Recent progress toward high performance above the Greenwald density limit in impurity seeded discharges in limiter and divertor tokamaks

Ongena, J.; Budny, R.; Dumortier, P.; Jackson, G.L.; Kubo, H.; Messiaen, A.; Murakami, Masato; Strachan, J.; Sydora, R. D.; Tokar, M. Z.; Unterberg, B.; Samm, U.; Vandenplas, P. E.; Weynants, R.; Asakura, N.; Brix, M.; Charlet, M.; Coffey, I.; Cordey, G.; Erents, S.K.; Fuchs, Gerhard; Hellermann, M. von; Hillis, D.L.; Hogan, J.; Horton, L. D.; Ingesson, L. C.; Itami, K.; Jachmich, S.; Kallenbach, A.; Koslowski, H. R.; Kraemer-Flecken, A.; Lawson, K.; Loarte, A.; Maddison, G.; Mank, G.; McKee, G. R.; Meigs, A.; Milani, F.; Monier‐Garbet, P.; Nave, M. F. F.; Puiatti, M. E.; Parail, V.; Rapp, J.; Sakurai, S.; Sharapov, S. E.; Sartori, F.; Stamp, M.; Tamai, Hiroshi; Telesca, G.; Valisa, M.; Wassenhove, G. Van; Weyssow, Boris; Zastrow, K.‐D.; Contributors, Efda‐Jet

doi:10.1063/1.1364513

Cited by 49 publications

(16 citation statements)

References 27 publications

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“…[14,15], instabilities driven by impurities were analyzed, and it was demonstrated that their growth rates are reduced with increasing A i . However, the importance of these instabilities for tokamak confinement has been cast in doubt by the discovery of plasma states where anomalous transport is reduced due to the deliberate seeding of impurities [16]. A positive isotope effect was found also in numerical modeling of collisional drift turbulence [17].…”

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confidence: 94%

Nature of the Isotope Effect on Transport in Tokamaks

2004

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The reduction of energy and particle losses with the increasing mass of the hydrogen isotope is more pronounced under conditions of improved confinement when the dominant ion temperature gradient instability is suppressed and other channels of anomalous transport are of importance. In this Letter, we reconsider the dissipative trapped electron (DTE) instability by taking into account finite Larmor radius effects in the analysis of the ion response to perturbations. By applying the improved mixing length approximation in order to estimate the transport coefficients, it is demonstrated that DTE contribution is intrinsically dependent on the isotope mass and provides a plausible explanation for the isotope effect. Contrary to the common belief, it is shown that the DTE turbulence may be of importance for reactor plasmas of low collisionality. As observed in practically all tokamaks, the energy and particle confinement improves with increasing atomic weight of the isotope used, from hydrogen to deuterium to tritium (see, e.g., Refs. [1][2][3]). This fact is of crucial importance for the performance of future thermonuclear reactors, which will operate with a mixture of deuterium and tritium rather than present devices which normally use either hydrogen or deuterium. However, in spite of its fundamental significance, the isotope effect is still one of the least understood phenomena in the physics of tokamak transport.Analysis of the experimental database shows that the isotope effect on confinement depends essentially on the mode of tokamak operation. Thus, the most recent scalings for the energy confinement time E [4] A pattern in this puzzling picture can be found by noting that the strongest improvement with A i takes place if the ion temperature gradient (ITG) instability, which is commonly considered as the main source of anomalous transport [6], is suppressed. This is the case both for the edge barrier in the H mode and for other regimes of improved confinement where ITG is subdued by the density gradient in a large part of the plasma volume (see, e.g., analysis of the L-RI transition in Refs. [7,8]). On the contrary, under conditions where the transport is dominated by ITG modes the confinement deteriorates with increasing A i . For the case of the plasma core in the H mode, this fact was explained in Ref. [9] by applying the mixing length approximation (MLA) [6,10,11], in which max =k 2 max is used as an estimate for transport coefficients. Here max is the maximum value of the instability growth rate considered as a function of the perpendicular wave number k and k max is the k-value where max is reached. For the ITG instability both max and k max vary as A ÿ0:5 i [6,12] and the characteristic heat diffusivity A 0:5 i . Since E 1= , the scaling above is in qualitative agreement with the decrease of the core confinement in the H mode with increasing isotope atomic weight. The same result follows from the so-called improved mixing length approximation (IMLA) [6,13], which takes into account the phase shift betw...

