Angioni, C.

Bernert, M.

Dunne, M.

Giroud, C.

Kappatou, A.

Lerche, E.

Neu, R.

Pütterich, T.

Sips, A.C.C.

Viezzer, Eleonora

English

Sips, A. C. C.

Dunne, M. G.

idUS. Depósito de Investigación Universidad de Sevilla

The effect of helium on plasma performance at ASDEX Upgrade and JETA. Kappatou1, C. Angioni1, A.C.C. Sips2,3, E. Lerche4, T. Pütterich1, M. Dunne1, R. Neu1,5,C. Giroud6, C. Challis6, J. Hobirk1, H.-T. Kim6, I. Nunes7, M. Tsalas8, K. Cave-Ayland6,D.F. Valcarcel6, S. Menmuir6, A. Lebschy1,5, E. Viezzer9, R.M. McDermott1, S. Potzel1,F. Ryter1, L. Frassinetti10, E. Delabie11, S. Saarelma6, M. Bernert1, the EUROfusionMST1 Team∗, the ASDEX Upgrade Team and JET Contributors+1Max-Planck-Institut für Plasmaphysik, Garching, Germany, 2JET Exploitation Unit, Culham ScienceCentre, Abingdon, UK, 3European Commission, Brussels, Belgium, 4LPP-ERM-KMS, TEC partner,Brussels, Belgium, 5Technische Universität München, Garching, Germany, 6Culham Centre forFusion Energy, Culham Science Centre, Abingdon, UK, 7Instituto de Plasmas e Fusão Nuclear, IST,Universidade de Lisboa, Portugal, 8FOM Institute DIFFER, Nieuwegein, The Netherlands, 9Universityof Seville, Seville, Spain, 10KTH Royal Institute of Technology, Stockholm, Sweden, 11Oak RidgeNational Laboratory, Oak Ridge, Tennessee, USA.IntroductionWhile it is well known that confinement in helium plasmas is lower than in deuterium plasmas[1, 2], helium is suspected to degrade confinement even when present at low, reactor relevant,concentrations in deuterium plasmas [3, 4]. This degrading effect of helium on plasma perfor-mance is demonstrated in dedicated experiments in ASDEX Upgrade (AUG) and JET and isinvestigated in detail in search of the physics reasons behind the confinement degradation.Helium seeding in JET baseline scenario plasmas0 5 10 152.02.53.03.5cHe [%]Wdia [MJ]PNBI~12MW PNBI~10MW PNBI~8MW, PICRH~2.5-3MW fELM control(60Hz, cHe=0 case) PNBI~10-12MW, no ICRH PNBI~8MW, PICRH~2.5-3MW Fig. 1: Reduction of the plasma dia-magnetic stored energy with increasinghelium concentration in JET plasmas.Helium was added to baseline scenario deuterium plas-mas at JET (Bt=2.1T, Ip=2MA, D-fuelling of unseededpulse 1.6 ·1022 e/s, βN∼1.5, q95∼3.1), in three types ofexperiments: with neutral beam injection (NBI) heatingonly, combined neutral beam and H-minority ion cy-clotron resonance heating (ICRH), and combined NBIand ICRH with ELM frequency control. The He concen-tration was scanned up to above 10% from pulse to pulse,controlled in real time using the divertor spectroscopysignals. The He concentration is estimated from divertorspectroscopy measurements (one He level benchmarkedalso with CXRS measurements, using an approximate correction for the He plume emissioncontribution).∗ H. Meyer et al, Overview of progress in European Medium Sized Tokamaks towards an integrated plasma-edge/wall solution, accepted for publication in Nuclear Fusion+ X. Litaudon et al, Overview of the JET results in support to ITER, accepted for publication in Nuclear Fusion44th EPS Conference on Plasma Physics O3.111Pe [kPa]ne [1019 m-3]90378: cHe=0%90379: cHe~6%90591: cHe=0%90592: cHe~6% (- - - fELM control)NBINBI+RFTe [keV]0.0 0.2 0.4 0.6 0.8 1.0ρtor00.51.01.52.0[108 cm-3 s-1]Beam-target neutrons TRANSP0.90 0.95 1.00ρ0.00.20.40.60.81.01.20.90 0.95 1.00ρ02460.90 0.95 1.00ρ02468Fig. 2: Fitted HRTS pedestal profiles of the JET plasmas, with (red and green lines) and without (blackand blue lines) He-seeding. The effect of the kinetic profile changes induced by the He-seeding on thebeam-target neutron production, simulated with TRANSP, is shown on the right.In plasmas with NBI-heating only (PNBI=10-12MW), a strong linear decrease in stored en-ergy (and normalised confinement factor H98(y,2)) was observed with increasing He concentra-tion, see Fig. 1 (red and orange points). The plasma stored energy is reduced by ∼11% at a Heconcentration of 5%. A significant