INSTITUTE OF PHYSICS PUBLISHING and INTERNATIONAL ATOMIC ENERGY AGENCY NUCLEAR FUSION

Nucl. Fusion 46 (2006) 214–224 doi:10.1088/0029-5515/46/2/004

Development on JET of advancedtokamak operations for ITERA.A. Tuccillo1, F. Crisanti1, X. Litaudon2, Yu.F. Baranov3, A. Becoulet2,M. Becoulet2, L. Bertalot1, C. Castaldo1, C.D. Challis3, R. Cesario1, M.R. De Baar4,P.C. de Vries3,4, B. Esposito1, D. Frigione1, L. Garzotti5, E. Giovannozzi1, C. Giroud3,G. Gorini6, C. Gormezano1, N.C. Hawkes3, J. Hobirk7, F. Imbeaux2, E. Joffrin2,P.J. Lomas3, J. Mailloux3, P. Mantica6, M.J. Mantsinen8, D. Mazon2, D. Moreau2,9,A. Murari5, V. Pericoli-Ridolfini1, F. Rimini2, A.C.C. Sips7, C. Sozzi6, O. Tudisco1,D. Van Eester10, K-D. Zastrow3 and JET-EFDA work-programme contributorsa

1 Associazione EURATOM-ENEA, CR ENEA-Frascati, Via E. Fermi 45, 00044 Frascati, Rome, Italy2 Association EURATOM-CEA, CEA/DSM/DRFC, Centre de Cadarache, F-13108 St Paul lez Durance, France3 UKAEA/EURATOM Association, Culham Science Centre, Abingdon, OX14 3DB, UK4 Associatie EURATOM-FOM, TEC, Cluster, 3430 BE Nieuwegein, The Netherlands5 Associazione Euratom-ENEA, Consorzio RFX, 4-35127 Padova, Italy6 Associazione EURATOM-ENEA, IFP-CNR, Via R. Cozzi, 53 - 20125 Milano, Italy7 Max-Planck-Institut fur Plasmaphysik, EURATOM-Assoziation, Garching, Germany8 Association EURATOM-TEKES, Helsinki University of Technology, FIN-02044, Finland9 EFDA-JET CSU, Culham Science Centre, Abingdon, OX14 3DB, UK10 LPP-ERM/KMS, Association ‘Euratom-Belgian State’, TEC, B-1000 Brussels, Belgium

E-mail: Tuccillo@frascati.enea.it

Received 18 January 2005, accepted for publication 24 November 2005Published 4 January 2006Online at stacks.iop.org/NF/46/214

AbstractRecent research on advanced tokamak in JET has focused on scenarios with both monotonic and reversed shear q-profiles having plasma parameters as relevant as possible for extrapolation to ITER. Wide internal transport barriers(ITBs), r/a ∼ 0.7, are formed at ITER relevant triangularity δ ∼ 0.45 and moderate plasma current, IP = 1.5–2.5 MA, with ne/nG ∼ 60% when ELMs are moderated by Ne injection. At higher current (IP � 3.5 MA, δ ∼ 0.25)wide ITBs sitting at r/a � 0.5, in the positive shear region, have been developed. Generally MHD events terminatethese barriers otherwise limited in strength by power availability. ITBs with core density close to Greenwald value,Te ∼ Ti and low toroidal rotation (4 times lower than standard ITBs) are obtained in plasma target preformedby opportune timing of lower hybrid current drive (LHCD), pellet injection and a small amount of NBI power.Wide ITBs, r/a ∼ 0.6, of moderate strength, can be sustained without impurities accumulation for a time close toneoclassical resistive time in 3 T/1.8 MA discharges that exhibit reversed magnetic shear profiles and type-III ELMyedge. These discharges have been extended to the maximum duration allowed by JET subsystems (20 s) bringing tothe record of injected energy in a JET discharge: E ∼ 330 MJ. Portability of ITB physics has been addressed throughdedicated similarity experiments. The ITB is identified as a layer of reduced diffusivity studying the propagationof the heat wave generated by modulating the ICRF mode conversion (MC) electron heating. Impressive results,QDT ∼ 0.25, are obtained in these deuterium discharges with 3He minority when the MC layer is located in the core.The ion behaviour has been investigated in pure LHCD electron ITBs optimizing the 3He minority concentration fordirect ion heating. Preliminary results of particle transport, studied via injection of a trace of tritium and an Ar–Nemixture, will be presented.

PACS numbers: 52.55.fa, 52.55.-s, 52.55.wq

(Some figures in this article are in colour only in the electronic version)

a See annex of Pamela J et al 2004 Proc. IAEA 20th Fusion Energy Conf.(Vilamoura, Portugal, 2004) IAEA-CN-116/OV/1-2.

