INSTITUTE OF PHYSICS PUBLISHING PLASMA PHYSICS AND CONTROLLED FUSION

Plasma Phys. Control. Fusion 44 (2002) B69–B83 PII: S0741-3335(02)52864-5

Steady state advanced scenarios at ASDEX Upgrade

A C C Sips1,12, R Arslanbekov1, C Atanasiu2, W Becker1, G Becker1,K Behler1, K Behringer1, A Bergmann1, R Bilato1, D Bolshukhin1,K Borrass1, B Braams3, M Brambilla1, F Braun1, A Buhler1, G Conway1,D Coster1, R Drube1, R Dux1, S Egorov4, T Eich1, K Engelhardt1,H-U Fahrbach1, U Fantz5, H Faugel1, M Foley6, K B Fournier7,P Franzen1, J C Fuchs1, J Gafert1, G Gantenbein8, O Gehre1, A Geier1,J Gernhardt1, O Gruber1, A Gude1, S Gunter1, G Haas1, D Hartmann1,B Heger5, B Heinemann1, A Herrmann1, J Hobirk1, F Hofmeister1,H Hohenocker1, L Horton1, V Igochine1, D Jacobi1, M Jakobi1, F Jenko1,A Kallenbach1, O Kardaun1, M Kaufmann1, A Keller1, A Kendl1,J-W Kim1, K Kirov1, R Kochergov1, H Kollotzek1, W Kraus1,K Krieger1, B Kurzan1, P T Lang1, P Lauber1, M Laux1, F Leuterer1,A Lohs1, A Lorenz1, C Maggi1, H Maier1, K Mank1, M-E Manso9,M Maraschek1, K F Mast1, P McCarthy6, D Meisel1, H Meister1, F Meo1,R Merkel1, D Merkl1, V Mertens1, F Monaco1, A Muck1, H W Muller1,M Munich1, H Murmann1, Y-S Na1, G Neu1, R Neu1, J Neuhauser1,J-M Noterdaeme1, I Nunes9, G Pautasso1, A G Peeters1, G Pereverzev1,S Pinches1, E Poli1, M Proschek10, R Pugno1, E Quigley6, G Raupp1,T Ribeiro9, R Riedl1, S Riondato1, V Rohde1, J Roth1, F Ryter1,S Saarelma11, W Sandmann1, S Schade1, H-B Schilling1, W Schneider1,G Schramm1, S Schweizer1, B Scott1, U Seidel1, F Serra9, S Sesnic1,C Sihler1, A Silva9, E Speth1, A Stabler1, K-H Steuer1, J Stober1,B Streibl1, E Strumberger1, W Suttrop1, A Tabasso1, A Tanga1,G Tardini1, C Tichmann1, W Treutterer1, M Troppmann1, P Varela9,O Vollmer1, D Wagner1, U Wenzel1, F Wesner1, R Wolf1, E Wolfrum1,E Wursching1, Q Yu1, D Zasche1, T Zehetbauer1, H-P Zehrfeld1 andH Zohm1

1 Max-Planck-Institut fur Plasmaphysik, Boltzmannstrasse 2, D-85748 Garching, Germany2 Institute of Atomic Physics, Romania3 Princeton University, Princeton, USA4 Technical University St Petersburg, CIS, St Petersburg, Russia5 University of Augsburg, Augsburg, Germany6 University of Cork, Cork, UK7 Lawrence Livermore National Laboratory, Livermore, USA8 IPF University of Stuttgart, Stuttgart, Germany9 Associacao EURATOM/IST, Lisboa, Portugal10 IAP, Vienna, Austria11 VTT Tekes, HUT Tekes, Helsinki, Finland

Received 30 August 2002Published 21 November 2002Online at stacks.iop.org/PPCF/44/B69

