Beurskens, M.

Bozhenkov, S.

Bähner, J.

Edlund, E.

Ford, O.

Grulke, O.

Hansen, S.

Huang, Z.

Lazerson, S.

Porkolab, M.

Stechow, A.

English

MPG.PuRe

Turbulence studies in NBI heated discharges by the Phase ContrastImaging diagnostic in the Wendelstein 7-X stellaratorZ. Huang1, E. Edlund2, M. Porkolab1, A. von Stechow3, J-P. Bähner3, S. K. Hansen1, O. Grulke3,4,O. P. Ford3, M. N. A. Beurskens3, S. A. Bozhenkov3, S. Lazerson3, and the Wendelstein 7-X Team1 MIT Plasma Science and Fusion Center, Cambridge, MA, USA2 SUNY Cortland, Cortland, NY, USA3 Max-Planck-Institut für Plasmaphysik, Greifswald, Germany4 Technical University of Denmark, Kongens Lyngby, DenmarkIntroductionOne of the major optimization criteria of the Wendelstein 7-X (W7-X) stellarator is reducedneoclassical transport compared to classical stellarators [1]. The optimization has been provento be successful from the experimental results in the first operation phase [2, 3]. As a result,a significant fraction of the transport of heat and particles is dominated by turbulence trans-port [4]. Turbulence in magnetized plasmas is usually generated by temperature and densitygradient driven electrostatic micro-instabilities. Ion-scale turbulence, in particular, ion temper-ature gradient (ITG) and trapped electron modes (TEM), are suggested by theoretical and gy-rokinetic simulation studies to be the primary transport-relevant instabilities in the W7-X coreplasma [5, 6], whereas transport by electron-scale instabilities is generally strongly reduced [7].Fluctuation diagnostics such as phase contrast imaging (PCI) [8, 9, 10] and Doppler backscat-tering (DBS) have been utilized on W7-X to identify and monitor broadband instabilities. PCImeasures line-integrated absolute electron density fluctuations, I ∝∫ndl, along an infra-redlaser beam across the W7-X vacuum vessel from inboard to outboard side intersecting the mag-netic axis. By imaging the 32 line-of-sights determined by the 32 channels of the detector array,fluctuations are spatially resolved along a roughly poloidal direction. The instrumental lowercutoff is at kc = 1.55 cm−1, the Nyquist wavenumber ranges from kN = 4 to 25 cm−1 thanksto the flexible magnification of the laser beam, allowing the PCI diagnostic to well resolveion-scale turbulence such as ITG and TEM with typical W7-X plasma parameters.Turbulence in standard ECH dischargesMost discharges in the first operation phase of W7-X are gas-fueled and heated by ECH only,without direct ion heating nor additional central fueling such as NBI or pellets. An overview of atypical gas-fueled ECH discharge is shown in Fig. 1. The line-integrated density is kept roughlyconstant, while the ECH power is reduced periodically during the discharge.47th EPS Conference on Plasma Physics P2.1062Figure 1: Discharge overview for a typical ECRHdischarge in W7-XAs a result, the central electron temperatureTe and diamagnetic energy Wdia decrease dur-ing the discharge, while the central Ti re-mains roughly unchanged. The absolute den-sity fluctuation amplitude measured by PCIis also reduced with the ECH power, as wellas the normalized fluctuation amplitude. Onthe other hand, the energy confinement timeτE increases with decreasing ECH power.Similarly, it has been found that the impu-rity transport time increases with decreasingECH power [11]. The PCI measurements in-dicate that decreasing turbulence level withECH power correlates with improved energyand particle confinement. Gyrokinetic simu-lations also suggest that ITG-type turbulenceacts as the main channel of the energy and im-purity particle transport, and the negative de-pendence of the ITG linear growth rate on the Te/Ti ratio causes the drop in turbulence levelwith lower ECH power and thus the improved confinement [11].Turbulence in pure-NBI dischargesFigure 2 shows the time evolution of a typical pure-NBI discharge. The plasma is initiallyheated by 2 MW ECH till t = 1 s, when the gas fueling stops as well. The central electrontemperature drops from 2 to 1 keV when ECH is switched off. At the same time, 3.5 MW NBIis switched on and the plasma is sustained by pure-NBI until t = 3 s. The line-integrated plasmadensity continues to increase during the pure-NBI phase. The diamagnetic energy also increaseswith constant NBI heating power, resulting in an increasing τE. The absolute density fluctuationamplitude measured by PCI features an initial jump during the heat switch, and then it falls backin about 0.5 s and keeps nearly unchanged after, even though the bulk density keeps increasing.As a result, the normalized fluctuation level continues to decrease during the pure-NBI phase.Figure 3 shows that the central density doubled from 5×1019 m−3 to 1×1020 m−3 while theedge plasma density profile outside reff/a= 0.5 remains nearly unchanged, leading to a stronglycentrally peaked density profile. This indicates that the density fluctuations measured by PCIare likely originated from the edge plasma, while the turbulence