1. Introduction

The JET programme is designed according to priorities focusedon ITER requirements. A large fraction of the experimental

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activity is devoted to the study and development of advancedtokamak (AT) scenarios suitable for ITER operations. JETplays a key role in developing these regimes as, with its sizeand shape, it bridges the gap in parameters space betweensmaller experiments and ITER, thus providing a sounder basisfor extrapolation. Internal transport barriers (ITBs) have beenstudied in JET discharges with different degrees of reversedmagnetic shear looking for optimization between performanceand steady state capability [1]. More recently the so-calledhybrid regime [2, 3] with monotonic/flat q-profiles, thoughtto be a good candidate for high fluence long burn operationon ITER, has also been addressed. Given the present poweravailability, the JET AT strategy has been oriented towards(i) the development of scenarios with ITER relevant ρ∗–ν∗–δ,including scenarios with Te ∼ Ti and low momentum input,(ii) the maximization of JET contribution to the AT databasethrough dedicated similarity experiments, (iii) the integrationof core-edge solutions to obtain wide barriers, a pre-requisitefor high fusion performance via the maximization of thevolume with good confinement and indirectly favouring betterstability and (iv) the real time control (RTC) techniques. Thelatter has become a routine tool in AT experiments in JET. Sincethe first successful control of plasma current profile achievedusing as actuator the lower hybrid current drive (LHCD) in thelow density prelude phase of the discharge, it has progressed tothe control also during the high power phase [4]. More recentlya simultaneous control of the plasma current and electrontemperature profile has been achieved in ITB plasmas. Actingwith LHCD, NBI and ICRH, the RTC has successfully reachedboth monotonic and reversed target q-profiles. At the sametime different ITB strengths and positions have been obtainedcontrolling the normalized electron or ion temperature gradient[5, 6]. As a measure of the strength of the barrier the ρ∗

Tparameter is routinely used in JET [7]; it is given by thelocal ion Larmor radius at the sound speed normalized to theelectron or ion temperature gradient scale length LT. It isworth noting that RTC of q-profile in the high power phase ofthe discharge is now possible thanks to the progress achievedin coupling LH waves in elmy edge and, generally, in ITERrelevant conditions [8–10]. The progress in the RTC techniqueat JET has been key to the development of AT scenariosand, in particular, wide barriers. Disruptions, due to extremepeaking of core barriers, have been limited by controlling thebarrier strength. Controlling q-profile optimizes localization,triggering and sustainement of the barrier. Light impurity [11]seeding and minority [12] concentration control allows formitigation of edge localized MHD activity, namely the ELMsand optimization of ion cyclotron radio frequency (ICRF)heating scenarios. In this paper, we will briefly review theactivity of the task force on advanced scenarios (S2) at JET inthe recent campaigns. To give the most complete panoramaof the undertaken experimental activity, in some cases, likehybrid regime with dominating ICRH [35] or tritium transport[39,42], results will only be summarized referring the readersto appropriate references for detailed discussion.

2. AT scenarios at JET

The next generation of fusion experiments, with 50%D–50%Tplasmas, will produce enough fusion power to study α

particles behaviour [13]. At the same time long operationswill be exploited both for checking scenarios on theresistive time scale and for checking technical impact on themachine structures [28]. This motivates the developmentof ‘steady-state’ ITB plasmas on JET with high magneticfield (BT) and plasma current (IP), to favour a high neutronyield. Additionally, a necessary condition for high fusionperformance is to obtain an ITB at large radius (r/a > 0.6),to maximize the volume confining the particles and energyand to improve stability. Furthermore the density needs to beincreased in the range of Greenwald values [14], i.e. to valuesclose to the maximum linearly averaged density compatiblewith mean plasma current density; the scenario also needs tobe developed at ITER triangularity. All these aspects have beenaddressed in JET AT research separately; some integration hasstarted for the more mature aspects and when compatible withthe present level of power and JET operational constraints.

2.1. Development of wide ITBs

Integrated optimization of the core and edge conditions hasbeen the key to the development of large barriers in ITERrelevant high triangularity (δ = 0.4–0.45) configurations.As shown by previous JET experiments [15] ELM free ortype I edge conditions are more favoured in high triangularity.Moreover the inward propagation of the perturbation generatedby large ELMs erodes the foot of the barrier up to its destructionthus limiting ITB duration and compatibility with the H-mode.The successful route to overcome this impasse has beena compromise between the reduction of edge confinement(lower pedestal amplitude for more friendly type III ELMs)compensated by the increased core confinement extended tothe large radius barrier. A relatively high magnetic field, BT �3.45 T, has been chosen as being far enough from the powerthreshold of type I-ELM. The plasma current limitations,imposed by the configuration, IP � 2.5 MA, allow then tooperate with q95 = 4.5–7.5. Narrow ITBs (r/a ∼ 0.3–0.4) areroutinely triggered at medium power level (PTOT � 20 MW)in deeply reversed current profile discharges (qmin = 3), butmitigation of edge MHD activity via light impurities (CD4

or Ne) injection is necessary for sustaining these ITBs atIP = 2 MA. Generally these internal barriers do not improveglobal confinement of the discharge. Very wide ITBs have beenobtained at lower current IP = 1.5 MA and high triangularityδ ∼ 0.45. As shown in figure 1, the foot of these barriers,i.e. the region where the temperature gradient increases abovethe threshold value, confines with the edge pedestal associatedto the H-mode. In the high power phase the magnetic axisis located at R = 3.13–3.16 m and the H-mode pedestal atR ∼ 3.85 m making the radius of the barrier r/a � 0.7.Indeed these ITBs survive the H transition if the total injectedpower is in excess of PTOT = 20–22 MW and the edge MHDactivity is moderate due to impurity seeding thus reducing theperturbation induced to the foot of the ITB by the ELM. Anequivalent flux of 1021 electron s−1 of Ne is injected 0.5 s afterthe start of the high power phase to maintain the ITB throughoutthe H-mode phase. As shown in figure 2, by the steep gradienton electron temperature profile, the ITB foot is located welloutside of the minimum q radius without appreciable changefrom the L to H phase. These discharges have H89βN ∼ 3.5,