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confidence: 94%

Nature of the Isotope Effect on Transport in Tokamaks

2004

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“…This is an important issue for future reactors, since, with increasing input power, the heating and the erosion of the first wall become a critical point. It has been found in TEXTOR and subsequently in other devices (Asdex-Upgrade, DIII-D, JT-60U) that in these highly radiative regimes the plasma performance improves when density limits are approached (for an overview of these experiments, see [1] and the references quoted there). Of course the accompanying risk is an increase of the plasma pollution, with large values of effective charge Z eff and of the radiated power density from the plasma centre.…”

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confidence: 99%

Radiation pattern and impurity transport in argon seeded ELMy H-mode discharges in JET

Puiatti

Mattioli

Telesca

et al. 2002

Plasma Phys. Control. Fusion

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This paper addresses the issues of impurity behaviour during the argon seeding experiments in JET, in which argon and D 2 have been simultaneously puffed in ELMy H-mode discharges to reach high density regimes, maintaining good confinement properties throughout the plasma discharge. The analysis is based mainly on a 1-D impurity diffusion model, which evaluates the impurity transport coefficients in three experimental scenarios.The available experimental data include, as a function of the time, the brightnesses of the soft x-rays and of several impurity lines, the effective charge Z eff , the profiles of the radiated power and the fully stripped Ar and C ion densities from charge-exchange recombination. From the simulation of these data it is possible to characterize the considered scenarios in terms of high-Z ion increase in the plasma centre and high radiation from the edge. In particular, it has been found that the configuration at high triangularity with continuous D 2 puffing, characterized by an outward impurity pinch velocity, features flat or slightly hollow impurity ion profiles and high radiation from a narrow region at the edge, possibly associated to the presence of a significant neutral density. In this case an edge radiative mantle is established without high radiation from the plasma centre.

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“…High density (n e /n Gr 0.85), confinement (H 98(y, 2) ≈ 1.0) and β (β N 1.8) were realized simultaneously together with a high radiation level. In low triangularity H-modes in the 'septum' configuration (x-point lying on the dome of the MkII-GB divertor), a peaking of the density profile was observed with argon injection, reminiscent of the observations made in the RI mode on TEXTOR [4]. The present work reports only on high triangularity (δ = 0.44) plasmas with the MkII-SRP divertor configuration, without the septum.…”

Section: Introductionmentioning

confidence: 79%

“…Can impurity be used to control/mitigate ELMs? A summary of the previous JET work on argon seeded H-modes is given in [4][5][6]. The experiments reported in these references were performed with the MkII-GB divertor configuration.…”

Section: Introductionmentioning

confidence: 99%

Impurity-seeded ELMy H-modes in JET, with high density and reduced heat load

Monier‐Garbet

Andrew

Belo³

et al. 2005

Nucl. Fusion

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Experiments performed at JET during the past two years show that, in high triangularity H-mode plasmas with I p = 2.5 MA, n e /n Gr ≈ 1.0, it is possible to radiate separately up to ≈40% of the total injected power on closed flux surfaces in the pedestal region (argon seeding) and up to ≈50% of the injected power in the divertor region (nitrogen seeding), while maintaining the confinement improvement factor at the value required for ITER, H 98(y, 2) ≈ 1.0. The total radiated power fraction achieved in both cases (65-70%) is close to that required for ITER. However, Type I ELMs observed with impurity seeding have the same characteristics as that observed in reference pulses without seeding: decreasing plasma energy loss per ELM with increasing pedestal collisionality. One has to reach the Type III ELM regime to decrease the transient heat load to the divertor to acceptable values for ITER, although at the expense of confinement. The feasibility of an integrated scenario with Type-III ELMs, and q 95 = 2.6 to compensate for the low H factor, has been demonstrated on JET. This scenario would meet ITER requirements at 17 MA provided that the IPB98 scaling for energy content is accurate enough, and provided that a lower dilution is obtained when operating at higher absolute electron density.

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Recent progress toward high performance above the Greenwald density limit in impurity seeded discharges in limiter and divertor tokamaks

Abstract: An overview is given of recent advances toward the realization of high density, high confinement plasmas with radiating mantles in limiter and divertor tokamaks worldwide.

Cited by 49 publications

References 27 publications

Nature of the Isotope Effect on Transport in Tokamaks

Nature of the Isotope Effect on Transport in Tokamaks

Radiation pattern and impurity transport in argon seeded ELMy H-mode discharges in JET

Impurity-seeded ELMy H-modes in JET, with high density and reduced heat load

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