Bielajew, R.

Cano-Megias, P.

Cavedon, M.

Cruz-Zabala, D.

Dunne, M.

Happel, T.

Hobirk, J.

Karagianni, C.

Maraschek, M.

McDermott, R.

Merle, A.

Meyer, H.

Oyola, P.

Solano, E.

Suttrop, W.

Vanovac, B.

Viezzer,, E.

Willensdorfer, M.

English

MPG.PuRe

Progress towards a quiescent, high confinement regime for the all-metalASDEX Upgrade tokamakE. Viezzer1, E. R. Solano2, P. Cano-Megias1, D. J. Cruz-Zabala1, P. Oyola1, C. Karagianni1,H. Meyer3, J. Hobirk4, R. Bielajew5, M. Cavedon4, M. Dunne4, T. Happel4, M. Maraschek4,R. M. McDermott4, A. Merle6, W. Suttrop4, B. Vanovac4, M. Willensdorfer4,the EUROfusion MST1 Team∗and the ASDEX Upgrade team†1 University of Seville, Seville, Spain2 CIEMAT, Madrid, Spain3 Culham Center for Fusion Energy, Culham, UK4 Max Planck Institute for Plasma Physics, Garching, Germany5 Plasma Science and Fusion Center MIT, Boston, USA6 Ecole Polytechnique Fédérale de Lausanne, Swiss Plasma Center, Lausanne, SwitzerlandIntroductionFuture magnetic fusion devices foresee the high confinement (H-mode) as the baseline op-erational scenario. However, the steep edge pressure gradient of the H-mode comes with adrawback – edge localized modes (ELMs) that expel particles and energy from the plasma,causing erosion and unacceptable power loads onto the divertor target plates. Several mitigationor even full suppression techniques are currently being developed across all machines to ensurehigh confinement without the transient degradation of the pedestal caused by ELMs. The qui-escent H-mode (QH-mode) [1] is one of the considered alternative scenarios as it is naturallyELM-free. The onset of the QH-mode is characterized by the edge harmonic oscillation (EHO),which is thought to increase the edge particle transport to allow natural stability against ELMs.Originally discovered at DIII-D [1], it was later also observed on ASDEX Upgrade (AUG) [2],JET [3] (both still equipped with a carbon wall) and JT-60U [4]. In a metal machine, a sustainedQH-mode proved to be more difficult to obtain.Experiments dedicated to the development of the QH-mode were carried out in the all-metalAUG tokamak and are presented here. For the first time, the appearance of the EHO was ob-served during transient QH-modes that lasted several confinement times (up to 500 ms).∗See author list of B. Labit et al, Nucl. Fusion 59 086020 (2019)†See author list of H. Meyer et al, Nucl. Fusion 59 112014 (2019)47th EPS Conference on Plasma Physics P1.1054Access conditionsThe QH-mode is typically accessed at low density and low collisionality, conditions that areusually obtained right after a fresh boronization. Stationarity has been achieved over a wide op-erational space [1], including pedestal collisionality, shaping, edge safety factor (q95), poloidalbeta (βpol) and Greenwald fraction [5]. However, steady operation at high Greenwald fractionand ITER relevant q95 as well as the compatibility with a metal wall have not been demonstratedyet and are current research targets.For a long time, a counter-current torque was believed to be a necessary ingredient for QH-mode access as the first QH-modes [6] were achieved with counter neutral beam injection (NBI).In 2009, DIII-D carried out dedicated experiments with co- and counter-injection and achievedthe QH-mode also with pure co-NBI [6]. This demonstrated that the counter edge rotation isnot a necessary condition, but rather the magnitude of the shear in the edge rotation needs to besufficiently large. While pure co-NBI QH-modes were observed in the past at DIII-D, they arefar more challenging to achieve and often some level of counter-injection is applied. At AUG,the counter-current NBI geometry can only be achieved in the reversed Ip/Bt configuration.In fact, when AUG was equipped with a carbon wall, the QH-mode was only achieved withcounter-current beams [2].Experiments in reversed Ip/Bt configurationSince counter-current beams seem to facilitate the access to the QH-mode, part of the QH-mode development programme at AUG was carried out in reversed Ip/Bt . As increased beam-ion orbit losses are observed in this configuration, beam operation is usually restricted. In orderto couple in more beams, the distance of the last closed flux surface to the wall was increasedby using a small plasma compared to an edge optimized contour plasma, which is usually used0480.00.20.402464.6 4.7 4.8 4.9 5.0Time (s)04•10558•10504080010203040-8-40Wmhd[km/s][MW][0m]119-2[]kHz[MJ][a.u.]#37289NBIECRHPradB31-14edge densitycore densityEdge tor. rotation#3#2Bolometry#7#6EHOcore mode(a)(b)(c)(d)(e)(f)Figure 1: Overview of discharge #37289.at AUG to obtain high resolved pedestal measure-ments. A low density scenario at 2.5 T and -800 kAwas successfully established with up to 8 MW counter-current beams. The EHO appeared in several dis-charges, with the most notable phase shown in figure 1.At the switch-on of the fourth beam the ELMs changein size and have a higher frequency (not shown here),until they are replaced by an edge harmonic oscilla-tion (see figure 1(d)), localized to the plasma edgeby means of the ECE and soft X-ray diagnostics.Throughout the four-beam phase the radiated power47th EPS Conference on Plasma Physics P1.1054and line-averaged density slowly increase, reaching values of 4 MW and 2.5· 1019 m−2.0.0 0.2 0.4 0.6 0.8 1.00246810Temperature [keV]ρpolTeTi# , [ , . ]s35776 3.2  3 4#3 , [ , . ]s7289  4.7  4 8AUG-W AUG-CFigure 