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Figure 1. Contour plot of the strength of a high tringularity ITBaccording to JET ρ∗

T criterion, for JET (shot 62293 bottom trace);power waveforms, (mid) and Dα emission, (top trace). The darkarea at r/a ∼ 0.7 indicates the position of the barrier very close toH-mode pedestal.

Figure 2. Electron temperature, density and q-profile for JET hightriangularity (shot 62293: left) during the L-mode phase; aftertransition to H-mode (right). It is very evident that the steep gradienton electron temperature at R ∼ 3.67 m is located well outside theminimum q radius.

linear averaged density around 60–70% of Greenwald value,last for ∼10τE and are only limited by the power pulse duration[16]. The fraction of radiated power is PRAD/PTOT ∼ 50–55%with no evidence of impurity accumulation even though adetailed study of impurity transport is still missing for thisscenario. From the JET database it is evident that these wideITBs are compatible only with plasma edge in L-mode or withsmall and frequent ELMs. Nevertheless the energy stored inthe core of these discharges compensates for the reductionassociated with the lower edge confinement.

The development of wide ITBs at high current (IP �3.5 MA, BT = 3.4 T) started from negative magnetic sheartarget plasmas for triggering the barriers in the region of lowmagnetic shear at large radius. In the selected scenario, the

Figure 3. q-profiles deduced from MSE measurements at 4.4 s(before high power phase) for ohmic preheat (shot 58092,diamonds), NBI preheat (shot 58220, squares) and LHCD preheat(shot 58383, circles).

ITBs are triggered at a relatively large radius (r/a > 0.5) whenthe minimum safety factor (qmin) reaches an integer value [17],with a relatively modest amount of power (PNBI + PICRH ∼15 MW). This ‘outer’ ITB is situated in the positive magneticshear region of the plasma and can coexist with an ITB situatedat a smaller radius, in the negative shear region. Three routesfor pre-forming the q-profile, before the high additional power(PADD) is applied, were compared: fast ohmic current ramp(0.5 MA s−1), NBI preheat (relying on the bootstrap currentfor shaping the q-profile) and LHCD preheat (relying on theexternal current drive). The first two routes led to weaknegative shear and the third one, to weak or deep negative sheardepending on the LH power. In figure 3 typical target profilesare reported for the three scenarios. Despite the differences inthe starting q-profile, by applying PADD ∼ 15 MW, an ‘outer’ITB is triggered when qmin reaches either 3 (NBI preheat andLHCD preheat scenarios) or 2 (fast ohmic ramp scenario). TheITB triggering time changes between scenarios, and the strongreversed q-profile case with lower hybrid preheat is the onlyscenario also exhibiting an internal barrier throughout all thehigh power phase. There is little difference in the ‘outer’ ITBlocation, indicating that it is not sensitive to the magnetic shearin the inner part of the plasma. At the additional power levelsused in the experiments described above, the ‘outer’ ITB hasa relatively weak pressure gradient and does not lead to highneutron yield. Higher power is needed to obtain strong ITB atwide radius in plasmas with monotonic q-profile in JET [18].PADD > 18 MW (NBI + ICRH) has been applied in plasmaswith LHCD preheat in a limited number of shots. At that powerlevel, large ELMs are triggered, which result in the terminationof the ITB. Neon puffing was used to control the edge andmaintain small ELMs throughout the high power phase. Thetime evolution of the ITBs obtained is shown in the upper partof figure 4, and in the bottom the additional powers and IP

waveforms are reported. The ‘outer’ ITB is triggered whenqmin reaches 3. It starts at R > 3.5 m and moves outwardsto R ∼ 3.65 m as the current is ramped; with magnetic axispositioned at R ∼ 3.05 m and separatrix at R ∼ 3.95 m, theeffective radius of the barrier is r/a = 0.55–0.65. This is inaddition to the ITB located in the region of negative magneticshear, which exists already during the LHCD preheat and

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Figure 4. Location of the ITB as a function of time for the high current JET (shot 65208, top); power and IP waveforms (bottom).

persists throughout the high power phase. The ‘outer’ ITBis terminated by an MHD event at 7.1 s, (figure 4, top) whichhas been identified as the so-called snake mode [19]. Snakeshave been linked to the presence of an integer q surface and asharp pressure gradient and are frequently observed in plasmaswith ITB [20]. In the experiments described above, snakemodes terminate the ‘outer’ ITB in several pulses. This iscorrelated to n = 1 edge MHD activity. Reduced NBI powercan prevent the earlier appearance of snakes (it generally occurswhen IP ∼ 2 MA, see also figure 4). A possible solution toavoid the later snake (7.1 s in figure 4) would be to crop thecurrent ramp before the edge MHD is reached. However, notethat the ‘outer’ ITB remains weak (near the empirical thresholdfor an ITB in JET [7]) even with PADD > 20 MW and doesnot lead to the performance hoped for. Possibly this is due tothe magnetic shear at the ITB location not being low enough,which in turn indicates the need for a larger off-axis currentcontribution.