12 For the ASDEX Upgrade Team.

0741-3335/02/SB0069+15$30.00 © 2002 IOP Publishing Ltd Printed in the UK B69

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AbstractRecent experiments at ASDEX Upgrade have achieved advanced scenarioswith high βN (>3) and confinement enhancement over ITER98(y, 2) scaling,H H98y2 = 1.1–1.5, in steady state. These discharges have been obtained ina modified divertor configuration for ASDEX Upgrade, allowing operationat higher triangularity, and with a changed neutral beam injection (NBI)system, for a more tangential, off-axis beam deposition. The figure of merit,βNH ITER89-P, reaches up to 7.5 for several seconds in plasmas approachingstationary conditions. These advanced tokamak discharges have low magneticshear in the centre, with q on-axis near 1, and edge safety factor, q95 in therange 3.3–4.5. This q-profile is sustained by the bootstrap current, NBI-drivencurrent and fishbone activity in the core. The off-axis heating leads to a strongpeaking of the density profile and impurity accumulation in the core. Thiscan be avoided by adding some central heating from ion cyclotron resonanceheating or electron cyclotron resonance heating, since the temperature profilesare stiff in this advanced scenario (no internal transport barrier). Using acombination of NBI and gas fuelling line, average densities up to 80–90%of the Greenwald density are achieved, maintaining good confinement. Thebest integrated results in terms of confinement, stability and ability to operateat high density are obtained in highly shaped configurations, near double null,with δ = 0.43. At the highest densities, a strong reduction of the edge localizedmode activity similar to type II activity is observed, providing a steady powerload on the divertor, in the range of 6 MW m−2, despite the high input powerused (>10 MW).

1. Introduction

In fusion research, ITER is planned as the first experiment operating in reactor relevantconditions. It integrates the physics of burning plasmas with key technological requirementsin order to demonstrate fusion as a viable source for generating electricity. The standardoperation regime of ITER at 15 MA and 5.3 T [1] is based on the ELMy H-mode, because of itsreproducibility and large experimental database from tokamak experiments. In this regime, areference scenario with a confinement compared to the ITER98(y, 2) H-mode scaling [2],H H98y2 = 1.0, has been defined at a normalized β, βN = 1.8 (βN = 〈β〉aBt/Ip in %mT/MA,with 〈β〉 the volume-averaged β, a the minor radius, Bt the toroidal field and Ip the plasmacurrent). The density of the plasma should be close to the Greenwald density, defined asnGW = 1020Ip(MA)/πa(m) [3], to ensure power exhaust and sufficient lifetime of the divertortarget.

Advanced scenarios in tokamaks focus on improving the confinement and stability overthe standard ELMy H-mode regime. Typically, H H98y2 � 1.2 and βN � 2.5 would be requiredto operate at reduced plasma current [4]. This would allow a substantial fraction of the plasmacurrent to be provided by the neoclassical bootstrap effect, for long pulse operation. However,these advanced scenarios would still have to be operated at high enough density for powerexhaust. In addition, repetitive transient power loads to the divertor from edge localizedmodes (ELMs) should be avoided. Several advanced scenario regimes have been studied inrecent years. Enhanced energy and particle confinement have been achieved in many tokamakexperiments [5–8], mainly through optimization of the current density profile.

Steady state advanced scenarios at ASDEX Upgrade B71

New hardware has been installed at ASDEX Upgrade to facilitate progress in developingadvanced scenarios. In a 9 months shutdown from July 2000 to March 2001, a new divertor wasinstalled (figure 1(a)) [9]. The new divertor geometry allows operation at higher triangularity,and permits comparison of discharges with a low triangularity (δ = 0.17) to discharges withhigh triangularity up to δ = 0.43, where δ is the average of the upper and lower triangularityat the separatrix. In addition, one of the two neutral beam injection (NBI) boxes, with foursources each, was modified to allow injection at a more tangential angle of the neutral beams.With this, the second neutral beam box (NI2, in figure 1(b)) has two NBI sources, which depositthe power off-axis with a total of 5 MW injection power. The power deposition of these beamshas a maximum at r/a ≈ 0.5, depending on plasma density and the height of the magnetic axisof the plasma (r/a is the normalized minor radius, with r the distance from the centre of theplasma, and a the minor radius of the plasma). The more tangential NBI should provide highercurrent drive efficiency. In combination with the other NBI sources, they provide control overthe power and particle deposition by the neutral beams [10]. Furthermore, the carbon tileson the inner wall of the ASDEX Upgrade vessel are coated with tungsten to provide a highZ component facing the plasma with a surface area of 7.1 m2 [11]. This is a test for theuse of reactor relevant components. These upgrades have not changed the main parametersof ASDEX Upgrade: major radius R0 = 1.67 m, minor radius a = 0.5 m, plasma currents(Ip) in the range 0.4–1.2 MA, toroidal field (Bt) of up to 3 T, total NBI heating of 20 MW(eight sources), ion cyclotron resonance heating (ICRH) of up to 7 MW at 30 MHz or 40 MHz,for central heating at 2.0 T and 2.5 T, respectively, and electron cyclotron resonance heating(ECRH) at 140 GHz, with a maximum power of 2 MW, employing adjustable mirrors to allowpower deposition variations. Results obtained with these hardware modifications are presentedin this paper. Some of the detailed data analyses in this paper are made with upgrades to thediagnostic systems of ASDEX Upgrade. However, one key diagnostic, the motional starkeffect (MSE) diagnostic, did not operate satisfactory during this campaign. Hence, the newresults presented here have been analysed without information on the current density profilein the centre from the MSE measurements.