2: Ti and Te profiles during theQH-mode at AUG-W (left) and compari-son to AUG-C (right) [2].Note that the plasma is ramped down at 5 s, to avoidoverheating of the heatshield caused by the NBI inthese low density plasmas.One important difference to the QH-modes in AUG-C is that these plasmas feature a much higher elec-tron temperature (Te) compared to the ion temperature(Ti), since central electron cyclotron resonance heat-ing (ECRH) is needed to avoid core tungsten accumu-lation. Figure 2 shows a comparison of the Te and Tiprofiles obtained in AUG-W and in AUG-C [2].0.51.52.53.5012340.000.100.20[MW][0m]119-2#38663NBIECRHPradEdgeCoreLine-av. density[MJ]1.5 1.6 1.7 1.8 1.9 2.0Time [s]02•1054•1056•1058•105[kA][a.u.]Divertor thermo-currents (outer/ )innerBolometry-4-2024Wmhd3035(a)(b)(d)(e)(f)(c)Figure 3: Overview of discharge #38663.Note that in discharge #37289 the EHO appearedafter entering the H-mode regime. Interestingly, infollow-up experiments the EHO also appeared duringthe early ELM-free phase. An example is given in fig-ure 3. As the plasma transits into a higher confine-ment regime, two small ELMs appear, and then con-tinues to evolve until, at 1.59s, it arrives into a regimewith a higher recycling level, as shown in one of thebolometry channels. At the same time, an EHO ap-pears and the stored energy, plasma density and radi-ated power stall. The phase with the EHO lasts for 320ms. At 1.827s the plasma density starts to increase (by10%) and evolves into a type-I ELMy regime at 1.92s.As shown in figure 3, the EHO appears with 1.8 MWcounter-NBI and up to 2.5 MW of ECRH. Future workfocusses on extending this phase to stationarity.Experiments in forward Ip/Bt configurationIn forward Ip/Bt , the most promising phase was identified in an upper single null (USN)discharge, corresponding to the unfavourable ∇B drift configuration. Similar as in reversedIp/Bt , this provides a higher L-H power threshold and a starting point at low density and higherrotation, as up to two beams (up to 4 MW) of NBI can be coupled into the L-mode phase. Noticethat AUG does not feature a pump in the upper divertor, making density control a challenge.Typically, a very small gas puff at the beginning of the discharges or a minimum feedforward47th EPS Conference on Plasma Physics P1.1054gas puff of 0.5 ·1021 e−/s is applied.Figure 4 shows an example discharge in USN with up to 6 MW NBI and 0.5 MW ECRH.0.01.02.0024680.00.20.40246804.0•1048.0•1041.2•105-10-505103.0 3.5 4.0 4.5 5.0Time (s)[kA][MW][0m]119-2[]km/s[MJ][a.u.][a.u.]0501001 05#38877NBIECRHPrad#3edgecoreDivertor thermo-currentsBolometryLine-av. densityβNH98#6#7WmhdEdge tor. rotation(outer/ )inner(a)(b)(c)(d)(e)(g)(f)Figure 4: Overview of discharge #38877.As soon as the second beam switches on, rotation andstored energy increase. The plasma exhibits 3 smallELMs and as the power ramp on NBI source #6 starts,it enters into a long ELM-free phase (about 500 ms)exhibiting an EHO which is then replaced by an n = 3mode. Both modes are localized in the pedestal region.During this phase a toroidal rotation up to 150 km/sis reached at the plasma edge (see figure 4(g)). Thedensity increases and at 3.76 s the plasma transits intoan H-mode regime with compound ELMs [7]. At 4 s,a shape-change transition to lower single null (LSN)is programmed, which brings the plasma into a type-I ELMy regime. Notice the lower density in LSN, asthe pump in the lower divertor is active. During thisQH-mode phase (3.25 - 3.76 s) an energy confinement factor up to H98(y,2) ≈1.4 is achieved.However, notice the evolution of the density, making this phase a transient. The installation ofthe upper divertor (and a cryo-pump) foreseen for 2022/23 at AUG may help to get a bettercontrol of the density and get a sustained QH-mode in USN.SummaryTransient QH-modes up to 500 ms were observed for the first time in the all-W tokamakASDEX Upgrade. The signatures of the EHO were observed in several diagnostics. The QH-mode appears at the low density, high temperature branch, with the pedestal close to the kink-peeling boundary. Future work focusses on extending the QH-mode to stationarity.Acknowledgments This work has been carried out within the framework of the EUROfusion Consortium andhas received funding from the Euratom research and training programme 2014-2018 and 2019-2020 under grantagreement No 633053. The views and opinions expressed herein do not necessarily reflect those of the EuropeanCommission. The support from the European Research Council (ERC) under the European Union’s Horizon 2020research and innovation programme (grant agreement No. 805162) is gratefully acknowledged.References[1] K. H. Burrell et al., Phys. Plasmas 8, 2153 (2001)[2] W. Suttrop et al., Plasma Phys. Control. Fusion 45, 1399 (2003)[3] E. R. Solano et al., Phys. Rev. Lett. 104, 185003 (2010)[4] Y. Sakamoto et al., Plasma Phys. Control. Fusion 46, A299 (2004)[5] E. Viezzer, Nucl. Fusion 58, 115002 (2018)[6] K. H. Burrell et al., Phys. Rev. Lett. 102, 155003 (2009)[7] H. Zohm, Plasma Phys. Control. Fusion 38, 105 (1996)47th EPS Conference on Plasma Physics P1.1054

Progress towards a quiescent, high confinement regime for the all-metal ASDEX Upgrade tokamak

http://hdl.handle.net/21.11116/0000-0009-A60E-D

Progress towards a quiescent, high confinement regime for the all-metal ASDEX Upgrade tokamak

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MPG.PuRe