Nature of high-Z impurity accumulation in tokamaks

Tokar, M. Z.; Rapp, J.; Bertschinger, G.; Könen, L.; Koslowski, H. R.; Kramer-Flacken, A.; Philipps, V.; Samm, U.; Unterberg, B.

doi:10.1088/0029-5515/37/12/i03

Cited by 44 publications

(62 citation statements)

References 21 publications

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“…The temperature gradient term is responsible for the temperature screening effect: the thermal force acts on the impurities in the direction of the thermal gradient and prevents them from penetration into the plasma core. The temperature screening effect has been observed in several tokamak experiments [39,43]. An interesting consequence of this effect is that in a stationary transport regime, its failure can result in an impurity accumulation instability.…”

Section: Discussionmentioning

confidence: 89%

“…The enhanced core power loss due to increased impurity radiation leads to further ion temperature decrease and further temperature screening effect failure. The instability threshold is defined in terms of the critical electron density n e (0): n I (0)/n e (0) ∼ 4 × 10 −2 × (D ⊥ /D PS neo ) 2 [43]. Observations in CDX-U plasmas prior to 1999 appeared to satisfy the criteria of the accumulation instability for low-Z impurities [46].…”

Section: Discussionmentioning

confidence: 99%

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Observation of neoclassical impurity transport in Ohmically heated plasmas of CDX-U low aspect ratio tokamak

Soukhanovskii

Finkenthal

Moos

et al. 2002

Plasma Phys. Control. Fusion

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High β, good confinement and stability properties of the low aspect ratio tokamaks, or spherical tori (ST), have been predicted theoretically and preliminarily confirmed in several large experiments recently. This paper reports on impurity transport experiments carried out in ohmically heated plasmas of the small spherical torus CDX-U with the aspect ratio of A 1.5. Vacuum ultraviolet and soft x-ray multichannel spectroscopic diagnostics are used to measure intrinsic carbon, oxygen and radiated power radial brightness profiles in plasmas with T e (0) 60-80 eV and n e (0) 2 × 10 13 cm −3 . The measurements are performed in both magnetohydrodynamically dominated and quiescent phase of the plasmas. The properties of the observed low m/n modes, sawtooth oscillations, and ST-specific reconnection events are discussed in the context of particle transport. The measured impurity profiles are modelled using one-dimensional impurity transport code MIST and a collisional-radiative package CRMLIN. Impurity diffusion of 0.2 m 2 s −1 D 0.6 m 2 s −1 and convection velocity of v 4-16 m s −1 are inferred from the modelling. These transport coefficients are very close to the neoclassical theory predictions obtained with the FORCEBAL code, which uses analytical plasma viscosity expressions valid for an arbitrary aspect ratio geometry. Neoclassical analysis indicates that both carbon and oxygen are in the collisional regime, and the Pfirsch-Schluter flux is the major fraction of the impurity flux. The causes of the observed strong non-diffusive transport are discussed, and it is concluded that the ∇n i /n i term, resulting from highly peaked ion density profile, makes the largest contribution to the inward pinch. Present analysis suggests that drift wave turbulence is reduced in CDX-U ohmically heated discharges within V A Soukhanovskii et al at least r/a 0.4, however more refined measurements are needed to interpret the results in the framework of ST ion transport.

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Section: Discussionmentioning

confidence: 89%

Section: Discussionmentioning

confidence: 99%

Observation of neoclassical impurity transport in Ohmically heated plasmas of CDX-U low aspect ratio tokamak

Soukhanovskii

Finkenthal

Moos

et al. 2002

Plasma Phys. Control. Fusion

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“…Therefore, impurity seeding of both divertor and core plasma is needed to mitigate the heat loads on the divertor [68]. However, radiation levels are limited by: dilution of the fuel in the core; impurity accumulation [69] and impurity sputtering of the tungsten target [70]. In particular, transient heat loads caused by instabilities, decrease significantly the operation domain of plasma facing components, due to thermal stresses and consequent enhanced erosion [71].…”

Section: 232mentioning

confidence: 99%

Efficiency and availability driven R&D issues for DEMO

Paméla

Bécoulet

Borba³

et al. 2009

Fusion Engineering and Design

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“…At JET, it has been found that the maximum achievable radiative power fraction in stationary type-I ELMy H-modes is f rad = 0.65. At higher radiative power fractions, the local core radiation power density exceeds the local heating power density and an accumulation instability on the basis of the neoclassical theory develops [21]. This has detrimental effects on the confinement [22], although central heating by ion cyclotron resonance heating (ICRH) might enable high impurity concentrations without accumulation in the plasma core [22].…”

Section: Iter Implicationsmentioning

confidence: 99%

Reduction of divertor heat load in JET ELMy H-modes using impurity seeding techniques

Rapp¹,

Monier‐Garbet²,

Matthews³

et al. 2004

Nucl. Fusion

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The main objective of this paper is investigation of methods for reduction of divertor heat loads in order to increase the lifetime of divertor tiles in future fusion reactors. Special emphasis is given to studies of reduction of transient heat loads due to edge localized modes (ELMs). Two methods are compared: argon seeded type-I ELMy H-modes and nitrogen seeded type-III ELMy H-modes. In both scenarios, the impurity seeding leads to a reduction in the pedestal energy and hence a reduction in the energy released by the ELM. This consequentially reduces the power load to the divertor targets. At high radiative power fractions in type-III ELMy H-modes, part of that released ELM energy (small ELMs, below 20 kJ) is dissipated by radiation in the scrape off layer (SOL). Modelling of the ELM mitigation supports the experimental findings. This ELM mitigation by radiative dissipation is not effective for larger ELMs. In between ELMs, the plasma is detached and radiates strongly from the X-point region. During an ELM, the nitrogen in the X-point and divertor region becomes ionized into more weakly radiating higher charge states and the plasma re-attaches for large ELMs. At JET, argon radiates predominantly in the main plasma and not so much in the cold divertor region. Hence, the effect of radiative dissipation of ELM heat fluxes by argon is very low due to the limited argon density in the divertor region. Nevertheless, both scenarios might be compatible with an integrated ITER scenario, with respect to acceptable divertor lifetime and acceptable confinement.

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Nature of high-Z impurity accumulation in tokamaks

Cited by 44 publications

References 21 publications

Observation of neoclassical impurity transport in Ohmically heated plasmas of CDX-U low aspect ratio tokamak

Observation of neoclassical impurity transport in Ohmically heated plasmas of CDX-U low aspect ratio tokamak

Efficiency and availability driven R&D issues for DEMO

Reduction of divertor heat load in JET ELMy H-modes using impurity seeding techniques

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