Impurity accumulation in plasma regimes with high energy confinement

Ran, Lingbing; Roberts, David E.; Yang, H.R.; Dodel, G.; Gentle, K. W.; Goeler, S. von; Holzhauer, E.; Hübner, K.; Keilhacker, M.; Korotkov, A.; Luce, T.C.; Miura, Y.; Tsois, N.; Würz, H.; Fußmann, G.; Hofmann, J.; Janeschitz, G.; Krieger, K.; Müller, E.; Nolte, R.; Röhr, H.; Steuer, K.-H.; Becker, G.; Bomba, B.; Bruhns, H.; Büchl, K.; Carlson, A.; Eberhagen, A.; Fahrbach, H.‐U.; Gehre, O.; Gernhardt, J.; Giannone, L.; Gierke, G. von; Glock, E.; Gruber, O.; Haas, G.; Herrmann, H. W.; Kaesdorf, S.; Karger, F.; Kaufmann, Michael; Klüber, O.; Kornherr, M.; Lackner, K.; Lang, R. S.; Lee, P.; Lisitano, G.; Mast, F.; Mayer, H. M.; McCormick, K.; Meisel, D.; Mertens, V.; Mürmann, H.; Neuhäuser, J.; Niedermeyer, H.; Noterdaeme, Jean-Marie; Poschenrieder, W.; Preis, R.; Rapp, H.; Rudyj, A.; Sandmann, W.; Schneider, F.; Schnider, U.; Siller, G.; Simmet, E.; Speth, E.; Söldner, F. X.; Stäbler, A.; Steinmetz, K.; Stroth, U.; Vollmer, O.; Zasche, D.

doi:10.1016/0022-3115(89)90254-7

Cited by 24 publications

(5 citation statements)

References 8 publications

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“…This may be due to that neoclassical transport becomes more dominant in the H-mode pedestal leading to stronger impurity confinement. Experimentally, the found increase in the impurity confinement is more than the improvement in energy confinement, maybe due to the effect of charge dependence of the neoclassical pinch which gives a stronger convection coupled with a relatively weak diffusive contribution with higher charged ions that has been discussed in [17,43,44].…”

Section: The Confinement Characteristic Of High-enhanced-recycling (H...mentioning

confidence: 90%

“…It is well known that high performance and steady state operation are necessities in future fusion. Nevertheless, in most H-mode regimes, the impurity accumulations are frequently observed [17], which would restrict the acquisition of high performance and steady state operation. Plenty of radio-frequency (RF) heating experiments for impurity transport study have indicated that the strong electron heating can be used to prevent the core impurity accumulation.…”

Section: Nuclear Fusionmentioning

confidence: 99%

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Investigation of impurity confinement in lower hybrid wave heated plasma on EAST tokamak

Zhang

et al. 2017

Nucl. Fusion

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The transient perturbation method with metallic impurities such as iron (Fe, Z = 26) and copper (Cu, Z = 29) induced in plasma-material interaction (PMI) procedure is used to investigate the impurity confinement characters in lower hybrid wave (LHW) heated EAST sawtooth-free plasma. The dependence of metallic impurities confinement time on plasma parameters (e.g. plasma current, toroidal magnetic field, electron density and heating power) are investigated in ohmic and LHW heated plasma. It is shown that LHW heating plays an important role in the reduction of the impurity confinement time in L-mode discharges on EAST. The impurity confinement time scaling is given as 42I 0.32 P B 0.2 t n 0.43 e P −0.4 total on EAST, which is close to the observed scaling on Tore Supra and JET. Furthermore, the LHW heated high-enhanced-recycling (HER) H-mode discharges with ~25 kHz edge coherent modes (ECM), which have lower impurity confinement time and higher energy confinement time, provide promising candidates for high performance and steady state operation on EAST.

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Section: The Confinement Characteristic Of High-enhanced-recycling (H...mentioning

confidence: 90%

Section: Nuclear Fusionmentioning

confidence: 99%

Investigation of impurity confinement in lower hybrid wave heated plasma on EAST tokamak

Zhang

et al. 2017

Nucl. Fusion

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“…Thereby, the temporal evolutions of n z (r, t) are obtained. Subsequently, the local radiation intensities are calculated based on equation (1). The necessary 'radiative powerloss coefficients' are also obtained from the ADAS database.…”

Section: General Considerationsmentioning

confidence: 99%

“…Impurity control and energy confinement are topics of crucial importance for achieving high-performance plasmas in both tokamak and stellarator fusion research. It has been observed that the improvement of energy confinement is often accompanied by the accumulation of impurities [1][2][3], which can limit the discharge duration due to radiative collapse [4,5]. Therefore, an important goal for fusion experiments is to find operating windows where impurities can be kept at acceptable levels while the plasma is in a high-performance phase, i.e.…”

