Fattorini, L.

Horton, L.

Kallenbach, A.

Lang, P.

Neuhauser, J.

Urano, H.

English

MPG.PuRe

Active edge localized mode (ELM) frequency control with pellets

Active edge localized mode (ELM) frequency controlwith pelletsP.T. Lang∗, A. Kallenbach, L. Fattorini1, L.D. Horton, J. Neuhauser, H. Urano,ASDEX Upgrade TeamMax-Planck-Institut für Plasmaphysik, EURATOM Association,Boltzmannstr. 2, 85748 Garching, Germany1 Centro de Fusão Nuclear, Associacão EURATOM/IST,Instituto Superior Técnico, 1046-001 Lisboa, Portugal∗ Corresponding author; e-mail: ptl@ipp.mpg.de1. IntroductionThe occurrence of edge localized mode (ELM) activity seems to be a common feature inmagnetically confined plasmas when accessing high confinement regimes. ELM events,characterized by a transient confinement collapse at the plasma edge, result in strongenergy and particle bursts from plasma. These bursts can form a hazard for in-vesselcomponents. Hence, a technique would be desirable to achieve external control of theELM frequency. Pellets are already known to trigger ELMs when injected during type-IELMy H-mode phases [1]. Here, we report on a successful experimental approach atASDEX Upgrade [2] to increase the ELM frequency and mitigate losses per ELM intype-I regime plasma discharges by repetitive injection of small pellets.2. Experimental setupFor the study presented, we choose a configuration with IP = 1MA, Bt = −2.7T ,q95 = 4.9, κ = 1.6, δu = 0.12 and δl = 0.37. The applied neutral beam injection (NBI)heating of 3.5 MW, just above the L-H transition power threshold, results in stableand robust operation in the type-I ELM regime with rather low natural ELM frequencyas desired for demonstration of active ELM frequency control with the existing pelletinjector. Depending on the actual wall conditions, these discharges developed a naturalELM frequency in the range 20 to 40 Hz for no or modest external gas puffing, with theELM frequency a bit higher and the ELM size accordingly smaller in the latter case.The pellet injector is based on a centrifuge pellet accelerator equipped with a storagecryostat capable of delivering up to 120 pellets. In order to deliver pellets sufficientlysmall to prevent too strong refueling at the desired high repetition rate, it was necessaryto further reduce the nominally smallest pellet size from the existing hardware originallydesigned for deep pellet fueling. This was achieved by exploiting the special features ofthe ”looping-II” set-up [3]. Nominal (1.4mm)3 cubic pellets injected through the 17 mlong guiding tube at a speed of 1 km/s are continuously reduced in size by transfer lossesto about 1 × 1019 D-atoms entering the plasma, while keeping the delivery reliabilitystill beyond 0.9. Applying this injector setting and omitting pellets on purpose, pelletrepetition frequencies fP =250n, n = 3, 4, 5... can be obtained.3. Results3.1. Controlling the ELM frequencyThe basic idea behind ELM control is to choose the parameters in such a way thatspontaneous ELMs are avoided, i.e. only ELMs initiated by a pellet appear. This requiresthat pellets can trigger ELMs reliably and, in addition, that the pellet frequency issufficiently high. In fact, it has been found that every pellet injected during such type-IELM phases triggered an ELM, with the minimum required pellet size obviously well30th EPS Conference on Contr. Fusion and Plasma Phys., St. Petersburg, 7-11 July 2003 ECA Vol. 27A, P-1.129below 1 × 1019 D-atoms. With respect to the second condition, it has been found thatfPel > 1.5× f0ELM is required.In this way, full ELM control is achieved withthe desired increase of fELM by increasingfPel as seen in fig. 1. Here, the ELM fre-quency (averaged over 0.2 s) and the pelletcontrolled ELM fraction c ≡ fELMfPelare plottedas obtained in a discharge series performinga fPel scan. In these discharges, f0ELM wasabout 30 Hz. For the lowest fPel of 20.8 Hz,an enhancement of fELM takes place duringthe pellet phase (indicated as shaded area),while c reaches 0.5 only. At fPel = 41.7Hz slightly above f 0ELM , c is already close tounity. Full external ELM control and essen-tially c = 1 is established at the highest fre-quencies. fELM is equal to the selected valuefor 62.5 Hz while for the maximum setting of83.3 Hz a few missing pellets reduce the realfrequency to about 