2.2. Pellet fuelled high density ITBs

Both NBI central fuelling and torque injection will bemuch lower or missing in ITER compared with the presentexperiments. A high additional current drive power willbe needed to maintain an optimal magnetic shear profileto stabilize the turbulence. Pellet injection is a promisingtool for creating steep density gradients that can contributeto turbulence stabilization and to raising the central density.Experiments in JET have successfully started exploring thispossibility [21]. The basic scenario makes use of LHCDapplied at the very beginning of discharges (BT = 3.2 T,IP = 2.0 MA) to produce a reversed shear configuration whichis maintained after a 1 s gap used for pellet pre-fuelling. Thepellets are injected at the speed of 80 m s−1 and frequency of5 Hz, but some pellet injection can be lost due to technical

Figure 5. Time traces of plasma current (top grid), electron densityat two different radii (middle grid) and powers (bottom grid) forpellet fuelled ITB JET shot 57941. Spikes in the off-axis density area good representation of the pellet injection timing.

unreliability of the system. At the end of the gap, thatis either ohmic or heated by low NBI power (4 MW), themain ICRH and NBI heating are switched on, see figure 5.A high density ITB, figure 6(a), is obtained at an initialtoroidal rotational shear which is four times lower than instandard ITB discharges with similar performances [22] withPLH/PNBI/PICRH = 1.9/8.6/6.6 MW, see figure 6(b). Withthe above recipe, current and density profiles are independentlycontrolled, and a variety of combinations have been produced.So far, it seems that both early LHCD pulse and pelletfuelling are needed to enter this regime, thus pointing tothe synergetic role of density gradient and magnetic shearfor turbulence quenching. Discharges without LH preheat,

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(a)

(b)

Figure 6. (a) Density and temperatures profiles of the high densityITB discharge #57941 after 1.1 s high power and refuelling withshallow pellets. (b) Toroidal rotation profile for the same dischargeand timing (——), for the standard ITB discharge #51573 in asimilar condition (- - - -).

featuring a monotone target q-profile, never produce an ITBdespite the peaked density profile generated by the same pelletinjection [21]. The above recipe has allowed the production ofITB discharges with core density close to the value foreseen bythe Greenwald limit. These discharges have equalized ion andelectron temperatures, figure 6(a), and the ITBs last for morethan 1 s corresponding to 4–6 times the energy confinementtime. The improved performance phase is typically terminatedby MHD events and by the decay of the core density. The firstattempts were made to refuel the already formed barrier duringthe main heating. So far, the barriers seem to survive only theinjection of shallow pellets (80 m s−1), efficiently refuelling theedge. These discharges have been simulated [23] by runningthe JETTO code [24] in the predictive mode. In this mode ofoperation the code computes the electron and ions temperatureprofiles using the semi-empirical shear dependent Bohm/gyro-Bohm transport model [25]. Furthermore to describe pelletinjection, JETTO has been equipped with a module based ona neutral gas and plasma shield (NGPS) ablation model [26].Analysis of particle deposition and transport shows that theablation is in agreement with code prediction without any

evident radial drift. This is expected to be due to the lowβ plasma target outside the barrier where ablation takes place.To simulate the barrier formation in JETTO, the particle andenergy transport coefficients D and χ can be reduced accordingto a criterion which takes into account the magnetic shears and the ratio ωE×B/γITG between the shear of the E × B

velocity and the growth rate of the ITG modes. Indeed, it hasbeen statistically observed that a barrier is formed where thecondition z = −0.14+s−1.47ωE×B/γITG < 0 is satisfied [27].Therefore the Bohm diffusion term in the mixed Bohm/gyro-Bohm transport model is multiplied by (z), where is theHeaviside step function.

During the gap between LHCD prelude and high powerphase, the post pellet density evolution is in agreement withthe mixed Bohm/gyro-Bohm diffusion including an anomalouspinch velocity as usually observed in L-mode. In the mainheating phase, the simulations show that pellet penetrationdepth and barrier strength are the main factors determiningwhether the barrier survives the pellet injection or not. Indeedit is seen that the density pulse induced by the relatively shallowpellet (80 m s−1) injected on a strong ITB (#57941) stops atthe barrier foot. The ITB survives but no fuelling inside thebarrier is observed. On the other hand when the density pulseassociated with faster pellets (160 m s−1) injected on a weakerITB (#55861) reaches the barrier foot, both the density gradientand the toroidal rotation shear are locally reduced, with anegative impact on turbulence stabilization and consequentconfinement degradation. The results of the simulation forthe two cases are shown in figure 7. The profiles of particlediffusivity, toroidal rotation, density gradient and ωE×B shearare simulated at pre- and post-pellet times. In the left case(57941: 80 m s−1 shallow pellet on strong ITB) the diffusioncoefficient does not change much after the pellet and it remainslow in the barrier region; this is a consequence of the littlechange in the subsequent parameters, which are involved inthe turbulence stabilization. However it is not so in the rightcolumn (55861: 160 m s−1 deep pellet on weak barrier) wherethe pellet penetrates inside the barrier destroying it as shownby the increase of the particle diffusion in the core.

