Kaufmann, M.

Lang, P.

Lang, R.

Mertens, V.

Murmann, H.

Neuhauser, J.

Salzmann, H.

Schweinzer, J.

English

MPG.PuRe

EDGE DENSITY CHARACTERIZATION CLOSE TO THEGREENWALD DENSITY LIMIT WITH THE NEW CLOSED DIVERTORIN ASDEX UPGRADEV. Mertens, M. Kaufmann, P. Lang, R. Lang, H.D. Murmann, J. Neuhauser, H. Salzmann,J Schweinzer, and ASDEX Upgrade TeamMax-Planck-Institut für Plasmaphysik, EURATOM Association,85748 Garching, Fed. Rep. of Germany1. IntroductionIn order to achieve thermonuclear burn, future fusion experiments must safely operate at ratherhigh density, while retaining sufficiently high energy confinement τE . Present experiments,however, show that the useful tokamak operation space is limited towards high density by variousprocesses, for example by exessive edge radiation cooling eventually causing a disruption, bythe onset of fast MHD-istabilities (e.g. ballooning limit) or simply by intolerable energyconfinement degradation (e.g. loss of H-mode). Despite the obvious role of the edge energybalance in most disruptive discharges, the empirical heating power independent Greenwaldline averaged density limit scaling (nGWe = Ip / (πa2) [1020m−3,MA,m]), which primarilyhas been developed for OH and L-mode discharges, has been shown to be quite successfulin describing experimental data. Greenwald et al. suggested a severe degradation of particleconfinement when approaching this limit [1].Meanwhile, ne beyond this limit have been reported from several limiter and divertortokamaks with gas-puffing. In practically all this cases centrally peaked electron density profileshave been observed [e.g. 2], indicating a limitation of the edge density rather than of the lineaveraged density.As often demonstrated injection of cryogenic pellets is also a successfull tool to surpassthe Greenwald limit. However, experiments have shown that in the most probable envisagedoperation scenario in ITER, the ELMy H-mode, the pellet particle fuelling efficiency degradesstrongly with increasing heating power. This holds for the standard injection scheme from themagnetic low field side (LFS).During the short process of pellet ablation these particles build a high pressure plasmoidwhich is subject to a radial force. This diamagnetic force points always outwards in major radiusdirection. Therefore we extended our system to inject pellets from the inner side of the torus,i.e. from the magnetic high field side (HFS). This new injection scheme leads to significantlyimproved instantaneous particle fuelling efficiency while the density build-up in the plasma maybe still limited by transport degradation phenomena.This paper presents global and local densities in the vicinity of the Greenwald limit. Sinceboth the H- and L-mode density limit (DL) is commonly believed to be an edge physics effect,we focus our investigations on edge density and transport behaviour of low Zeff deuteriumplasmas in the new closed divertor configuration DV-II. The results are compared with thoseobtained with the old open divertor configuration DV-I.2. Discharge Parameters and DiagnosticsOur investigations concentrate on lower single null discharges (R = 1.65 m, a = 0.5 m, κ ∼ 1.6)in deuterium with plasma currents Ip of 0.6-1.2 MA and toroidal magnetic fields Bt of 1.5-3 T,corresponding to safety factors q95 between 3 and 7. NBI heating powers up to 15 MW havebeen applied. ne ranges between 0.5 · 1020m−3 and 1.4 · 1020m−3. The solid deuteriumpellets were injected from HFS with an inclination angle of ≈ 45o with respect to the midplanedirecting to the plasma centre. Both feed-forward and density feed-back injection are performedwith injection velocities of 240 m/s and repetition rates of up to 60 Hz, yielding penetrationdepths of up to half plasma radius.The densities and temperatures mainly presented are measured by Li-beam and Thomsonscattering and FIR interferometry diagnostics.3. High Density Operation with Pure Gas-puffingThe H-mode is generally accessible when the input heating power Pheat exceeds a certain limitdepending on density and magnetic field PL→Hheat = c · ne Bt where the constant c depends pri-marily on ion species and ion∇B drift direction [3]. Closely above the threshold the H-mode ischaracterized by high frequency type-III ELM’s (∂ νELM/∂ Pheat < 0). Deeper in the H-modethe ELM activity changes to low frequency type-I ELMs (∂ νELM/∂ Pheat > 0) [4]. Duringdensity rises of H-mode plasmas up to the non disruptive H-mode density