The remainder of the paper is structured as follows: in section 2, the advanced scenariosdeveloped at ASDEX Upgrade are described. Section 3 discusses the use of the modifiedNBI geometry in scenarios with improved confinement in H-mode (‘improved H-modes’).Operation at higher density is presented in section 4. The reduction of the ELMs in advancedscenarios at the maximum triangularity at ASDEX Upgrade (δ = 0.43) and densities close

NI-1NI-2

NI-2/CD

(a) (b)

Figure 1. New divertor at ASDEX Upgrade (a) and the modified NBI geometry (b) of the secondneutral beam injector (NI 2).

B72 A C C Sips et al

to the Greenwald density limit are described in section 5. The paper finishes with a shortsummary and a discussion (section 6).

2. Advanced scenarios in ASDEX Upgrade

Internal transport barrier (ITB) discharges are a candidate for advanced scenarios [6, 7]. ITBsare characterized by good confinement in the plasma centre due to suppression of turbulenttransport. This regime requires low or negative magnetic shear s = (r/q)(dq/dr), and, inmost cases, strong heating in the centre. In ASDEX Upgrade, auxiliary heating (mainlyNBI) during the current rise phase is used, creating ITBs with a negative magnetic shear inthe plasma centre [10]. This regime was initially developed with an L-mode edge, usingthe inner wall as a limiter or an upper single-null configuration (figure 2(c)) with plasmacurrent Ip = 0.8–1.0 MA and toroidal magnetic field Bt = 2.5–3.0 T [13]. Steep pressureprofiles are formed and the ion transport is reduced down to neoclassical levels in the centre.Confinement enhancement factors over ITER89P L-mode scaling [14], H ITER89-P = 1.9,have been obtained in L-mode. However, βN is limited to a maximum of 1.8 due to theoccurrence of ideal MHD modes [15, 16]. This scenario has been extended, recently, by usinga high triangularity (δ = 0.4), lower single-null configuration (figure 2(d)). Transiently, theconfinement and stability have been increased to H ITER89-P = 2.9 and βN up to 4.0. This isachieved by broadening the ITB region at higher input power and adding an H-mode edge inorder to increase the edge density and pressure. A comparison between the old results withan L-mode edge and the new results with an H-mode edge is made in figure 2. Here the iontemperature profile as measured with charge exchange recombination spectroscopy and thedensity profile measured with Thomson scattering. The improvement in ITB performanceand stability is achieved by broadening the pressure profile. This has been reported before byDIII-D [17]. However, maintaining these conditions in steady state have been unsuccessful,despite a large effort in many tokamak experiments during the past 7 years. For example, ithas been difficult the merge ITB discharges and an H-mode edge with strong ELM activityin steady state [18]. So far, ITB discharges with reversed shear have only been sustained at arather low fusion performance and at plasma densities too low to be reactor relevant. In JETthe product of βNH ITER89-P = 3.0, at ne/nGW = 0.5, has been sustained for 37 confinementtimes [19, 20], while in JT-60U reversed shear discharges have been sustained with values ofβNH ITER89-P < 4.5 for up to ten confinement times [21].

ITBs in a reversed shear configuration would not be required if an advanced scenariocould be developed with more current in the centre; so the magnetic shear could be close tozero. If it would be possible to achieve H H98y2 � 1.2 and βN � 2.5 in these conditions,then this would provide an easy and reproducible experimental set-up. Compared to scenariosthat require the formation of ITBs in a reversed shear configuration, such a scenario wouldprovide a more conservative regime for a steady-state reactor [21–23]. At ASDEX Upgrade,these scenarios with low magnetic shear in the centre, and the central safety factor, q0, closeto unity have been developed [24]. In H-mode, this allows a more peaked pressure profiledue to absence of sawteeth. This improved H-mode scenario is obtained at an edge safetyfactor, q95, of up to 4.5 at 1 MA/2.5 T. At a lower plasma current of 400 kA (higher q95),so-called high βpol discharges have been pursued as advanced tokamak regimes [25] (whereβpol = 2µ0〈p〉A/〈Bpol〉2 with 〈p〉A the poloidal cross-section-averaged plasma pressure and〈Bpol〉 the average poloidal magnetic field on the plasma boundary). In these discharges upto 93% of non-inductive current drive has been achieved. Figure 3 gives and overview of theresults obtained at ASDEX Upgrade in experiments before the hardware upgrades described inthe introduction (before July 2000). Plotted in figure 3 is the product βNH ITER89-P against the

