ASDEX-Upgrade Team

Gruber, O.

Mertens, V.

Murmann, H.

Ryter, F.

Salzmann, H.

Schweinzer, J.

Suttrop, W.

English

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H-Mode and Confinement Studies in ASDEX Upgrade

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F1-CN-69/EXP2/06H-mode and confinement studies in ASDEX UpgradeW. Suttrop, F. Ryter, V. Mertens, O. Gruber, H. Murmann, H. Salzmann,J. Schweinzer, and ASDEX Upgrade team,Max-Planck-Institut für Plasmaphysik,EURATOM-IPP Association, D-85740 Garching, GermanyAbstractH-mode operational boundaries and H-mode confinement are investigated on ASDEX Upgrade. The local edgeparameter threshold for H-mode holds independent of divertor geometry and changes little with ion mass. Thedeviation of the H-mode power threshold at densities near the Greenwald limit can be understood as a consequenceof a confinement deterioration, caused by “stiff” temperature profiles and lack of core density gradients in gas pufffuelled discharges. Ion and electron temperature profiles can be described by a lower limit of gradient lengthLT = T=T 0.1. IntroductionThe plasma edge, in particular region near the outermost closed flux surfaces, affects in variousways confinement and stability of tokamak operation. Operational limits, such as the boundary betweenthe L-mode and H-mode confinement regimes, onset of MARFE radiation instability, occurence of MHDinstability (Edge Localized Modes) can be mapped out in edge parameters, in particular edge temperatureand pressure gradient, as reported on the previous IAEA conference [1] from results of ASDEX Upgrade.Since then, ASDEX Upgrade has undergone several modifications, a new, closely baffled “Lyra”-shapeddivertor (divertor II, [2]), increased neutral beam heating power of 20 MW and additional diagnosticscapabilities. These possibilities have enabled to study the effect of edge parameters in a regime extendedtowards higher density. Here, we focus on the H-mode boundary and the confinement behaviour fordensities approaching the Greenwald density limit [3]. Global and local parameters are compared asthey reflect a change in confinement behaviour.2. H-mode operational limitsThe H-mode power threshold in ASDEX Upgrade has been investigated previously in global [4] andlocal edge [5] parameters. The new divertor II allows to compare the H-mode threshold in ASDEXUpgrade in two divertor geometries. Figure 1 shows the loss power (heating power with change of storedenergy subtracted) at the L-mode to H-mode transition as a function of line averaged densityne (a) andedge densityne;edge(b), taken atr = a 2 cm. While the loss power threshold scaling withneBt differsfor the two divertor geometries, the scaling againstne;edgeBt is identical. This reflects the observationthat in divertor II the density peaking is smaller than in divertor I, i.e. higher edge densities are achievedwith the same core density. The local edge parameters, in particular the critical edge temperature for theH-mode transition, has not changed with divertor geometry as figure 1 (c) shows. The edge temperaturemeasured just before the L- to H- mode transition for deuterium at low density (3:81019 m 3, opensquares) is in accordance with the scaling derived in divertor I [5]. The figure also shows profiles withand without use of the newly installed divertor cryompump in both hydrogen and deuterium. Additionalpumping has no effect on the transition condition other than by changing the edge density for a givenneutral flux and therefore does not modify the scaling. The difference of critical edge temperatures inhydrogen and deuterium at low density is limited to about 20%, whereas the difference in heating powera)n           B     [10    m     T]e,edge t20 -30.00.51.01.52.02.53.03.54.04.55.00.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6divertor IIdivertor IASDEX UpgradeP       [MW]heatb)0.00.51.01.52.02.53.03.54.04.55.00.0 0.5 1.0 1.5 2.0 2.5 3.0divertor IIdivertor IASDEX UpgradeP       [MW]heatn   B     [10    m     T]e t20 -3c)-20 -10 0 10midplane distance from separatrix [cm]02004006008001000electron temperature [eV] #9904   D      3.8        off #9905   D      3.8        on #9654   H      4.7        off #9657   H      6.5        off #9698   H      4.7        on  #9699   H      4.7        ongas ne19 cryopumpshotASDEX UpgradeDivertor IIB  = 2.5TtI   = 1.0 MApdivertor IscalingThomson scatteringFigure 1: H-mode threshold in ASDEX Upgrade (a) loss power vs. line-averaged densityne and (b)edge density (taken at r= a 2 cm), multiplied by the toroidal field Bt ; (c) edge temperature profiles justbefore the transition for deuterium and helium in divertors I and II.is approximately a factor of two. These comparisons show that the edge