limit (i.e. H→L modeback-transition) the discharges pass normally the following phases : at high density the ELM’srevert from type-I back to type-III, the density at the separatrixnsepe increases monotonically withne but tends to saturate in the high density type-III ELM phase and the divertor detaches [5], τEdegrades with increasing recycling monotonically down to L-mode levels, the electron tempera-ture closely inside the separatrix at the H-mode transport barrier approaches≈ 200 eV at the DL.02460.4 0.6 0.8 1 1.2 1.4H→L-moden    /  ne eGWL → H thresholdP      / (I    q) [MW / MA  ]heatp220.6 MA0.8 MA1.2 MA1.0 MA≈L-modeFigure 1. The operation di-agram shows the strong devi-ation of the power needed toachieve H-mode from the usualPheat ∝ ne Bt scaling closebelow nGWe . Note that Ip andBt are varied by at least a factorof 2.Generally, slighly below the Greenwald limit the L↔H-mode threshold power deviatesfrom the above scaling and rises dramatically (see Fig. 1), i.e. the H-mode DL becomes nearlyindependent of Pheat. Earlier experiments in ASDEX Upgrade have shown [6] this detrimentaleffect at one particular plasma current of Ip = 0.8 MA. To get more insight into the responsibleprocesses further parameter scans have been performed, especially in Ip, Bt and Pheat. Figure 1gives an overview over these experiments. To combine the data of different Ip in one picturewith a common H-mode threshold we replace in thePheat ∝ ne Bt scaling ne by the Greenwaldexpression. This leads to Pheat / (I2p q95) ∝ ne / nGWe . Figure 1 shows nicely that the earliesfindings [6] hold also for a significantly enlarged parameter space. It is interesting to note thatthe increase of Ip by a factor of 2 yields the same normalized densities ne / nGWe , i.e. at 1.2 MAdouble the line averaged density than at 0.6 MA with comparable ITER scaling normalized τE .This is remarkable since at these high densities the edge particle fuelling profiles shift markedlyoutward into the scrape-off layer (SOL) and also the NBI particle ionization profiles move closeto the edge. Nevertheless, with gas-puffing alone the Greenwald limit could not be exceeded.DV-IIDV-I00.40.81.20 0.4 0.8 1.2 1.6n   [10    m   ]e20 -3n      [10    m   ]e20-3sepFigure 2. The figure shows thedensity profile similarity over alarge density variation at the H-mode DL.To gain further insight into the H-mode limit physics we discuss the related edge densitiesin more detail. The ratio of the edge density nsepe to ne is found to be practically constant,indicating a strong self similarity of the bulk density profile in a wide density range (see Fig. 2).This is in contrast to findings with the open divertor DV-I, where nsepe increased quadraticallywithne and also the edge densities at a givenne were clearly lower [6, 7]. On the other hand, nsepeversus other typical SOL moments like density profile decay length λSOLe and the line averageddensity in the SOL nSOLe show similar linear relationships as found in DV-I [8]. Typical SOLdensity decay lengths are 3-5 cm for the densities discussed here (see Fig. 2).To deduce from the experimental knowledge of a constant pedestal TDLe at the H-mode DL and empirical ITER confinement scalings a predictive scaling for the correspond-ing pedestal density nDLe , we rewrite the pressure under the assumption of profile similarity< p > ∝ TDLe nDLe ∝ Pheat τE . This yields nDLe ∝ I≈1p P 0.3−0.5heat , the exponent of Pheatdepending on the actual confinement scaling used. The Ip scaling agrees with the experimentalfindings, whereas the observed heating power dependence is with nDLe ∝ nsepe ∝ P 0.1heatnoticeably weaker than the above derived expression. This might reflect the fact that dischargeswith densities close to the Greenwald limit are only a small fraction of those used in the ITERconfinement regressions.4. High Density Operation with HFS Pellet InjectionUsing the pellet injector in ASDEX Upgrade in density feedback mode long lasting (>> τE)density phases beyond nGWe in H-mode are achieved (see Fig. 3). The energy confinement,however, still degrades with increasing neutral particle density [9]. Within the density build-upto e.g. ne ≈ 1.2 nGWe the density profiles do, in first order, not change their shape despitethe much deeper particle deposition compared to gas-puffing. The profiles remain quite flatin the core plasma with steep gradients in the edge pedestal region (see Fig. 3). These edgegradients increase, typically, strongly