Steady state advanced scenarios at ASDEX Upgrade B73

ITB in upper SNITB in lower SN

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Figure 2. Typical profiles for the ion temperature (a) and the electron density (b), for an uppersingle-null discharge (c) with an L-mode edge, indicated by the red points and a lower single-nulldischarge (d) with an H-mode edge labelled by the blue points. The ion temperature is measuredwith charge exchange recombination spectroscopy, the electron density is measured with Thomsonscattering.

duration of the improved confinement regime normalized to the energy confinement time (τE).Again the figure of merit βNH ITER89-P is used and can be compared to βNH ITER89-P = 3.6 [1]for the ITER reference scenario. Presenting the data in this way, it is clear that the improvedH-modes and high βpol discharges achieve steadier conditions compared to ITB dischargeswith an L-mode edge. However, values of βNH ITER89-P > 3.6 are achieved in the improvedH-mode discharges at low density (ne), with ne normalized to the Greenwald density, ne/nGW,in the range 0.3–0.4. For high βpol plasmas the value of q95 is too high (the best results are atq95 = 9); this would require operation at even higher values of H H98y2 (>2) and βN (>4) forthis scenario to be advanced.

In contrast, the results of this campaign (after March 2001) are presented in figure 4.A marked progress has been made in the past year, by taking full benefit of the capabilities

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Figure 3. Overview of advanced scenarios in ASDEX Upgrade before July 2000. The figureof merit βNH ITER89-P is plotted against the duration of the advanced scenarios normalized to theenergy confinement time τE . Given are values for high βpol discharges (green symbols), improvedH-modes (red symbols) at low density and ITB discharges with an L-mode edge (transient, bluesymbols).

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Figure 4. Overview of advanced scenarios in ASDEX Upgrade after March 2001. The figureof merit βNH ITER89-P is plotted against the duration of the advanced scenarios normalized to theenergy confinement time τE . Given are values for high βN discharges at high density (greensymbols), improved H-modes (red symbols) at low density and recent ITB discharges (transient,blue symbols). The ITER reference scenario has βNH ITER89-P = 3.6.

of the modified NBI system, and operation at higher triangularity with the new divertor. Asdescribed in the introduction, this is the subject for the remainder of this paper.

3. Improved confinement H-mode discharges with off-axis NBI heating

Key to achieving an improved H-mode scenario obtains a q-profile with zero magnetic shear inthe centre, and q0 close to 1. To create these conditions in ASDEX Upgrade, one NBI source

Steady state advanced scenarios at ASDEX Upgrade B75

at 2.5 MW is used during the current ramp-up phase. The rise phase is performed in a limiterconfiguration at low density. The heating reduces the current diffusion and delays formation ofa (m = 1, n = 1) resonant surface. At the start of the current flat top (1 MA), a lower single nullis formed and a second NBI source is added. Strong (m = 1, n = 1) fishbones occur, drivenby NB injection, which prevent sawtooth activity. This gives improved core confinement incombination with a type I ELMy H-mode edge at densities in the range (4–5) × 1019 m−3. Ata toroidal field of Bt = 2.5 T, the start of the flat top and the second NBI source are just beforeq0 reaches 1, provided the limiter rise phase can be kept at low density (machine conditioning).This scenario has been extended in parameter range with the use of the off-axis tangential NBIsources. Figure 5 shows an example of an improved H-mode with off-axis NBI sources atlow triangularity, δ = 0.17. The current rise phase is not changed from the description above,but at the start of the flat top (t = 1.0 s), one off-axis NBI source is added. At t = 2.0 s,another off-axis NBI source is added which has the maximum tangential injection angle. Thisdischarge is run at 2.1 T and has sawteeth before the start of the flat top. The off-axis NBIsources provide discharge conditions in which the sawteeth are suppressed from 2.1 s onwards(so the sawteeth are stabilized within 0.1 s after applying the third NBI source). The dischargemakes a transition to H-mode at 1 s and has a further improvement of H H98y2 of up to 1.4, whenthe sawteeth are suppressed. In most cases, fishbones remain the main magnetohydrodynamic(MHD) activity in the core. At an edge safety factor q95 = 3.3, this improved H-mode isnot possible without the use of off-axis NBI heating to suppress the sawteeth. When centralNBI sources are used, the sawteeth are not suppressed and provide a seed for neoclassicaltearing modes (NTMs). Figure 5 illustrates the use of 1.2 MW ICRH with a resonance in thecentre of the plasma. The reason for this becomes clear in figure 6. Plotted in figure 6(a) plotsdensity profiles just after 2 s for two improved H-mode discharges at different triangularity.The improved H-mode at the highest triangularity has a higher edge density, but both show avery pronounced peaking of the density profile. Although peaking of the density profile is oneof the main aims of advanced scenarios, this strong peaking leads to NTMs even without asawteeth trigger. Here the density gradient term in the Rutherford equation [26] becomes themain destabilizing term, generating NTMs even without a seed island [27]. This strong peakingis a result of the off-axis NBI heating. Here the explanation is that a reduction of the heating tothe core reduces the turbulent-driven transport [28, 29]. This reduction in transport reduces theparticle transport and leads to a reduced outward flux compared to the inward particle fluxes.The density in the centre can be reduced, and controlled, by adding ICRH power in the centreproviding an increase of heating to the core (figure 6(b)). This behaviour and explanation hasbeen published extensively for standard H-mode scenarios at ASDEX Upgrade [30]. In thefollowing example, ECRH is used to control the impurity accumulation in the centre. Figure 7shows an improved H-mode at 1 MA/2.5 T with one NBI source on-axis and one NBI sourceoff-axis (both at 2.5 MW). With NBI heating alone, the density peaks and the concentrationof Tungsten in the centre (CW) rises continuously. For peaked density profiles, the high Z