parameter threshold scaling isrelatively robust and global scalings can be affected by heat and particle transport.The difference between local and global scalings becomes especially apparent at high densities closeto the Greenwald density limit, where the H-mode power threshold increases strongly above the usualneBt -scaling encountered at low and medium densities [6]. With the rec ntly upgraded neutral beamheating of now 20 MW injected power in deuterium a series of high edge density experiments with gas-puff fuelling was carried out. It is found that high edge densities can be achieved only if the heatingpower is increased proportionally with the total neutral gas influx (external gas flux plus wall recycling).The operational space at high density and high heating power is mapped out by the trajectories shownin figure 2 (a) in global parameters, heating power vs. neutral gas influx, and (b) in local edge parameters,electron temperature and density atr = a 1 cm (inside the H-mode barrier region). Stable operationat high density is achieved above the boundary labelled “HL” which denotes the H- to L- backtransition.At high edge densities, the backtransition is recognized by a fast drop of the edge temperature gradient,effectively destroying the edge transport barrier [7]. This transition is quickly followed by MARFE ex-tension onto closed flux surfaces. If fuelling is continued, the edge temperature can drop further, leadingto tearing mode formation and a disruption, which can be avoided if the heating power is increased toabove the “HL”-boundary. A feedback control algorithm has been implemented in the ASDEX Upgradecontrol system which, after MARFE detection by bolometry, reduces the external gas flux and increasesthe heating power. With this algorithm, disruptions are avoided reliably in density limit experiments.In ASDEX Upgrade divertor II, at high density the “HL” boundary almost coincides with the onset ofstrike point detachment during type III ELMs both in global and local edge parameters. Detachment isdetected by a drop ofC III line intensity, a measure of power flux into the divertor. Langmuir probemeasurements (Isat at the target plate) reveal that still a particle flux to the target exists during ELMs.a)0 2 4 6 8 10 12Γ          [10    m    s   ]051015P       [MW]sep-2 -10,main21IIIMHDHLASDEX UpgradeI    = 1 MAptype Itype I+IIItype IIIL-modeMARFE  ELMyH-mode}attached during ELMspartiallyfully } detached alsoduring ELMsD b)n      (a-1 cm)   [10    m  ]T     (a-1 cm)  [eV]e-3e19III MHDHLASDEX UpgradeI    = 1 MApL-modeMARFEtype Itype I+IIItype III  ELMyH-mode}attached during ELMspartiallyfully }detached, alsoduring ELMs0 2 4 6 8 100100200300400υ*=constT=constDFigure 2:Operational space at high edge densities: a) separatrix power and main chamber neutral gasflux, b) edge density and edge temperature (taken at r= a 1 cm). Regime boundaries marked “III”,“MHD”, “D”, and “HL” are discussed in the text.Nevertheless, the pressure balance at the separatrix between target plate and mid-plane is lost in betweenand during ELMs. From the edge measurement it is difficult to infer a scaling of the “HL” boundaryat the edge. Possibilities are aT = constor collisionality boundary,ν = const, in contrast to low andmedium densities.If the heating power is much in excess of the H-mode threshold, the edge temperature increases. If thepressure gradient imposed by ELMs is reached (“MHD” boundary), further increase of edge temperatureis accompanied by a reduction of edge density. At sufficiently high edge temperature, small type IIIELMs are lost and only type I ELMs can be obtained. Type I ELMs are characterized by lower ELMfrequency and larger momentary heat flux during the ELM event. Predictions for the FDR-ITER basedon extrapolation of JET and DIII-D data [8], result in a power loss from the main plasma of 25-80 MJper type I ELM at a frequency of 1 Hz. While detachment during type III ELMs can be achieved, duringtype I ELMs detachment has not been observed to date, thus a large fraction of the type I ELM energycan be expected to be deposited on the divertor target, which probably leads to an unacceptable peakpower load in a reactor-sized plasma.a)           2.0 2.5 3.0 3.51.5time  (s)    4.01.0PauxWtot 2 × Wth,eNBI  60 kVn eASDEX Upgrade #1076104812MW0481210    m19 -300.40.20.6MJ0123H ITER-89P H ITER92P(y)10.50n / nGWb)0 0.10 0.20 0.30 0.40 0.50minor radius [m]ASDEX Upgrade #10761Thomson scattering t = 2.0 - 2.1 s t = 2.9 - 3.0 s t = 3.6 - 3.7 selectron density [10    m    ]19-3151050Density rampnGWc)0.00 0.10 0.20 0.30 0.40 0.50minor radius [m]100100010000ion temperature [eV]ASDEX Upgrade #10761Charge exchange spectroscopy t = 2.0 - 2.1 s t = 2.9 - 3.0 s t = 3.6 - 3.7 sTiFigure 