Alonso, A.

Baldzuhn, J.

Beidler, C.

Biedermann, C.

Bozhenkov, S.

Brezinsek, S.

Brunner, K.

Burhenn, R.

Buttenschön, B.

Damm, H.

Drewelow, P.

Effenberg, F.

Feng, Y.

Fuchert, G.

Gao, Y.

Giannone, L.

Hammond, K.

Hirsch, M.

Höfel, U.

Jakubowski, M.

Killer, C.

Klinger, T.

Knauer, J.

Krychowiak, M.

Kwak, S.

König, R.

Laqua, H.

Laube, R.

Niemann, H.

Pablant, N.

Pasch, E.

Pavone, A.

Pedersen, T.

Penzel, F.

Rahbarnia, K.

Reimold, F.

Svensson, J.

Thomsen, H.

Wagner, F.

Winters, V.

Zhang, D.

English

Baldzuhn, J. 

Brunner, K. 

MPG.PuRe

First observation of a stable highly-dissipative divertor plasma regime on theWendelstein 7-X stellaratorD. Zhang,1 R. Ko¨nig,1 Y. Feng,1 R. Burhenn,1 S. Brezinsek,2 M. Jakubowski,1 B. Buttenscho¨n,1H. Niemann,1 A. Pavone,1 M. Krychowiak,1 S. Kwak,1 J. Svensson,1 Y. Gao,2 T.S. Pedersen,1A. Alonso,3 J. Baldzuhn,1 C.D. Beidler,1 C. Biedermann,1 S. Bozhenkov,1 K.J. Brunner,1 H.Damm,1 M. Hirsch,1 L. Giannone,4 P. Drewelow,1 F. Effenberg,5 G. Fuchert,1 K.C. Hammond,1 U.Ho¨fel,1 C. Killer,1 J. Knauer,1 H.P. Laqua,1 R. Laube,1 N. Pablant,6 E. Pasch,1 F. Penzel,4 K.Rahbarnia,1 F. Reimold,1 H. Thomsen,1 V. Winters,5 F. Wagner,1 T. Klinger,1 and the W7-X team11Max-Planck-Institut fu¨r Plasmaphysik, 17491 Greifswald, Germany2Forschungszentrum Ju¨lich GmbH, IEK-4 52425 Juelich, Germany3Laboratorio Nacional de Fusio´n – CIEMAT, 28040 Madrid, Spain4Max-Planck-Institut fu¨r Plasmaphysik, 85748 Garching, Germany5University of Wisconsin – Madison, Wisconsin 53706, USA6Princeton Plasma Physics Laboratory, Princeton, NJ 08540, USA(Dated: May 26, 2019)For the first time, the optimized stellarator Wendelstein 7-X has operated with an island diver-tor. An operation regime in hydrogen was found in which the total plasma radiation approachedthe absorbed heating power without noticeable loss of stored energy. The divertor thermographyrecorded simultaneously a strong reduction of the heat load on all divertor targets, indicating al-most complete power detachment. This operation regime was stably sustained over several energyconfinement times until the pre-programmed end of the discharge. The plasma radiation is mainlydue to oxygen and is located at the plasma edge. This plasma scenario is reproducible and robustat various heating powers, plasma densities and gas fueling locations. These experimental resultsshow that the island divertor concept actually works and displays good power dissipation potential,producing a promising exhaust concept for the stellarator reactor line.Reduction of the heat flux density on plasma facingcomponents to a technically feasible level is one of themost challenging issues for magnetic confinement fusiondevices on the path to a reactor. A promising approachis the creation of a controlled impurity radiation zonein the vicinity of the plasma-surface-interaction (PSI)area by means of a divertor[1–8]. The basic idea of adivertor configuration[9–11] is to separate the PSI re-gion from the plasma confinement region by establishingan appropriate separatrix-bounded magnetic configura-tion. In the advanced stellarator Wendelstein 7-AS (W7-AS)[12] and the optimized stellarator Wendelstein 7-X(W7-X) [13, 14], this separation is realized by an intrinsic,low-order magnetic island chain which forms the basis forthe so-called island divertor configuration. The island di-vertor concept has been successfully tested on W7-AS. Inparticular heat flux reduction onto divertor targets with‘partial’ plasma detachment has been observed[15–18].W7-X is an optimized quasi-isodynamic stellarator[19]with shaped non-planar superconducting coils. It is de-signed for long-pulse steady-state plasma operation withthe goal of bringing the optimized stellarator to reactormaturity. Development of a reactor-relevant island di-vertor concept is one of the main scientific objectives ofW7-X [20]. The essential question is the following: Howmuch power can be removed via line radiation of low-Zimpurities without serious impurity contamination anddegradation of energy confinement at the core plasma?Plasma detachment from the divertor targets can playa decisive role here. In this paper, we present first ob-servations of a stable, highly-radiative divertor regime,in which the radiation power loss approaches the ab-sorbed heating power, leading to almost complete powerdetachment while the stored plasma energy is preserved.We note the difference between the stable detachmenton W7-AS and W7-X: Most significant is that the heatflux reduction on the targets in W7-AS is inhomogeneousand a certain target range still receives a notable heatflux even at the maximum radiation level and is