impurities tend to accumulate stronger in the centre of the plasma [31, 32]. The fishboneactivity is not enough to prevent the build-up of the tungsten in the core. At 2.5 s ECRHis applied. This increases the central electron temperature, reduces the density peaking andreduces the tungsten concentration by one order of magnitude to below the detection level [9].ECRH is stopped at 4.5 s. Immediately the density starts to peak and CW rises until the end ofthe improved H-mode at 6 s, when the NBI heating is switched off. From these two examples,it is evident that the improved H-mode scenarios have so- called stiff temperature profiles (noITB) [33, 34]. Furthermore, this is supported by the results presented in figure 8. Analysis ofstandard H-mode discharges (figure 8(a)) shows a correlation of the edge ion temperature to thecore ion temperature [35]. Here variation of the ion temperature is obtained from scans of the

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Figure 5. Improved H-mode scenario with 7.5 MW NBI heating at 1 MA/2.1 T (one NBI at thecentre and two off-axis NBI sources). Plotted are the NBI heating and ICRH (both in MW), thecentral electron temperature (Te0) measured with the electron cyclotron emission (ECE) diagnostic(in keV), βN, the confinement enhancement factor HH98y2, the electron density (ne) normalizedto the Greenwald density limit (nGW), the plasma current (in MA) and the Dα emission from thedivertor plasma.

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Figure 6. (a, b) Electron density (ne) profiles in units of 1019 m−3, for two improved H-modeswith NBI heating only. In both cases >50% of the NBI power is deposited off-axis. The increase incentral density can be reduced (controlled) with ICRH heating at the centre. The electron densityis measured with Thomson scattering.

Steady state advanced scenarios at ASDEX Upgrade B77

time (s)

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Figure 7. Improved H-mode scenario at 1.0 MA/2.5 T, with one NBI at the centre and one off-axis NBI source. Plotted are the NBI heating and ECRH (both in MW), the electron temperature(Te at ρ = 0 and ρ = 0.5, ρ is the normalized flux label) measured with the ECE diagnostic, iontemperature (Ti at ρ = 0 and ρ = 0.5) measured with charge exchange recombination spectroscopy(all temperatures are in keV), the electron density (ne) evolution at ρ = 0 and ρ = 0.5 in units of1019 m−3, from Thomson scattering and the tungsten concentration (CW) at ρ = 0 and ρ = 0.75from spectroscopy measurements.

density, input power and plasma current. For temperature-gradient-driven transport, like iontemperature gradient (ITG) and trapped electron mode (TEM)-driven turbulence, the transportincreases with increasing temperature gradient length (∇T/T ), so all the temperature profilescan be normalized to a unique profile in the region 0.3 < r/a < 0.9 [36, 37]. This behaviour of(ion) temperature profile stiffness is also observed in advanced scenarios at ASDEX Upgrade[38]. In figure 8(b), several time points are selected from an improved H-mode and improvedH-modes at high density (high βN mode, see section 4). Both data from the experiment(open symbols in figure 8(b)) and from transport code simulations using the Weiland model[36, 38] (closed symbols in figure 8(b)) are presented. The simulations of the temperatureprofiles with the Weiland model are in agreement with experimental observations, within theerror bars of the measurements. The Weiland model uses ITG mode and TEM as a basis forthe energy transport of the ions and electrons, respectively, and is a so-called ‘stiff transportmodel’.