No experimental data of the 160 m s−1 pellet injected onstrong ITB are available so far; however, JETTO simulations,performed with an artificial injection of such pellets on shot#57941 featuring a strong barrier, show that the ITB wouldsurvive the perturbation. Nevertheless, even in this case,no central fuelling is expected. More experimental data areneeded to better assess this issue.

Magnetic islands with an m/n = 3/1 topology and doubletearing features are destabilized after pellet injection whichcauses as well a braking of the edge rotation [22]. Furtherstudies are planned in the future for better clarifying theseparate role of density peaking and current profile in thebarrier formation and for obtaining a more steady performance.

2.3. Long pulse

The ITB experiments in JET have also attempted to extend ITBdischarges to the duration close to or exceeding the currentdiffusion time [28]. The resistive time here is assumed as thevolume averaged plasma resistivity. To realize this scenarioNBI power is split into two sequential pulses each of 10 s

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Figure 7. JETTO simulation: (left) #57941 (shallow pellet), (right) 55861 (pellet destroying the ITB). Full lines pre-pellet, dashed linespost-pellet profiles. (a) particle diffusion coefficient, (b) plasma toroidal rotation velocity, (c) density gradient, (d) ωE×B shear.

and with half the available beams. Firstly a wide ITB regime(IP � 1.8 MA, BT = 3 T) has been developed at the poweravailable in these conditions (PNBI +PICRF +PLHCD � 18 MW).Wide ITBs are required in order to increase the confinementsignificantly by their larger volume thus limiting operationalrequirements. The required scenario is achieved by carefultiming of the main heating power at the time when theminimum q value in the plasma reaches 3. To generate awide ITB in the positive shear region, the LH preheat phaseis tuned to form a moderately reversed shear q-profile in theplasma core. In this case, the ITB is triggered at qmin = 3 andthen evolves in the positive shear region up to r/a = 0.6, andthe central ITB in the negative shear region does not developor vanish. Due to the combination of LH, NB and bootstrapcurrent the q-profile can be maintained above q = 2 for a longtime. Although this discharge is not fully non-inductive andstill has a βN of 1.6, the ITB is sustained for more than 7 s.The NBI power is generally used under RTC [5] to stabilizethe ITB strength at moderate value, ρ∗

T < 0.02 comparedwith a JET threshold of 0.014 [7], and in this condition noimpurity accumulation is observed by soft x-ray signals. Thistype of discharge has been used as a target to experimentwith the new real time techniques for the simultaneouscontrol of the current and pressure profile [6]. Additionalexperiments have attempted to extend this ITB to times upto 20 s i.e. significantly longer than the resistive time ∼8–10 s. The previous experiments reported in [1, 29] were justapproaching this duration, but did not have all the technologicalconstraints already fulfilled to allow long operation in JET [28].

Although the creation of wide ITB is quite reproducible inthis scenario, the worse machine conditions, due to an aircontamination, determined higher oxygen concentration in theplasma discharge making it difficult to maintain the ITB forlong times. In fact the increased plasma resistivity made theapplied LH power insufficient to drive enough off-axis currentto maintain the required q-profile. Consequently a peaking ofthe current profile occurs as indicated by the continuous rise ofthe plasma inductance in the bottom trace of figure 8. Howevera record energy of 326 MJ was injected in this dischargedemonstrating on the one hand the JET capability of handling alarge amount of power for the purpose of long pulse dischargesand on the other the criticality of steady state operations.

2.4. Mode conversion ICRF heating on ITBs

An ITB scenario has been developed at JET in deuteriumplasmas with minority concentrations of 3He up to 20% ofthe total electron density [30] for using ICRF heating in modeconversion (MC). The MC power provides a well-localizedsource of electron heating thus allowing both to obtain highelectron temperatures and to infer transport characteristics ofthe ITB [31] through the well-assessed modulation techniqueof the coupled power. The reference scenario makes use ofdischarges at BT = 3.2–3.6 T and IP = 2.6–2.9 MA; thebracket values allow for the localization of the ICRF poweroutside/inside the ITB. Target plasmas, with deep reversedmagnetic shear profiles, are obtained by applying 2–3 MW ofLHCD power in the early phase of the discharges. Barriers,sitting in the region of negative shear, are then triggered

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Figure 8. Time traces of shot 62065, IP = 1.6 MA, BT = 3.0 T. ITB end at 9 s. E = 326 MJ inject energy.