reduction of ∼30% in measured neutrons is also observed,which cannot be solely attributed to the dilution by He.The addition of He in plasmas with pure NBI heating leads to an increase of the pedestalelectron density and a corresponding decrease of the pedestal electron temperature (Fig. 2,black and red lines). The loss in plasma confinement is not connected to a loss of pedestalpressure, as this remains almost the same with increasing He concentration. The core pressuredoes decrease, as the peaking of the electron density and temperature profiles reduces. Thecore ion temperature follows the changes in the electron temperature. Pedestal ion temperaturemeasurements were not available for these shots. However, consistency checks have shown thatthe pedestal ion temperature follows the behaviour of the pedestal electron temperature. In thepulses with He seeding, two bands of ELM frequencies appear, for example, 35 and 80Hz withthe first level of He, while only the lower ELM frequency is present in the unseeded case.When part of the NBI power is replaced with central H-minority ICRH heating (PNBI=8MW,PICRH=2.5-3MW), the electron temperature as well as the total neutron rates are not as stronglyreduced (Fig. 2, blue and green solid lines). The pedestal electron density increases morestrongly with the addition of He in comparison to the NBI-only heated case. The pedestal elec-tron density could be kept at the levels of the unseeded pulse (Fig. 2, green dashed lines), whenthe ELM frequency was controlled in real time to the frequency of the non He-seeded pulse(∼60Hz, with ICRH and NBI heating), by reducing the D gas puff.The higher edge density induced by He in the NBI-only plasmas leads to a stronger atten-uation of the NBI in the plasma and reduced central heating, which in combination with thereduced core temperature leads to a significant reduction of beam-target neutrons (TRANSPsimulation shown on the right of Fig. 2, thermal DD neutron contribution is <15% of the total44th EPS Conference on Plasma Physics O3.111neutrons). With the addition of ICRH, the degrading effect of He on the plasma performanceis limited. The temperature is kept high, which, in combination with increased electron density,increases the pressure in comparison to the unseeded case. With the reduction of the D-fuelling,the NBI attenuation was no longer affected and the confinement of the plasma without He-seeding was recovered. The D gas, however, was reduced much more than the added He gas.Helium seeding in ASDEX Upgrade high confinement plasmasHelium was added with 0.5s long gas puffs to high confinement AUG deuterium plasmas,(PNBI=10MW, PECRH=1.3MW, Bt=2.5T, Ip=1MA, βN∼1.9−2.3, q95∼4.3) with two differentlevels of D fuelling (5 ·1021 and 7.5 ·1021 e/s), as well as to N-seeded deuterium plasmas, lead-ing to He concentrations of up to ∼14%. The He density profile is measured with CXRS, takinginto account the plume effect [5]. A clear reduction of the plasma stored energy, shown in Fig. 3,as well as a clear effect on the pedestal and core profiles were observed when He was puffed.0 2 4 6 8 10 12 14cHe [%]500600700800900WMHD [kJ]He- & N-seedingHe-seedinghigher D-gaslower D-gaslow N-seedinghigh N-seedingFig. 3: Plasma stored energy versus Heconcentration in AUG plasmas.Both with and without N-seeding, the He-puff leadsto no or small increase of the electron density pedestaland a reduction of the electron and ion temperatures.These changes are stronger at lower D-gas fuelling. Fit-ted pedestal profiles for such a discharge are shown inFig. 