3:(a) Time traces, (b) ion temperature and (c) density profiles of an H-mode density ramp exper-iment. As the density limit is approached, the increasing edge density causes an edge temperature drop.Core temperatures fall proportionally and, together with a flat density profile, the volume integratedpressure (stored energy) decreases.3. High power/high density H-mode ConfinementIt is generally found experimentally that H-mode confinement depends on plasma density, in partic-ular near the Greenwald density limit a strong degradation compared to the ITER H-mode confinementscalings is found. This is illustrated in figure 3 for a density ramp experiment conducted as describedin the previous section. Even with increased heating power, the H-factors relative to L-mode (HITER89P)and ELMy H-mode (HITER92Py) decreases by about 50% when a density of 90%nGW is approached (fig.3 a). The origin for this degradation can be inferred from the profiles, taken at 3 different times. Withincreased gas puffing, mainly the edge density increases (fig. 3 b). Due to the limitation of the edgepressure gradient, this requires that the edge temperatures decreases. A profile similarity causes the coretemperature to decrease as well (fig. 3 c). Shown is the ion temperature, but electron temperatures showsimilar behaviour. The logarithmic temperature gradientLT =T=T 0 has a lower limit, which is reached athigh heating powers that usually lead to H-mode and, at all but highest densities, type I ELMs. The edgedensity profiles in gas puffed H-mode discharges are typically flat, and the density peaking decreaseswith increased gas puff, i.e. the core density increases slower than the edge density. Therefore, if thetemperature reduction in the core cannot be compensated with a density increase by the same fector, thecore pressure, and the stored energy must decrease.4. Summary and DiscussionAt high densities, the edge MHD pressure gradient limit approaches the critical temperature formaintaining an edge transport barrier (H-mode) and avoiding radiation instability. This leaves a narrowparameter space for stable H-mode operation at high density. The intersection of the MHD limit and theH-mode boundary formally defines a maximum edge density which in reality is not achieved experimen-tally even with high heating power. A model for the density limit based on pressure balance and a heatflux limit that determines the separatrix temperature has been proposed in Ref. [9] to explain the H-modedensity limit. A saturation of edge density is predicted which has been observed in JET [9] and ASDEXUpgrade [7] at limited heating power. However, the breakdown of the transport barrier inside closed fluxsurfaces, a change of radial transport, has not yet been explained conclusively. Simulations of anomaloustransport in the relevant parameter regime of drift-Alfven-ballooning instabilities, disagree on whetherat highβ a transport enhancement with collisionalityν occurs [10] or not [11]. The observed edge-corerelation of the temperature profiles leads to a degradation of confinement at high densities. However,one can conceive of several possibilities to overcome this limitation. The ASDEX Upgrade programincludes investigations of scenarios with peaked density profiles in the core, e.g. by pellet fuelling fromthe high-field side [12], spontaneous density peaking during CDH-mode [13,14], and the physics of in-ternal transport barriers [15], with the goal to overcome the limitation of the temperature gradient lengthobserved in the conventional H-mode regime.References[1] KAUFMANN, M. et al., Plasma Physics and Controlled Nuclear Fusion Research 1996, volume 1, pages79–94, Vienna, 1997, IAEA.[2] KAUFMANN, M. et al., IAEA-CN-69/EX3/02, this conference.[3] GREENWALD, M. et al., Nucl. Fusion28 (1988) 2199.[4] RYTER, F. et al., Plasma Phys. Controlled Fusion40 (1998) 725.[5] SUTTROP, W. et al., Plasma Phys. Controlled Fusion39 (1997) 2051.[6] MERTENS, V. et al., Nucl. Fusion37 (1997) 1607.[7] SUTTROP, W. et al., PSI conference (San Diego, 1998) to be published in J. Nucl. Mater.[8] LEONARD, A. W. et al., PSI conference (San Diego, 1998) to be published in J. Nucl. Mater.[9] BORRASS, K. et al., Contrib. Plasma Phys.38 (1998) 130.[10] ROGERS, B. N. et al., IAEA-CN-69/THP2/01, this conference.[11] SCOTT, B. D., IAEA-CN-69/TH1/05, this conference.[12] LANG, P. T. et al., Phys. Rev. Lett.79 (1997) 1487.[13] GRUBER, O. et al., Phys. Rev. Lett.74 (1995) 4217.[14] NEUHAUSER, J. et al., Plasma Phys. Controlled Fusion37 (1995) A37.[15] GRUBER, O. et al., IAEA-CN-69/OV4/3, this conference.

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H-Mode and Confinement Studies in ASDEX Upgrade

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