×

Figure 9. Temperatures, density and q-profiles of a BT = 3.6 T ITBdischarge. 12% 3He concentration in D for core deposition ofminority and MC ICRF power (33 MHz).

injecting up to 18 MW of NBI power. 4 MW ICRF power,50% amplitude modulated, are also coupled to these plasmasat different radial positions. The effect of the ICRF poweris strongly enhanced by the good transport properties of thebarrier when it is coupled in the core region. In figure 9 theprofiles of one of these discharges at BT = 3.6 T are reported,where a 12% 3He concentration generated a mixed minority-MC heating regime. In these conditions both minority heating(r/a ∼ 0.3) and MC (r/a ∼ 0.06) deposition locations

are inside the barrier radius (r/a ∼ 0.5) producing Te0 �13 keV, Ti0 ∼ 24 keV. This performance is significantly highercompared with similar discharges where H minority or small3He minority concentration, n(3He)/ne � 5%, schemes areused. The ITB strength in this discharge is further revealed bythe hysteresis effect seen on the barrier when the main poweris stepped down by the RTC for avoiding disruption inducedby extreme pressure profile peaking [32]. Neutron emissionreaches its maximum 1 s (∼3τE) after the NBI power has beenreduced from 14 to 11 MW and is still close to the maximumwith the barrier surviving for 300 ms after a further reductionto 8 MW. A simulation of this discharge with the TRANSPcode estimated a transient equivalent QDT = 0.25. From themodulation analysis the barrier is seen to be behaving as anarrow layer of reduced diffusivity embedded in plasma withhigher diffusivity. This layer strongly damps the heat wavesregardless of the propagating direction. Detailed transportresults deduced from the detected heat waves moving inwardswhen the power is deposited outside the ITB location oroutwards when centrally deposited are reported in [33].

2.5. ITB similarity experiments

The objective of this study is to compare the dynamics ofthe same type of ITB on JET and ASDEX Upgrade (AUG),using neutral beam heating in current ramp with low magneticshear. The parameters for the two experiments are matched asfar as possible, using the same low triangularity (δ ∼ 0.22)plasma shape, similar q-profiles (with qmin ∼ q0 ∼ 2) andclosely matched values of ρ∗, ν∗ and β for the target plasma.Just before the start of the neutral beam heating, at similarline averaged densities, neutral beams have comparable powerdeposition profiles.

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Table 1. Discharge parameters for AUG–JET ITB comparison.

AUG #16147 @ 0.7 s JET #62175 @ 2.5 s

BT = 3.0 T BT = 1.8 TIP = 0.78 MA IP = 0.81 MAq95 = 6.8 q95 = 7.6〈ne〉 = 2.3 × 1019 m−3 〈ne〉 = 1.1 × 1019 m−3

〈Te〉 = 1.2 keV 〈Te〉 = 0.6 keV

Some more details of the experiment are reported intable 1. In this comparison, the target electron temperaturein JET is too low without additional heating. The resultsof the experiments show that both devices generate an ionITB at 7–10 MW input power; neither machine exhibits anelectron ITB in this regime. Both experiments made transientITBs, which collapsed coincident with the onset of largeELMs (always the case at AUG). This suggests similar ITBphenomenology in the two tokamaks. Differences betweenthe experiments mainly result from the low target temperaturein JET. Hence the neutral beams in JET begin mainly as anelectron heater while the AUG beams heat ions dominantlyfrom the outset. AUG achieves higher ratios of the iontemperature over the electron temperature, than the JETcases, throughout the main heating phase (including the ITBphase). In follow up discharges in JET, the scenario usedweak LHCD heating in the prelude to obtaining higher targetelectron temperature, to overcome the mismatch between theexperiments. Moreover, by effectively mitigating the edgeMHD activity with neon seeding, an ITB was sustained for10 energy confinement times, with barriers both on ions andelectrons. Similar mitigation techniques in ITB discharges onAUG were not successful so far.

2.6. Hybrid regime and low momentum input scenarios

Activity on hybrid scenarios started in 2003 at JET with theobjective of developing the regime towards non-dimensionalparameters achievable on ITER. Firstly the AUG regimewas reproduced in an identity experiment where magneticconfiguration, q-profile, ρ∗ and β were matched andperformance verified up to βN = 2.8 at BT = 1.7 T. Stationaryconditions with H89βN/q95 = 0.42 have been achieved. Thenthe scenario has been tested at high triangularity δ = 0.45and at ITER magnetic configuration before being developed atlower ρ∗ ∼ 0.4 ×10−2. Here all the signatures of the scenariohave been reproduced, but the performances are limited by theavailable power. Detailed results of the hybrid research at JETwill be found in [34].