referredto as “partial” detachment[15–18]. By contrast, the de-tachment achieved in W7-X is more complete in the sensethat the heat flux reduces uniformly on all the targets.W7-X (major radius of 5.5 m, effective minor radius0.5 m, and plasma volume 30 m3 ) is currently thelargest advanced stellarator worldwide. After initial op-eration with a limiter configuration[21–23], the devicewas subsequently equipped with ten uncooled graphitedivertor units (figure 1(a)), presently being replaced bycorresponding water-cooled divertor modules. The firstwall (made of stainless steel) is partially covered withgraphite tiles in regions exposed to high thermal loads(> 0.3 MW/m2). Wall conditioning was done by meansof glow- and ECRH-discharges in helium (boronizationbecame possible only after this initial divertor campaign).The plasma is generated by electron cyclotron resonanceheating (ECRH). Carbon and oxygen have been identi-fied as the main impurity species. The experiments de-scribed here were carried out in a magnetic configuration2with a 5/5 island chain at the plasma edge and 2.52 Tmagnetic field on axis. The island chain has an effectiveradial dimension of about 6.5 cm[24].HBC (32 chn)VBC (33 chn )M3M2M4M5M1bolometer(a)(b)FIG. 1. (a) The stellarator W7-X equipped with ten graphitedivertor units. The islands in the scrape-off layer (SOL) in-tersected by the divertor elements. (b) The lines of sightof the horizontal and vertical bolometer cameras (HBC andVBC) installed at the triangular cross-section for monitor-ing the plasma radiation from both confinement region andSOL. The Poincare´ plot of the magnetic flux surfaces at thiscross-section is also shown).Plasma radiation is measured by two bolometer camerasinstalled at a triangular cross-section of W7-X with aspatial resolution of 3-4 cm (figure 1(b))[25]. No di-vertor modules are installed at this toroidal position.To obtain the total radiated power loss, Prad, the ra-diation from the observation volume is extrapolated tothe whole plasma volume. Possible toroidal variationsof the radiation strength are not considered, as sup-ported by EMC3-EIRENE[26] calculations at differenttoroidal positions. Although a tomographic reconstruc-tion is possible, the line-integrated signals are used forPrad calculations in order to avoid uncertainties fromthe deconvolution procedure. Other plasma parame-ters and diagnostics involved [27–29] are: an interfer-ometer for the line-averaged plasma density, a diamag-netic loop for the plasma stored energy, an electron cy-clotron emission (ECE) radiometer for electron tempera-ture, a Thomson scattering (TS) system for electron den-sity and temperature profile, infrared (IR)-cameras fordivertor thermography measuring the divertor heat load,and spectroscopic diagnostics for impurity line emissionsand bremsstrahlung radiation. The Minerva Bayesianmodelling framework[30] has been used for fitting the TSmeasurements and deriving the effective ion charge Zeffbased on the bremsstrahlung radiation.FIG. 2. Time traces of plasma parameters of discharge#20171109.045. From top to bottom: ECR-heating powerPECRH and total radiation power Prad, line-averaged density,stored energy WP and energy confinement time τE, the ef-fective ion charge Zeff , two representative ECE channels forrespective core and edge electron temperature Te, total heatload on all targets. Pellets are injected in the time intervalfrom 1.7 to 2.1 s.Fig. 2 shows diagnostic time traces for the hydrogendischarge #20171109.045, in which at t > 2 s a highly-radiative regime is established with a quasi-stationaryphase between t = 3.3 and 4 s. At constant ECRH powerof ∼ 3 MW, a plasma density of 2.5 × 1019 m−3 is firstobtained via gas-puffing (t = 0 to 1.2 s) and afterwardsfurther increased by repetitively injecting hydrogen pel-lets (from 1.7 s to 2.1 s). Shortly after pellet injection,the stored energy Wp reaches a maximum (cf. also on thestellarator LHD[31, 32]), followed by a decay on a timescale larger than the energy confinement time τE of 0.15s, but corresponding to the particle confinement time τP3of around 0.5 s. After the initial transient phase for t >3 s, the line-averaged plasma density as well as WP andτE approach the levels prior to pellet injection. The totalradiation Prad, however, does not return to its pre-pelletvalue but stays at a higher level close to the total heatingpower PECRH, with a fraction frad = Prad/PECRH = 90%(estimated error ±10%). Meanwhile, the total heat loadon all divertor targets obtained from IR-camera measure-ments (integrated over the entire divertor surface) dropsstrongly to Pdiv ∼ 0.2 MW , corresponding to a heatflux of 0.15 MW/m2. For