70 Hz. The scan provesthat full external ELM frequency control ispossible at least in the frequency range ofabout 1.5− 3× f 0ELM , with the upper bound-ary only given by technical limitations.2 3 4 5TIME  (s)1000ELM frequency (Hz)Pellet controlled ELM fraction120.8 Hz111100010010041.7 Hz62.5 Hz83.3 HzFigure 1: Pellet frequency scan.Increasing fPel during pellet phase(shaded area) results in fELM increasebeyond f 0ELM . A pellet controlled ELMfraction c ≡ fELMfPelapproaching unityindicates full ELM control.3.2. Impact on the plasma performanceThe external enhancement of fELM causes confinement reduction. This effect, observedwhen increasing f 0ELM by increasing the gas puff rate or the heating power, takes place alsofor external ELM frequency enhancement. However, the observed confinement reductionis quite modest, as shown in figure 2. The left part shows the temporal evolution of thedischarge with a requested fELM = 83.3 Hz phase. Before and after the pellet sequence,reference phases were performed, the latter one including an additional gas puff Γgas =7× 1021/s, almost equal the pellet particle flux. As can be seen, the additional gas puffcauses a density enhancement like the pellet imposed one while causing only a weak ELMfrequency rise and confinement reduction with respect to its unpuffed counterpart. Forall three phases the key parameters fELM , WMHD and n̄e are extracted during quiescentphases. All data obtained from the pellet frequency scan are given in the right partof figure 2, where n̄e and WMHD data are plotted versus fELM . Squares and trianglesrepresent for the Γ = 0 and the gas puff reference phases, respectively; circles indicatepellet phases. Again, the slight density increase and energy decrease imposed by the gaspuff as well as the mild refueling constraint of pellet injection becomes visible. However,most important, there is only a quite modest confinement decrease with pellet enhancedfELM , WMHD ∼ f−0.16ELM , much weaker than predicted from the empirical scaling derivedfor AUG and JET type-I ELMy mode plasmas, yielding WMHD ∼ f−0.6ELM [4]. This pelletimposed confinement reduction, e.g. about dWW0≈ 0.10 for fPel = 68Hz, might be causedmostly by convective losses due to enhanced edge particle throughput: A flux of 7× 1021D atoms with a temperature of about 1 keV would result in an additional loss power ofabout 0.34 MW ≈ 0.096 × Pheat, consistent with the observed energy reduction. Thus,further reduction of the energy loss seems possible by a reduction of the pellet size.30th EPS Conference on Contr. Fusion and Plasma Phys., St. Petersburg, 7-11 July 2003 ECA Vol. 27A, P-1.129 2 of 42 3 4 50.8n   (10    m   )e 20 -3TIME  (s)W        (MJ)MHD0.50.40.7D   ELM monitor(a.u.)α0.6Pellet monitor#1748833 Hz29 Hz68 Hz509 kJ604 kJ567 kJ20 400 8060f        (Hz)ELMf        - 0.6f        - 0.16W        (MJ)MHD0.50.60.80.7n   (10    m   )e 20 -30.810.760.82Figure 2: Left: Density, energy and ELM frequency evolution during phases withoutpellets and external puff, with small pellets and no puff, and with external gas puffonly. Right: Data obtained from no gas (squares), gas puffed (triangles) and pelletphases (circles) versus fELM . Increasing fPel imposes slight refueling and confinementdegradation. A fit to data unveils WMHD ∼ f−0.16ELM much less pronounced than ∼ f−0.6ELMpredicted for natural ELMs.3.3. ELM featuresOn important motivation for external ELM controlwas the hope to mitigate the energy loss and maxi-mum power flux per ELM and prevent potential haz-ards for in-vessel components. Indeed, this desiredeffect could be verified. All ELMs detected in thisinvestigation - both in pellet and reference phases -fit well into a dataset derived for ASDEX Upgradeand JET data [6]. This scaling unveils that about20% of the heating power is expelled by ELMs. Since,for pellet triggered ELMs, this is not on the expenseof a strong confinement degradation, the energy lossper ELM reduces with increasing fELM . With a vir-tually unchanged ELM duration, also the power fluxis mitigated. This is shown in fig.3, where a typicalnatural and a pellet triggered ELM are compared ina case where pellets raised f 0ELM by more than a fac-tor 2. Whereas the pellet triggered ELM obviouslystill shows type-I features - as indicated by Mirnovcoil and Dα radiation signal - the energy loss and themaximum power flux into the divertor is mitigated.Energy and power flux reduction takes place for boththe inner and the outer divertor leg. The release ismore pronounced for the outer divertor, further in-creasing the in/out power flux asymmetry.101020Pellet monitor (a.u.)5#16716f         = 24.6 HzELM0W        (MJ)MHDDivertor energy deposition outer/inner/total (kJ)Pf         = 62.5 HzELM1 ms0-202020 kJMHD activity (T/s)D    outer/inner div. (a.u.)