More recently, in a low activation campaign, thedevelopment of the hybrid scenario with dominant ICRHand negligible momentum input has started [35]. In thisscenario the central q value is maintained above 1 by moderatepower of LHCD. Time traces of a typical hybrid regimedischarge with dominant ICRH power are reported in figure 10.While standard hybrid regime discharges generally exhibitbenign neoclassical tearing mode with toroidal mode numbern = 2 and fishbones, those with strong electron heatingby ICRH feature additional MHD activities in the form ofslowly growing n = 1 modes and Alfven eigenmodes [36].In the same condition of absence of momentum input, theion transport has been probed maximizing the ion heating

Figure 10. Time trace of the ICRH dominated hybrid regime JETdischarge 62789. From top: plasma current and lower hybrid power,(first grid); NBI and ICRH power, (second grid); Diamagneticenergy, (third grid); Normalized beta and enhancement factor H89,(fourth grid).

via high concentration 3He minority ICRH scheme. A well-developed electron-ITB is pre-existing in the core of thesedischarges, sitting in the region of negative shear. A strongE×B shearing rate is generally present in plasmas with a well-developed ion ITB and it is thought to be the main factor forthe turbulence stabilization. However, so far, it is not yet clearwhether this sheared flow is also the trigger of the turbulencereduction. In our experiments the bulk ion heating inducedby the ICRF is sufficient to create an ion ITB in conditions ofnegligible momentum input and shear flow whenever the qmin

approaches the q = 2 rational surface. Unfortunately, the goodconfinement phase, although very reproducible, was alwaystransient [37]. The absence of substantial levels of auxiliaryheating (less than 3 MW of ICRH coupled to the ions) maybe part of the reason for the difficulty in sustaining the ITB.Moreover, a systematic study of the dependence of the triggerof the barrier on the q-profile and on external momentumwould be necessary. A possible explanation of the observedphenomenology could be (at least at the available additionalpower level) that a negative magnetic shear (with a small s = 0region), by itself, is not sufficient for the onset of an ion ITB;the appearance of a rational q surface can further reduce theturbulence growth rate, facilitating the transition.

3. Fuel and impurities transport in AT in JETplasmas

The control of plasma density, purity and fuel concentrationswill be the key issue in optimizing and maintaining fusion

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Figure 11. D and V for tritium inferred by transport code (negativeinwards) compared with neoclassical predictions. Also reported areeffective diffusivity and neoclassical prediction for deuterium D.

performances in the next generation experiments. The studyof these issues has been especially addressed in advancedscenarios at JET as the presence of ITB could strongly affectthe transport of fuel and impurities. Transient injection oftrace amount of tritium allowed us to separately determineits diffusion coefficient and convection velocity. Moreovera new technique has been developed at JET that allows thestudy of impurities transport virtually in all the discharges byparasitically injecting a calibrated mixture of Ar/Ne [38].

3.1. Trace tritium injection

The high current scenario with deeply reversed magnetic sheargenerated by LHCD, described in section 2.1, was used tostudy tritium transport and fuelling in ITB plasmas [39]. Byengineering the NBI and LHCD waveforms, plasmas witheither a single ITB in the negative magnetic shear regionof the plasma or a double ITB with additionally an ‘outer’ITB, were obtained. Tritium was injected either by gaspuffing or by neutral beam injection, in both types of ITBplasmas for comparison. The tritium evolution was monitoredwith collimated vertical and horizontal neutron cameras thatseparately measure DD and DT neutron emission [40]. Thespatial resolution of the two systems allows monitoring ofboth the regions of the core and outer ITB. The diffusioncoefficient (DT) and convection velocity (vT) are determinedby fitting the spatial and temporal evolution of the neutronemissivity with the transport code UTC/SANCO [41], usingthe more complete DT neutron calculation by TRANSP [42].This analysis has been done only for the single ITB plasmaup to now and is reported in figure 11. It shows that DT

decreases to the neoclassical value in the region of the ITB, butcontrary to previous TFTR results obtained with perturbativetransport studies [43] that found a reduced diffusivity of tritiumto neoclassical value also in the core region here inside theITB DT remains higher. The inward convection of tritium also

decreases at the ITB location by about a factor of 3 but remainshigher than the neoclassical prediction. In hybrid regimes DT

remains higher than the neoclassical value on the whole minorradius while measured diffusion and convection of T in theedge show a strong correlation with q95 [42]. Preliminaryresults of the comparison of the fuelling methods indicated amuch higher efficiency of the beam injected tritium to fuel thecore of a strong ITB than the gas puffed tritium. Comparingthe neutron emission in two similar ITB discharges it is foundthat the beam is about 25 times more efficient than the gas puffin fuelling the core. This difference is much less pronouncedin discharges with a modest strength of ITB [39].

3.2. Impurity transport

Diffusion coefficient D and convection velocity V , both for Arand Ne, have been found to be strongly anomalous in the hybridregime discharges at fixed q95 = 4 and different ρ∗. A changeat the plasma edge from outwards to inwards convection isfound, for the investigated ρ∗, with triangularity increasingfrom δ ∼ 0.2 to δ ∼ 0.4 [38]. It remains to be clarified ifthis change is connected to different ELMs frequency or to thechange in edge temperature and density gradient.

Preliminary analysis of Ar and Ne transport has beencarried out in discharges with and without barrier at low andhigh current. It indicates that both the diffusion and convectioncoefficients for Ar and Ne are reduced in the region of thebarrier to values close to the neoclassical ones. The analysisof discharges with double barriers (narrow barrier generallylocated at r/a � 0.35 and external one at r/a � 0.6–0.7)presents an additional difficulty due to the structure in theconvection and diffusion coefficient. A systematic analysisof Ar and Ne transport with the code UTC-SANCO [41]and comparison to the neoclassical values calculated with thecode JETTO-NCLASS [44] is in progress in all types of ITBdischarges and will be the object of a dedicated publication.