comparison before pellet injec-tion, its maximum value was ∼ 3 MW/m2. This dropof the heat load (by a factor of more than 10) to a levelslightly above the diagnostic resolution indicates almostcomplete power detachment. The full recovery of the coreplasma is seen in the time evolution of Te and the evo-lution of electron temperature and density profiles mea-sured with Thomson scattering (figure 3). From t = 3.3 son, the plasma reaches a quasi-stationary, highly radia-tive divertor plasma regime, during which the plasmastored energy WP remains constant at WP ∼ 280 kJ.The global energy confinement time τE≈ WP/ PECRH isslightly above 0.1 s, almost the same as that before pel-let injection. This regime is self-sustained over severalenergy confinement times terminated only by the pre-programmed end of the discharge. The remnant heatload on the targets is 7% of the 3 MW ECR-heatingpower including the contributions of photons and charge-exchange neutrals. This implies that, at least 93% of thetotal power is removed before reaching the targets. Thisresult is consistent with the bolometer results within er-ror bars, presenting a good power balance. The effectiveion charge in the bulk plasma Zeff (figure 2), as deter-mined from bremsstrahlung radiation, is 3 before pelletinjection (with an uncertainty of 1); it has a slight incre-ment after pellet injection approaching 3.5 in the detach-ment phase. This variation may indicate that impuritytransport is partially involved in establishing the regime,which is a topic requiring further studies.In the transient phase from t = 1.7 to 3.0 s, two maximaof Prad (figure 2) have been obtained: one at t = 2.1 scorresponding to the maximum density and the other att = 2.7 s (with Prad > PECRH) associated with the en-hancement of Zeff and the decline of WP.It is noteworthy that the Thomson scattering measure-ments (together with Gaussian process fits[33], i.e. non-parametric fits of the profiles through covariance func-tions carried out within the Minerva Bayesian modellingframework[30]) shown in figure 3 demonstrate an edge-density increment at t = 3.5 s (in red). A rise of ne,LCFSup to 1.7±0.3 1019 m−3 with a flatter density profile incomparison to that at t = 1.7 s is observed. The electrontemperature has a slight reduction at the LCFS, compa-rable, however, to the diagnostic uncertainty of about 20eV.FIG. 3. (a) Electron density and (b) temperature profilesmeasured with Thomson scattering (solid points) at t = 1.7 s(before pellet injection), 2.6 s (just after pellet injection), and3.5 s (in the regime) in the discharge #20171109.045. Theradial position ρ marks the normalized effective radius of themagnetic flux surface and ρ = 1 corresponds to the last closedflux surface. The solid curves are Gaussian process fits[33] tothe measured data points and the shadowed region representsthe standard deviation of the fits. It shows the pronouncedcentral peaking of the density profile caused by pellet fuelingfollowed by a recovery at the plasma center and, in particular,an increment of ne,LCFS at t = 3.5 s; the temperature profilechange is comparatively small, however.Figure 4 (a) shows the corresponding temporal evolutionof the chord brightness of the vertical bolometer camera(VBC). After pellet-injection (t > 2.1 s), the brightnessof the bolometer channels that detect the SOL regionsharply drops due to cooling of the peripheral plasma, es-pecially the inboard side. After detachment ( t > 3.3 s),the edge bolometer channels at or slightly outside theLCFS show the strongest signals, indicating that the ra-diation is mainly located at the plasma edge. This isconfirmed by the data from the horizontal camera (HBC)and also a preliminary tomographic reconstruction of theradiation intensity distribution (not shown here) usingboth the HBC- and VBC-measurements with the Gaus-sian Process Tomography method[34]. Additionally, apoloidal asymmetry that has more intensive radiationfrom the plasma region below the midplane has beenobserved. This emissivity asymmetry is often observed4FIG. 4. Temporal evolution of the brightness of the VBCchannels labeled by their tangential radii of closed flux sur-faces (reff on the vertical axis with negative numbers indi-cating the inboard side). They are flux surfaces in the con-finement region, and linearly extended from a smooth fluxsurface near the separatrix into the edge islands.for high-density, high-radiation phases. It may resultfrom unevenly distributed impurity sources originatingfrom plasma-surface-interaction regions. Further analy-sis shows that approximately 80% of the power is radiatedfrom the near-separatrix region.During this divertor experimental