B78 A C C Sips et al

T (0.8) [keV]iexp

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Figure 8. (a) Correlation of the edge ion temperature to the core ion temperature in standardH-mode discharges. (b) This linear relation (stiff profiles) is also observed in advanced H-modesat ASDEX Upgrade. The closed symbols in (b) are the results of simulation with the ASTRAtransport code, using a Weiland transport model (see main text). The open symbols in (b) are fromexperimental observations. The ion temperature is measured with charge exchange recombinationspectroscopy.

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Figure 9. Increase of the density in advanced scenarios. The red symbols are for an improvedH-mode at low triangularity (δ = 0.17) with NBI fuelling only. The blue symbols show the increaseof the density at the same triangularity with additional gas fuelling up to 60% of the GreenwaldH-mode density limit. At the same NBI power and gas flow rate, the green symbols give thedensity profile at a triangularity of δ = 0.43. Indicated by the dashed line is the density profile foran advanced scenario scaled to the Greenwald density limit. The electron density is measured withThomson scattering; for each scenario two identical discharges are taken, combining measurementsat the edge and core.

4. Operation at high density: the high βN regime

The experience gained from the improved H-modes (and high βpol discharges) is used to extendthe operation of steady-state advanced scenarios to higher density, closer to the Greenwalddensity limit, while maintaining good confinement. Figure 9 shows the increase in density thatcan be achieved with NBI and gas fuelling. An improved H-mode scenario at low triangularity

Steady state advanced scenarios at ASDEX Upgrade B79

(δ = 0.17) with NBI fuelling only reaches 5×1019 m−3. This can be increased to 8×1019 m−3,which is ne/nGW = 0.65 at 1 MA, using a gas flow rate ten times the NBI fuelling rate.Operation at higher densities and low triangularity leads to a progressive loss of confinement.At a high triangularity for ASDEX Upgrade (δ = 0.43), the density reaches 10.5 × 1019 m−3,ne/nGW = 0.83 at the same gas flow and NBI fuelling rate used for the low triangularity. At thishigh density the confinement is reduced to H H98y2 = 1.1–1.2, compared to H H98y2 = 1.4–1.5,at low density. However at these high densities the stability against NTMs is improved andβN = 3.5 can be obtained in steady state. At ASDEX Upgrade, this regime is called the ‘highβN regime’ [39]. Similarly to the improved H-mode, this regime starts with early NBI heating,and off-axis NBI heating is used to stabilize the sawteeth. In these conditions, the densityis increased together with an increase of the heating power, up to heating powers of 14 MW,using 12.5 MW NBI heating and 1.5 MW ICRH in the centre. The βN values achieved inadvanced scenarios at ASDEX Upgrade is plotted against q95 in figure 10. These βN values arecompared to the values obtained in improved H-modes and recent ITB discharges (transient).High βN values are obtained over a range of q95 = 3–4.5. Hence, the choice of the optimumq95 in an advanced scenario with low shear in the centre does not depend on stability, but ratheron the requirements of maintaining the q-profile, maximizing the confinement, and in whichconditions small ELMs (type II) can be obtained (see section 4).

5. Advanced scenarios with type II ELMs

One consideration for advanced scenarios is the reduction of the size of ELMs. In theadvanced scenarios presented here, the confinement and stability improvements are obtainedby optimizing the profiles in the core. The edge of the plasma is similar to a standard H-modescenario. So, techniques developed to obtain small ELMs in standard H-modes can be directlyapplied to these advanced scenarios. At ASDEX Upgrade, these techniques are: operationat maximum triangularity (δ = 0.43) in a near double-null plasma shape and at high density>85% of the Greenwald density limit [40]. The observation of different ELM types in advancedscenarios is presented in figure 11. The increase in density and plasma shaping (from rightto left in figure 11) leads to a reduction of the ELM frequency, going from type I ELMs, to amixed type I/type II and at the highest value of ne/nGW = 0.88, to continuous type II ELMs.Figure 11 also shows the heat flux to the inner and outer divertor targets. The transient heatload during large ELMs approaches 60 MW m−2 (figure 11(c)). This is reduced to a continuouspower flux of 6 MW m−2 during the type II ELM phase (figure 11(a)). The continuous type IIELMs are observed at 800 kA/1.7 T (q95 = 3.7) with 10 MW of NBI heating (5 MW is off-axis). In this discharge, βN remains steady at 3.5 for 40τE . The electron density is at 88%of the Greenwald density limit and δ = 0.43 in a near double-null configuration. ASTRA[41] code simulations for this pulse show that 50–60% of the current is non-inductively drivenand comes from the neutral beams (tangential injection angle) which provide 15% of the totalplasma current, in combination with the bootstrap current which contributes to 35–45% of thetotal plasma current depending on the bootstrap current model applied [38, 42, 43].