after the L→H transition but change only little duringfurther ne build-up. ne and edge densities rise concomitantly up to ne ≈ 0.9 nGWe but thannsepe saturates. The constancy of the edge density (and pressure) gradient indicates a critiallimit caused perhaps by e.g. ballooning induced transport. The additional ne rise via pelletsis achieved by an extension of the radial width of the steep gradient zone further inwards, seeFig 3.The significantly different particle fuelling rates between pure gas-puffing and pellet in-jection on one hand and the mainly unaltered density profile form on the other hand indicatesa drastic change of the particle transport behaviour during pellet injection in the vicinity andabove nGWe . For comparison, the total particle fuelling rate inside the steep gradient zone owingto pellet injection (Φ ≈ 1022/s) is roughly one order of magnitude larger than the NBI fuelling1.4 1.8 2.2 2.6 3Time [s]# 929601201010 0.2 0.4 0.6 0.8 1ρne-3[10    m   ]20ne- 4[10    m   ]21-3[10    m   ]20∇n  (ρ~0.95)e➀ ➁OH + L-mode    H-mode1.6 s2.3 s3.0 sFigure 3. The figure shows the density development before and during the injection of a string of HFSpellets. With begin of the H-mode the edge density gradient (and pressure gradient) increases and staysconstant also during the further ne increase. The corresponding density profiles show clearly a wideningof the edge gradient zone after pellet injection.rate (Φ ≈ 1021/s at 10 MW) or even the recycling fluxes. Taking the slow density rise ofthe discharge in Fig. 3 as example and neglecting an inward pinch velocity, the particle fluxacross the edge can be approximated by Γe = D ∇ne, D being the effective particle diffusioncoefficient in the edge. Since the edge gradient does not change noticeably between the purelygas fuelled (phase 1 in Fig. 3) and the pellet fuelled phase (2), but the particle source strength, i.e.the necessary particle flux through the boundary, does, it follows that the diffusion coefficientincreases here by roughly a factor of ≈ 5 to explain the particle balance. In contrast, deter-mination of the edge diffusion coefficient during pellet injection clearly below the Greenwaldlimit exhibits values consistent with corresponding purely gas fuelled cases. Analyses for highdensities are in progress.5. SummarySystematic global and local analyses of H-mode density limit plasmas (H→L-mode back-transition) with pure gas refuelling and with pellet injection from the magnetic high field sideare performed in the new closed divertor configuration. With gas-puffing alone, nGWe could notbe surpassed. In the new divertor higher edge and SOL densities are normally found comparedwith the old open divertor. With gas-puffing the L↔H-mode power scaling Pheat ∝ neBt is notlonger valid if one approaches the Greenwald limit. The threshold power deviates dramaticallyfrom the prediction to higher powers which makes it difficult to maintain the H-mode at highdensities. The steep edge density gradient which establishes after the L→H-transition does notincrease further with concomitant ne increase indicating the reach of a critial limit. With pelletinjection nGWe can be exceeded. These discharges show ne-profiles with the same edge gradientbut a radially enlarged gradient zone. The strongly enhanced particle fuelling rate during pelletinjection compared with the gas-puff case induces in the vicinity ofnGWe a significantly increasededge particle diffusivity. Pellet injection, however, does not prevent the degradation of τE athigh ne. Further optimizations can probably combine pellet injection with conserved high τE .References[1] M. Greenwald et al.: Nuclear Fusion 28 (1988) 2199[2] P.C. de Vries et al.: Phys. Rev. Lett. 80(16) (1998) 3519[3] F. Ryter et al.: Physica Scripta, 51 (1995) 643[4] H. Zohm: Plasma Phys. Control. Fusion 38 105 (1996)[5] A. Herrmann et al.: this conference[6] V. Mertens et al.: Nucl. Fus. 37 1607 (1997)[7] J. Schweinzer et al.: PSI (1998), San Diego, USA[8] V. Mertens et al.: Proc. 24rd Europ. Conf. Controlled Fusion and Plasma Physics,Berchtesgaden 1997) 21A IV 1389[9] P.T. Lang et al.: this conference

Edge Density Characterization Close to the Greenwald Density Limit with the Closed Divertor in ASDEX Upgrade

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Edge Density Characterization Close to the Greenwald Density Limit with the Closed Divertor in ASDEX Upgrade

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