campaign carbon andoxygen have been identified as main impurity species.Their absolute concentration is however unknown. Fig-ure 5 shows the time-traces of the line emissions fromCIII (117.5 nm), CIV (31.2 nm), OV (22.02 nm) andOVI (12.98 nm). These are the most intensive radiatingion stages of carbon and oxygen and have therefore beenselected as representative for the total carbon and oxygenradiation. The temporal evolution of Prad is comparedwith that of the four selected impurity ion emissions.For this purpose, each line emission curve is aligned withthe normalized Prad at t = 1.7 s (just before the pellet in-jection). The measured line-emissions from OV and OVIfollow well the time evolution of Prad, whereas those fromCIII and CIV do not. In particular, the CIII and CIV lineemission intensities do not rise in the high radiation phaseafter the pellet injection; the CIV brightness even sharplydrops. (Additional analysis treating line emissions fromimpurity ions with even lower ionization stages, such asCII and OIII, close to the PSI region also indicates nocorrelation with the Prad evolution). Considering thatCIV is the ionization stage of carbon that radiates mosteffectively, the comparison suggests a dominant role ofoxygen in contributing to the total radiation. This islikely due to the high radiation potential of oxygen, whichis about three times larger than that of carbon[35], butFIG. 5. Line-radiation of OVI, OV, CIV and CIII for thesame discharge as shown in Figs. 2-4. For comparison Prad isshown as black lines.could also be due to a higher oxygen influx. The im-purity influx is not known in this discharge. However,according to the mass spectrometry data, relatively highH2O and CO partial pressure has been measured in thisdischarge. Clarification of the origins and recycling pro-cesses of oxygen impurities in W7-X is beyond the scopeof this paper.In summary, the first island divertor experiments on W7-X have demonstrated that a stable highly-radiative diver-tor plasma regime can be established. Oxygen radiation,which is located at the edge plasma region, plays a keyrole. Strong reduction of heat flux on the divertor targetsshows plasma detachment although the plasma stored en-ergy is well-maintained. This regime was first observed inthe studied discharge with pellet injection, and was laterfound in discharges fueled via gas-puff and even in dis-charges with only a prefill. A certain amount of particlefueling is needed to provide the necessary recycling andplasma conditions in the scape-off layer, while the fuelinglocation does not seem to play a role. In this regime, twotarget Langmuir probe arrays, which were out of opera-tion in the discharge mentioned above (#20171109.045)but available for other relevant discharges, have measuredsignificantly reduced ion saturation currents indicatingstrongly reduced particle flux. Nevertheless, there is noexperimental evidence for noticeable volume recombina-tion, so far. 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Wolf et al., Plasma and Fusion Research 5, S1011(2010).[21] T. Klinger et al., Plasma Physics and Controlled Fusion59, 014018 (2016).[22] T. Sunn Pedersen et al., Physics of plasmas 24, 055503(2017).[23] A. Dinklage et al., Nature Physics 14, 855 (2018).[24] Y. Feng, F. Sardei, P. Grigull, K. McCormick,J. Kisslinger, and D. Reiter, Nuclear fusion 46, 807(2006).[25] D. Zhang, R. Burhenn, R. Ko¨nig, L. Giannone,P. Grodzki, et al., Review of Scientific Instruments 81,10E134 (2010).[26] Y. Feng, F. Sardei, J. Kisslinger, P. Grigull, K. Mc-Cormick, and D. Reiter, Contributions to Plasma Physics44, 57 (2004).[27] M. Krychowiak et al., Review of Scientific Instruments87, 11D304 (2016).[28] K. Rahbarnia, H. Thomsen, U. Neuner, et al., NuclearFusion 58, 096010 (2018).[29] M. Jakubowski et al., Review of Scientific Instruments89, 10E116 (2018).[30] J. Svensson and A. Werner, in 2007 IEEE InternationalSymposium on Intelligent Signal Processing (IEEE, 2007)pp. 1–6.[31] N. Ohyabu et al., Physical review letters 97, 055002(2006).[32] T. Morisaki et al., Physics of plasmas 14, 056113 (2007).[33] C. E. Rasmussen and C. K. I. Williams, Gaussian Pro-cesses for Machine Learning (The MIT Press, 2006).[34] J. Svensson, JET Internal report, Tech. Rep. (EFDA-JET-PR (11) 24, 2011).[35] U. Samm et al., Journal of Nuclear Materials 176, 273(1990).

First Observation of a Stable Highly Dissipative Divertor Plasma Regime on the Wendelstein 7-X Stellarator

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First Observation of a Stable Highly Dissipative Divertor Plasma Regime on the Wendelstein 7-X Stellarator

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