αDivertor power flux outer/inner (MW)Figure 3: Comparison of nat-ural and pellet triggered ELM.The pellet induced ELM stillshows typical type-I features,but causes reduction in en-ergy loss and mitigates divertorpower flux and energy deposi-tion30th EPS Conference on Contr. Fusion and Plasma Phys., St. Petersburg, 7-11 July 2003 ECA Vol. 27A, P-1.129 3 of 43.4. Application of ELM control in a high performance radiative feedback scenarioA high performance scenario for simul-taneous feedback control of the divertorneutral flux (separatrix density, pump-ing) and the divertor temperature us-ing impurity injection controlled by ar-gon puffing has been developed at AS-DEX Upgrade [5]. The ‘effective‘ di-vertor temperature is obtained from on-line measurements of the thermoelectriccurrents flowing between the inner andouter divertor [6], which are a measurefor the difference of the divertor temper-atures. The thermoelectric Te,eff wasfound to be a sensitive measure for di-vertor conditions of plasmas with goodH-mode confinement. During the inves-tigation of radiative feedback scenariosin the type-I ELMy regime, a radiativerun-away situation is often observed inthe vicinity of the H-L threshold: Radi-ation rise in between type-I ELMs leadsto a further delay of the next ELM, withongoing radiation increase until a shortH-L transition occurs. This is quicklycured by the feedback, but does not leadto stationary conditions.     02468[MW]          0.00.51.01.52.02.53.0[  ]        0246810Γ [1e21 el/s]        0123   2 3 4 5time [s]010203040[a.u. ~ eV]   2 3 4 5time [s]Tdiv from thermocur.Set valueD2Ar⊗5PheatPNIPrad_totPrad_mainFeedback successfully handles Pheat transients caused by ICRH couplingβNf_ITER89pf_GreenpelletmonitorASDEX Upgrade # 17732      2 3 4 5 6time [s] Dα02468[MW]     # 17325reference without pellets:Prad_mainne [1e20  m-3]Figure 4: Discharge with feedback of the di-vertor temperature using Ar puffing. Thedivertor neutral D flux is controlled via anionization gauge measurement and D puff-ing. Small pellet injection with 40 Hz ensuresa corresponding minimum ELM frequency.Without the pellet ELM trigger, longer ELM-free periods develop, which lead to increasedAr radiation further delaying te next ELMwith a possible H-L transition (see lower rightpicture).Adding external ELM triggering by small pellets, the radiative run-away situation isavoided. Figure 4 shows an example for a discharge with simultaneous divertor neutralflux and temperature feedback combined with ELM frequency control. These dischargescombine high βN , H-factor and Greenwald fraction and retain low central tungsten con-centrations despite the full tungsten coating of the inner wall and the addition of animpurity with high sputtering capability (Ar). The lower right figure inset shows timetraces of a comparable discharge without ELM control, exhibiting radiative runawayswith short H-L transitions.In conclusion, maintaining of a minimum ELM frequency of 40 Hz enforced by small HFSpellet injection results in an integrated, fully controlled scenario with favorable divertorconditions which should also be applicable in a future full-tungsten ASDEX Upgradetokamak.References[1] P.T. Lang et al., Nucl. Fusion 36 (1996) 1531.[2] H. Zohm et al., ”Overview of ASDEX Upgrade Results”, IAEA FEC 2002, Lyon, Sept. 2002.[3] P.T. Lang et al., Nucl. Fusion 41 (2001) 1107.[4] A. Herrmann, Plasma Phys. Control. Fusion 44 1 (2002).[5] V. Mertens et al., Fusion Science and Technology in press.[6] A. Kallenbach et al., J. Nucl. Mat. 290-293 (2001) 1184.30th EPS Conference on Contr. Fusion and Plasma Phys., St. Petersburg, 7-11 July 2003 ECA Vol. 27A, P-1.129 4 of 4

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Active edge localized mode (ELM) frequency control with pellets

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