in the core plasma is stronglyreduced with the increasing density gradient. However, the temperature in the pure-NBI phase47th EPS Conference on Plasma Physics P2.1062remains low because the increasing density due to fueling absorbs much of the available NBIpower.Figure 2: Discharge overview for discharge#20181009.34 in W7-X.The correlation between the peaked den-sity profile and improved confinement hasbeen observed in W7-X ECH plasmas un-der certain conditions, for example by in-jecting fueling pellets into the plasma core,or by massive Laser Blow-off to reducethe edge density, thus transiently creating apeaked density profile and improved confine-ment regimes [12]. Gyrokinetic simulationsshowed that increased density gradient withpeaked profile brings down ηi =δTiTi/δnini andthus stabilizes ITG turbulence. While a highdensity gradient would destabilize TEM tur-bulence in tokamaks, the maximum-J mag-netic geometry of W7-X high-mirror config-uration stabilizes TEM turbulence by averag-ing "good" and "bad" curvature regions alongthe particle orbit [13].Figure 3: Density (top) andtemperature profiles (bottom)for discharge #20181009.34 inW7-X.In contrast to other pure-NBI discharges in which the peakeddensity profile continues to develop till the end of the discharge,in Fig. 2,3 we show what happens when injecting 1 MW ECHpower to the pure-NBI plasma after t = 3 s. This leads to a fastdrop in central plasma density to 8× 1019 m−3 from t = 3 to3.3 s. At the same time, both electron and ion temperature in thecore plasma increase from 1 to 1.75 keV, leading to an increasein Wdia. On the other hand, the broadband density fluctuation am-plitude increases during this time, and τE starts to decrease. Aftert = 3.3 s, the temperature and diamagnetic energy both fall backto a new state and then remain stable. Figure 3 shows that in thisstate the density profile is still peaked in the core, roughly thesame as t = 2.2 s in the pure-NBI phase. However, the additionalECH now provides enough heating power to peak up the ion tem-47th EPS Conference on Plasma Physics P2.1062perature profile, creating a Ti gradient at around the same radiallocation as the density gradient, where ηi is around 1.5. This indeed enhances ITG modes andtherefore the confinement slightly deteriorates from the pure-NBI phase, however τE = 140 msis still higher than ≈ 100 ms with similar total input power in the ECH-only discharge.ConclusionA highly centrally peaked density profile can be created by pure NBI heating in W7-X. Whilethe central density and diamagnetic energy continues to increase, the absolute density fluc-tuation amplitude measured by PCI remains surprisingly unchanged, leading to a decreasingrelative fluctuation amplitude and increasing confinement time. Adding low power ECH to thepure-NBI plasma slightly increases the turbulence, creates both peaked density and ion tempera-ture profiles and improved confinement comparing to standard ECH discharges. The turbulencemeasurements are qualitatively consistent with particle and heat transport studies [14].AcknowledgmentSupport for the MIT and SUNY-Cortland participation was provided by the US Departmentof Energy, Grant DE-SC0014229. This work has been carried out within the framework ofthe EUROfusion Consortium and has received funding from the Euratom research and trainingprogramme 2014-2018 and 2019-2020 under grant agreement number 633053. The views andopinions expressed herein do not necessarily reflect those of the European Commission. S.K. Hansen acknowledges support by an Internationalisation Fellowship (CF19-0738) from theCarlsberg Foundation.References[1] G. Grieger, et al., Physics of Fluids B: Plasma Physics 4, 2081 (1992)[2] T. Klinger, et al., Nuclear Fusion 59, 112004 (2019)[3] R. C. Wolf, et al. 2019 Physics of Plasmas 26, 082504 (2019)[4] S. A. Bozhenkov, et al., Nuclear Fusion 60, 066011 (2019)[5] J. Proll, et al., Physical Review Letters 108, 245002 (2012)[6] P. Xanthopoulos, et al., Physical Review X 6, 021033 (2016)[7] G. Plunk, et al., Physical Review Letters 122, 035002 (2019)[8] E. Edlund, et al., Review of Scientific Instruments 89, 10E105 (2018)[9] Z. Huang, et al., Journal of Instrumentation 14, P01014 (2021)[10] J-P. Bähner, et al., accepted by Journal of Plasma Physics (2021)[11] Th. Wegner, et al., Nuclear Fusion 60, 124004 (2020)[12] A. von Stechow, et al., Invited talk, 22nd International Stellarator and Heliotron Workshop 2019 (Madison,WI)[13] J. Alcuson, et al., Plasma Physics and Controlled Fusion 62 035005 (2020)[14] O. P. Ford, et al., 47th EPS conference on plasma physics (2021)47th EPS Conference on Plasma Physics P2.1062

Turbulence studies in NBI heated discharges by the Phase Contrast Imaging diagnostic in the Wendelstein 7-X stellarator

http://hdl.handle.net/21.11116/0000-0009-A999-C

Turbulence studies in NBI heated discharges by the Phase Contrast Imaging diagnostic in the Wendelstein 7-X stellarator

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