Here we report in figure 12 an example of intrinsicimpurities behaviour in a double barrier discharge (JETshot 61353). The carbon concentration profile is measuredwith the charge-exchange diagnostic, while nickel radiationis deduced from the high-resolution soft x-ray spectrometer.An interesting behaviour is evident in correlation with barriersevolution. In the upper part of figure 12 the electron ITBcriterion is reported showing that a double barrier is presentfrom 5 to 7 s at radii r/a ∼ 0.3 and r/a ∼ 0.6. Similarbehaviour can be found, with lower time/space resolution, alsoin the ion criterion. At 7 s, the outer barrier disappears andthe inner barrier gains in strength. The carbon concentrationat 3.7 m outside both barriers is reasonably constant in timethus suggesting that a constant influx from the separatrix canbe assumed. Inside the inner barrier and in between theinner and outer barrier, the carbon concentration is constantduring the time of existence of the double barrier, but it clearlyincreases when the outer barrier collapses. A similar behaviouris indicated by the Ni26+ concentration measured with the x-rayspectrometer. It must be noted that the shell of existence of theion Ni26+ is not exactly determined but is expected to be closeto the dimension of the inner barrier or slightly larger. Thephase after the outer barrier collapse is also accompanied byan increase of the ZEff as seen in the bottom part of figure 12

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Figure 12. Time traces of shot 61353, Ip = 3.0 MA, BT = 3.2 T.The ITB criterion for electrons is shown in the upper part of thefigure; then following from the top are the input powers, theneutrons emission, the diamagnetic energy, Dα emission, carbonconcentration measured by CXRS and the concentration of theNickel 26+ ions measured by the x-ray spectrometer.

for a time greater than 7 s. This apparent accumulation ofimpurity in the core is lost at 8 s when the barrier collapsesmomentarily. The detailed analysis of Ar and Ne transportshould answer if this apparent accumulation is linked to anincreased ITB strength and consequent steepness of gradientsin the core of the plasma or if there is an effective benefit ofthe outer barrier in controlling impurity influx.

4. Conclusions

Recently the research on advanced scenarios at JET hasfocused more on scenarios development than on performanceachievement; this strategy also follows the evolution of poweravailability at JET. Most of the parameters, required for anadvanced scenario to be promoted as an effective candidatescenario for ITER, have been separately reached. Wideand very wide barriers have been obtained in a variety ofplasma parameters. These barriers represent an importantstep forward to producing an advanced scenario with enoughimprovement of the core confinement to compensate for the

reduction in the pedestal amplitude necessary for moderatingELM activity. Small amplitude ELMs allow wide barriers tocoexist with H-mode and will alleviate plasma wall interaction.Furthermore, associated with these wide barriers there willbe the possibility of increasing poloidal beta, βP, withoutpushing plasma pressure to extreme values. The double benefitof increasing the bootstrap current (IBoot scale as βP), thusreducing the need of external current drive systems, and ofincreasing performances, without affecting MHD stability,would be intrinsic to the wide barrier scenario. Nevertheless,some external current drive power will be needed for steadystate operations. In this respect the use of lower hybrid hasbeen very effective in JET, allowing at the same time fullynon-inductive discharges and current profile control capability.More generally, RTC techniques with all heating systems asactuators, to control ITB location and performances, have beenthe key to the progress obtained at JET in advanced scenarios.Most importantly the widest barriers in JET have been obtainedwith ITER relevant shaping (δ ∼ 0.45), these barriers arecompatible with moderate ELM activity only and exhibit goodconfinement and plasma density above the densities generallyobtained in ITB plasmas at lower triangularity. High densityITBs at low triangularity, (δ ∼ 0.25), have been obtained inpreliminary experiments using the pellet; more developmentwill be necessary to find a reliable scheme for deep re-fuellingwithout destroying the ITB.

The different techniques (modulate RF power in MCscheme, impurity injection, tritium beam blip and gas puff)employed to infer transport characteristics seem to agree indescribing the ITB as a layer of reduced diffusivity. Noclear evidence of impurities accumulation is found at leastin the scenarios presented here. The presence of an outwardconvection located on the more external ITB, in dischargeswith double ITB, will represent, if confirmed by the ongoinganalysis, a very positive aspect of this scenario.

The role of turbulence on advanced scenarios has beenpreliminarily addressed in particular in a low activationcampaign where the development of scenarios with dominantRF heating, hence low momentum input and Te close to Ti, hasstarted. The role of q-profile and/or of the magnetic shear intriggering an ion–ITB has been investigated.

The integration of the different issues, successfullyaddressed till now, will be the central objective of futureexperimental JET campaigns. Moreover the ongoing powerupgrade will allow us to push these scenarios to higherperformances in terms of the figure of merit H ∗βN/q2

95 in anITER relevant space parameter.

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