6. Summary and discussion

Discharges with high βN (>3) and good confinement H H98y2 = 1.1–1.5 have been obtained atASDEX Upgrade. Table 1 provides an overview of the most important parameters obtained inthese discharges. In ELMy H-modes, with low shear in the centre, and q0 close to 1, the core

B80 A C C Sips et al

2 4 6

0

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Figure 10. Overview of the βN values, achieved in advanced scenarios at ASDEX Upgrade, plottedagainst q95. Given are values for high βN discharges at high density (green symbols), improvedH-modes (red symbols) at low density and recent ITB discharges (transient, blue symbols). TheITER reference scenario has βN = 1.8 at q95 = 3.

# 14521, Pin= 10 MW # 15486, Pin= 14 MW # 15524, Pin= 5.8 MW

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(a) (b) (c)

Figure 11. Heat load on the divertor plates for advanced scenarios and the Dα measurement inthe outer divertor. Infrared diagnostic data for the inner divertor is displayed on the top and datafor the outer divertor below this. Discharge: (a) with δ = 0.43 and ne/nGW = 0.88, q95 = 3.6,βN = 3.5, (b) with δ = 0.43 and ne/nGW = 0.83, q95 = 3.7, βN = 3.2, and (c) with δ = 0.33 andne/nGW = 0.50, q95 = 4.4, βN = 2.3. The increase in density and plasma shaping (from rightto left in the figure: (c)–(a)) results in a reduction of the ELM and coincident high transient heatloads (type I). At higher density this changes to a mixed type I/type II and at the highest densitiesne/nGW = 0.88, continuous type II ELMs are obtained.

confinement and stability is improved due to the absence of sawteeth. Off-axis heating withtangential NBI sources facilitates the creation of the required q-profile. However, the off-axisheating leads to a strong peaking of the density profile and impurity accumulation in the core.This can be explained due to the reduction of the turbulent transport in the core as a result of the

Steady state advanced scenarios at ASDEX Upgrade B81

Table 1. Parameters for advanced scenarios at ASDEX Upgrade. Given are the values achieved forthe high βN regime, an improved H-mode at low triangularity (0.2), an improved H-mode at mediumtriangularity 0.34 and an ITB discharge. For each regime, the values are obtained simultaneouslyin one discharge. Plasma current (Ip) is in MA and toroidal field (Bt ) is in T. For the high βN andimproved H-mode scenario, the duration is limited by technical constraints (PF coils energy limit,and NBI injection time).

δ q95 Ip Bt ne/nGW βpol βN βNH ITER89-P HH98y2 duration/τE

High βN 0.43 3.7 0.8 1.7 0.83 1.8 3.5 7.5 1.2 <40Impr. H-mode 0.17 3.3 1.0 2.1 0.40 1.4 2.8 6.5 1.4 <40Impr. H-mode 0.33 4.5 1.0 2.5 0.50 1.3 2.3 5.6 1.5 <20ITB 0.42 4.5 0.8 2.0 0.43 2.1 3.6 10.5 1.5 <3

reduced central heating (stiff temperature profiles). Too strong density peaking and impurityaccumulation can be avoided by adding central heating from ICRH or ECRH, providing anaddition tool to control this scenario. This improved H-mode can be obtained with q95 inthe range 3.3–4.5. High density, up to 80–90% of the Greenwald density limit, is possibleusing a combination of NBI and gas fuelling in a highly shaped plasma configuration withδ = 0.43. Even at these densities, the confinement is significantly improved over the ITERreference scenario, while βN reaches values of 3.5 in steady state. At the highest densities, astrong reduction of the ELM activity similar to type II ELMs is observed, in a near double-nullconfiguration, eliminating transient heat loads to the divertor target.

As mentioned in the introduction of this paper, the new results presented here havebeen analysed without information on the current density profile in the centre from MSEmeasurements. However, the improved H-modes obtained during this campaign are verysimilar to the previous campaign (before July 2000), when MSE data were available [13]. Insome discharges, MHD behaviour in the centre (fishbones) and NTMs provide the locationsof rational surfaces. These can be used to constrain the q-profile [43]. The few discharges thathave been analysed in this way indicate a similar q-profile as was presented in [13], low shearin the core with q0 close to 1.