4 (profiles before the He-puff in blue and immedi-ately after in green). Dilution due to He (and N) can-not by itself explain the observed reduction in storedenergy. Rather, the reduction in plasma stored energyis mainly due to the changes to the kinetic profiles in-duced by the He puff. In contrast to JET, the pedestalpressure is slightly reduced, with the effect being larger for the ions. The peaking of the kineticprofiles in the confinement region does not change significantly with the He-puff and as a resultthe core electron density increases up to a maximum of 5%, while the ion temperature is re-duced more strongly than the electron temperature with the addition of He. These observationsagree with those reported in [3], where an erosion of the ion pedestal was also observed.In the plasmas with the higher level of D-fuelling and in the absence of N-seeding, two dis-tinct ELM frequencies are present and a high field side high density front is observed in theSOL (HFSHD [6]). The He puff affects neither the ELMs nor the HFSHD. In the low D-fuelling cases, without N-seeding, two ELM frequencies are also observed, and the upper ELMfrequency band is populated more with the He-puff. With N-seeding, the HFSHD is reduced,leading to an increase of the plasma stored energy and confinement improvement [7, 8]. When44th EPS Conference on Plasma Physics O3.111ne [1019 m-3]0.90 0.95 1.00024680.90 0.95 1.000.00.20.40.60.81.01.21.40.90 0.95 1.000.00.20.40.60.81.01.21.40.90 0.95 1.0002468101214Te [keV] Ti [keV]Pe Pi[kPa]ρpolρpolρpolρpolbefore/after the He-puffFig. 4: Pedestal profiles for AUG discharge 34176 (low D-fuelling), before (blue, at 2.7s) and immedi-ately after (green, at 3.5s) the He puff injected during 3.0-3.5s.He is added, the HFSHD reappears and the ELM frequency increases, accompanied by a reduc-tion of the N concentration across the plasma radius. The reason for this interplay between thetwo impurities needs to be investigated further.Discussion and conclusionsThe degrading effect of He on plasma performance has been demonstrated at JET and AUG.Despite the fact that the two experiments are not directly comparable (different approaches tothe He puffing, lower power flux in the JET case), the effect of the He-seeding on the kineticprofiles has a phenomenological similarity: an increase in the electron density, accompanied bya reduction of the electron and ion temperatures. However, in AUG, a constant pedestal pressureis not sustained and the reduction of the produced neutrons is stronger in the JET plasmas. Withthe use of ICRH heating and a reduction of the D gas, it was possible to minimise the effect ofHe on the confinement at JET. In both experiments, the stored energy recovers after a He gaspuff on the same time scale as the He concentration decays, which is defined by wall recyclingand pumping and is different for the two experiments.Compared to future fusion reactors, where He will be produced in the core, He can only beadded as an extrinsic impurity in present day experiments. Thus, not only transport effects, butalso He pumping and recycling mechanisms, which will determine the He density [9], have tobe understood in order to make valid predictions for future reactors.References[1] F. Ryter et al, Nucl. Fusion 49, 062003 (2009)[2] D.C. McDonald et al, Plasma Phys. Control.Fusion 46, 519 (2004)[3] R. Neu et al, Proc. 35th EPS Conf. (2008)[4] E. Stefániková et al, H-mode workshop (2015)[5] A. Kappatou, PhD Thesis, Eindhoven Universityof Technology (2014)[6] S. Potzel et al, J. Nucl. Mat. 463, 541 (2015)[7] S. Potzel et al, Proc. 42nd EPS Conf. (2015)[8] M. Dunne et al, Plasma Phys. and Control. Fusion59, 025010 (2017)[9] C. Angioni et al, Nucl. Fusion 49 055013 (2009)This work has been carried out within the framework of the EUROfusion Consortium and has received funding from theEuratom research and training programme 2014-2018 under grant agreement No 633053. The views and opinions expressedherein do not necessarily reflect those of the European Commission.44th EPS Conference on Plasma Physics O3.111

The effect of helium on plasma performance at ASDEX Upgrade and JET

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The effect of helium on plasma performance at ASDEX Upgrade and JET

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