Type increase with plasma triangularity (i.e. shear), asIELMi a.” law and high a discharges, an expected (fig 6). Moreover, as shown in fig 7 for a gas‘steady state scan (subset of fig 6), there is a correlation between theconfinement of a discharge and its position in the stability diagram, with lowconfinementcorresponding to a cool and dense edge. In fact, the increase of the gas fuelling results in therise of the edge density at the expense of the edge temperature, at approximately constantpressure. The lower confinement point shown in the figure corresponds to a change from typeIto type HI ELMs. The crucial role of the edge temperature is confirmed by the analysis of theI low hiangularlty {a = 0.21}o high triangular'rty ta = £1.31}Edge n, no” m4]52time evolution of the edge 11,, and T, for low and high amo discharges. This shows that for low d, as and Tou. t m =,, increase similarly between ELMs, until the stability. 1' ¢:=mmnsd * limit is reached, whereas for high at, the pedestalI afiemsfi" “9:32- temperature clamps after an initial rise, and the stability* a e_=saae'r mi lim1t is reached due to the increase in ne. The clamping"“' + ”Hamill” ass+ of T3 is correlated with the occurrence of high frequencyMHD modes (60-80 kHz), possibly located in the edge.The clamping of the edge temperature is consistent withthe observation that high a plasmas have a lower energy' I i I t I t I 4 I 5 I 6 I t I * confinement between ELMs than low a discharges.Edgawflwmql In the high density ELMy I-I modes, partial detachmentFir-ii“ 7" Variation “fH93f0r‘13353m” at the inner divertor is observed between ELMs. Theat low at H93 is calculated withoutsubtraction afriie fast ions energy. degree of detachment [$28] is quite low in all cases andthere is no unique correlation between detachment andconfinement degradation. At high triangularity, some detachment can be sustained withoutloss of confinement...6. SummaryThe H mode density limit is a confinement limit. The decrease in energy confinement at highdensity cannot be explained by the loss of fast ion contributions (typically around 10% orless). The maximum plasma density achieved in H mode is = acts of the (3131., with H93 r:0.75 — 0.8. Discharges with high triangularity (edge shear) reach higher densities than lowtriangularity plasmas, for the same global confinement.The core confinement of high density, low edge temperature pulses (H9320.65) is comparableto the unfuelled reference case (H93>0.33}, and the reduced confinement is accounted for bythe enhancement of the edge transport (outer 20% of plasma radius).Experimental data in steady state discharges confirm that the edge stability for type I ELMsincreases with edge shear. The degradation of confinement is related to a trajectory in thenJTe space toward low pedestal temperature, consistent with the existence of a minimumcritical edge temperature for H mode confinement.Clamping of the edge T, between ELMs is observed in high triangularity discharges. In thiscase, the stability limit is determined by the density rise between ELMs, and not by re-heating.Divertor detachment and MARFE formation do not account for the observed loss ofconfinement.References[I] R rhythm" and the ITER Team, [ABA -CN~64iUI*l , lIE-UII IAEA Fusion Energy Confi, Montreal£Canada}, 1996[2] G Saibene et al, Journal of Nuclear. Materials, in press.[3] M Kaufmann and the ASDEX Upgrade Team, LAEA-Fl- CN—64i01-5, as ref. [1][4] M Kotschenreuther and W Doriand, Communication to FESAC ITER Confinement reviewers, 14i02i199?[5] H Zohm et a1, IAEAv-Fl-CN—fitliAS— l, as ref. [1][6] J Lingertat et a1, these proceedings[T] A Loarte et al, submitted to Nuclear Fusion.[8] R D Monk et at, these proceedings.

Steady StateH-Modes at High Plasma Density in JET

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Steady StateH-Modes at High Plasma Density in JET

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