Section: Introductionmentioning

confidence: 99%

“…So far, in the neoclassically optimized W7-X stellarator [10], particle and energy transport have been dominated by turbulent transport, especially in plasmas generated by electron cyclotron resonance heating (ECRH). It is characterized by (1) a transport time of impurities (∼50-100 ms) [11][12][13] much shorter than predicted by neoclassical theory (∼ several seconds) and (2) an energy confinement time somewhat below the ISS04 stellarator scaling [14,15]. In addition, the ion temperature T i is usually limited to 1.5 keV [16,17], except in some special cases with the T i -limitation being removed (cf figure 10 [16]).…”

Section: Introductionmentioning

confidence: 99%

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Observation of impurity accumulation and its compatibility with high plasma performance in W7-X

Zhang,

Buttenschön,

Jablonski

et al. 2023

Plasma Phys. Control. Fusion

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At the W7-X stellarator, the bolometer system has measured an intensive radiation zone in the inner plasma region (at a normalized radius ρ~0.3-0.4) in the hydrogen plasma generated by electron cyclotron resonance heating (ECRH); it differs from the normal plasma radiation distribution with an edge-localized emission zone. Spectroscopic diagnostics have recorded high-Z elements such as iron. This phenomenon happens in the plasma phases after the gas supply turn-off, which results in all impurity relevant diagnostic signals increasing for several seconds. Despite the enhancement of the core radiation, the plasma energy confinement is improved. A transport analysis shows that this impurity radiation behavior is associated with a low diffusion coefficient (D ~ 0.02 m2/s) and a reversal of the convection around the radial position of the emission peak, which, under normal conditions, separates the zones of outward convection in the central (│V│~ 0.1 m/s) and inward convection in the outer region (│V│~ 0.3 m/s). An impurity accumulation around this radial position has been identified. The transport coefficients obtained are comparable with the theoretical predictions of collisional impurity transport. In the plasma phases studied, both impurity and energy confinement are enhanced. The mechanism responsible for the improvement is believed to be a reduction of micro-instabilities associated with the observed steepening of the density profile, initiated by a low-edge plasma density (< 1.0×1019 m-3) after switching off the gas fueling. The normalized temperature and density gradients fulfill the condition for the suppression of ITG turbulence.

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Contribution of the Different Erosion Processes to Material Release from the Vessel Walls of Fusion Devices during Plasma Operation

Behrisch

2002

Contrib. Plasma Phys.

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In high temperature plasma experiments several processes contribute to erosion and loss of material from the vessel walls. This material may enter the plasma edge and the central plasma where it acts as impurities. It will finally be re‐deposited at other wall areas. These erosion processes are: • evaporation due to heating of wall areas. At very high power deposition evaporation may become very large, which has been named “blooming”. Large evaporation and melting at some areas of the vessel wall surface may occur during heat pulses, as observed in plasma devices during plasma disruptions. • At tips on the vessel walls and/or hot spots on the plasma exposed solid surfaces electrical arcs between the plasma and the vessel wall may ignite. They cause the release of ions, atoms and small metal droplets, or of carbon dust particles. • Finally, atoms from the vessel walls are removed by physical and chemical sputtering caused by the bombardment of the vessel walls with ions as well as energetic neutral hydrogen atoms from the boundary plasma. All these processes have been, and are, observed in today's plasma experiments. Evaporation can in principle be controlled by very effective cooling of the wall tiles, arcing is reduced by very stable plasma operation, and sputtering by ions can be reduced by operating with a cold plasma in front of the vessel walls. However, sputtering by energetic neutrals, which impinge on all areas of the vessel walls, is likely to be the most critical process because ions lost from the plasma recycle as neutrals or have to be refuelled by neutrals leading to the charge exchange processes in the plasma. In order to quantify the wall erosion, “Materials Factors” (MF) have been introduced in the following for the different erosion processes.

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Impurity accumulation in plasma regimes with high energy confinement

Cited by 24 publications

References 8 publications

Investigation of impurity confinement in lower hybrid wave heated plasma on EAST tokamak

Investigation of impurity confinement in lower hybrid wave heated plasma on EAST tokamak

Observation of impurity accumulation and its compatibility with high plasma performance in W7-X

Contribution of the Different Erosion Processes to Material Release from the Vessel Walls of Fusion Devices during Plasma Operation

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