The use of the off-axis NBI injection has certainly widened the operation space of theimproved H-modes (low q95, and more peaked density profiles at high density). However, themechanisms for the sawtooth stabilization have not been clearly identified, since the detailedq-profile evolution could not be measured. Taking into account that the sawteeth are stabilizedwithin 100–300 ms, it is likely that the sawtooth stabilization is a combination of current driveby the tangential NBI sources combined with a change of the fast particle density in the coredue to the off-axis NBI deposition.

Fishbones are the dominant MHD activity in these advanced discharges maintaining q

on-axis near 1. Here the use of the off-axis NBI injection is also beneficial. With central NBIheating alone, the fishbone activity is crucial to expel the central current and to prevent sawteeth,also the main heating (around 1 s in the discharge) needs to be timed before the sawteeth start.With off-axis NBI heating the timing of the main heating can be more flexible; also the fishboneactivity becomes less pronounced and is, in some cases, absent, while maintaining the goodcore confinement. Here a careful documentation of the q-profile and the off-axis NBI-drivencurrent is required to confirm that in some conditions the q-profile can be maintained withoutthe need for fishbone activity. This would go in the direction of discharges in DIII-D presentedin [22], where q on-axis is maintained just above 1 without fishbone activity. Furthermore,at ASDEX Upgrade, experiments are planned with ICRF minority heating in the centre ofthese plasmas, aiming at PICRH/Ptotal > 50%. With the use of the off-axis NBI, to control the

B82 A C C Sips et al

heating and q-profile, this would resemble, at least in the centre of the plasma, more closelythe conditions obtained in a reactor.

A crucial point of these advanced scenarios is that the temperature profiles are stiff. Thisallows control of the density peaking as illustrated in this paper, and a control of the impurities.Especially the use of tungsten as a plasma-facing component at ASDEX Upgrade has elucidatedthis potential problem for advanced scenarios, since this high Z impurity accumulates strongercompared to carbon or oxygen [32]. This kind of control of the core confinement has notyet been demonstrated in discharges with strong ITBs. Moreover, the ITB scenario with anL-mode edge at ASDEX upgrade, using the inner wall (tungsten coated) as a limiter, does notproduce the same results as with a carbon inner wall (before July 2000) due to a strong tungstenaccumulation in the core [9]; even in the rather short duration the ITB exists (200–400 ms).

Interesting to note here is that at JET experiments with low magnetic shear in the centreand q on-axis near have been performed [45]. In these conditions in JET, fishbone activityhas been observed with dominant NBI heating; however, an ITB is formed and sustained for0.5–1.0 s. At the lowest densities in ASDEX Upgrade the ion temperature profiles deviatefrom the stiff profiles in the first few 100 ms of the main heating phase, at 1.0–1.2 s (pulse13679 in [38, 45]). The ion temperature profiles relax back to stiff temperature profiles for theremainder of the heating phase with type I ELM activity. So the observation of an ITB at JETis not in contradiction with the ASDEX Upgrade results, especially since the discharges in JEThave not yet reached steady conditions. However, these results demonstrate that (1) similarityexperiments are important, since they provide a better basis for extrapolation to a next stepdevice, (2) they can be used to address the question if the tools to control these advancedscenarios are unique for ASDEX Upgrade or can be used in a more general sense. Third, theseresults provide an overlap of the experimental programs, and can be used to make comparisonswith other advanced scenarios developed at JET and ASDEX Upgrade (like ITB formation fordifferent q-profiles in JET).

The advanced scenarios presented here and in [22] provide a reproducible experimentalset-up, since only moderate changes to the q-profile are required compared to standard H-modes with sawtooth activity. In addition, they can benefit from developments made in thestandard scenario. This is highlighted by merging the improved H-mode at high density with anedge that has type II ELMs, at high triangularity, in a near double-null plasma configuration atq95 = 3.6–4.0. Moreover, if type II ELMs can only be obtained for q95 > 3.5 (further researchis definitely required), then the advanced scenarios presented here could provide the increasein confinement and stability that would be required to achieve operation at Q = 10 in ITER,without changing the parameters (size or magnetic field) of the device. On the other hand, thesesteady-state scenarios do not provide enough confinement enhancement (H H98y2 = 1.1–1.5)over the standard H-mode scenario to achieve full non-inductive operation, possibly requiredfor a steady-state commercial reactor. Hence ways to further improve confinement and stabilityof advanced scenarios should still be pursued. This requires a balanced research program onsteady-state operation [46] and the physics of improved confinement modes.

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