Model for the Transition to the Radiatively Improved Mode in a Tokamak

Tokaŕ, M. Z.; Ongena, J.; Unterberg, B.; Weynants, R.

doi:10.1103/physrevlett.84.895

Cited by 94 publications

(113 citation statements)

References 23 publications

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“…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 .…”

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“…The DTE instability is caused by the specific velocity dependence of the collision frequency of trapped particles [11,[18][19][20]. DTE induced transport has been identified as the main channel of particle and energy losses in the linear Ohmic [21], improved Ohmic [22], and RI [8] modes of confinement. Its enhancement is believed to be the main cause of confinement deterioration in the RI mode when hydrogen puffing is applied instead of deuterium puffing [23].…”

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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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Nature of the Isotope Effect on Transport in Tokamaks

2004

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“…A careful adjustment of the edge and heating conditions is necessary to reach RI-mode confinement, proportional to the electron density, leading to high confinement at high density. The confinement improvement observed in the RI-mode has been interpreted as resulting from the suppression of ion thermal gradient (ITG) modes caused by the presence of impurities in the edge and by the resulting core density peaking [9]. TEXTOR-94 is a medium size tokamak (R 1.75 m, a 0.46 m) equipped with the toroidal belt limiter ALT-II which consists of eight pumped blades.…”

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“…The steepening of the density profile is considered together with the presence of impurities at the edge to trigger the confinement improvement in the RI-mode through stabilization of the ITG-modes [9].…”

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Quasistationary High Confinement Discharges with trans-Greenwald Density on TEXTOR-94

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et al. 2000

Phys. Rev. Lett.

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Confinement quality as good as ELM-free H-mode at densities substantially above the Greenwald density limit (n e,0 ͞n GW 1.4) has been obtained in discharges with a radiative boundary under quasistationary conditions for 20 times the energy confinement time. This is achieved by optimizing the gas-fueling rate of RI-mode discharges which tailors their favorable energy confinement and leads to discharges with beta values just below the operational limit b n 2 of TEXTOR-94, thereby effectively avoiding confinement back transitions or disruptions. In addition, this high-density regime is favorable for helium removal and results in figures of merit t ‫ء‬ p,He ͞t E ഠ 10 15, relevant for a future fusion power reactor. A subject of intense investigations in present day fusion research is the development of an operational scenario, providing at the same time (i) confinement with at least H-mode quality and (ii) high densities around or above the empirical Greenwald density value n GW I P ͞pa 2 [1] [with I P the plasma current (MA), the minor plasma radius (m), and n GW in units 10 20 m 23 ]. Success in this direction has been obtained in the past years on limiter [2,3] and divertor tokamaks by pellet injection or adjusting deuterium fueling and pumping [4,5]. Recently, high energy confinement at a Greenwald numbern e,0 ͞n GW 1.4 (n e,0 being the central averaged chord density) was obtained at DIII-D by optimizing divertor pumping in gas puffed H-mode discharges with edge localized modes (ELM) [6,7]. This is in contrast to most H-mode discharges which undergo a confinement degradation when approaching this value [8]. In this paper we show that it is possible in addition to substantially overcome the Greenwald limit under quasistationary conditions with confinement of ELM-free H-mode quality, high b, q 3.4, and a regime favorable for heat and particle exhaust. This is obtained by optimizing the gas-fueling rate in radiatively improved (RI) mode plasmas on the limiter tokamak TEXTOR-94 in order to maintain a sufficiently low neutral pressure in the scrape off layer (SOL).The RI-mode on TEXTOR-94 is obtained with edge radiation cooling due to neon seeding under boronized wall conditions or sputtering of silicon under siliconized wall conditions. A careful adjustment of the edge and heating conditions is necessary to reach RI-mode confinement, proportional to the electron density, leading to high confinement at high density. The confinement improvement observed in the RI-mode has been interpreted as resulting from the suppression of ion thermal gradient (ITG) modes caused by the presence of impurities in the edge and by the resulting core density peaking [9]. TEXTOR-94 is a medium size tokamak (R 1.75 m, a 0.46 m) equipped with the toroidal belt limiter ALT-II which consists of eight pumped blades. Some blades are equipped with gas inlet apertures located at the surface of the limiter tiles. The gas flow through the ALT fueling apertures is smoother than it is for the standard TEXTOR gas feeders equipped with a fast feedback ...

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“…The mechanisms considered presently to lead to confinement improvement in the RI mode are largely based on the reduction of the growth rate of the toroidal ion temperature gradient ͑ITG͒ mode when the plasma Z eff increases. 8 It is argued that the plasma density profile peaks when the ITG mode gets suppressed owing to the pinching effects associated with the dissipative-trapped electron ͑DTE͒ mode. 9 The density peaking further suppresses the ITG and the plasma undergoes a bifurcation into the improved confinement, high-density regime.…”

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Asymmetric radiation-induced toroidal flow and improved confinement in tokamaks

et al. 2006

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The role of impurity radiation in influencing the toroidal flow and radial electric fields ͑parameters critical for determining turbulent transport͒ has been studied on the edge of a tokamak plasma. It is demonstrated for the first time that the impurities distributed in an asymmetric ͑poloidally͒ manner may lead to significant density and temperature perturbations on magnetic surfaces. These, in turn, interact with the dependent toroidal field variations and yield a mean divergence of the stress tensor driving strong neoclassical toroidal flows. A self-consistent theory of interplay of equilibrium, fluctuations, neoclassical flows, and E ជ ϫ B ជ shear rotation in a tokamak is also presented. It is shown that the resulting enhanced toroidal velocity shear on the outer radiative layers produces a stabilizing effect on the well known instabilities ͑which determine edge transport͒ such as the drift resistive ballooning mode, the drift trapped electron mode, and the ion temperature gradient mode. For various values of the radiation asymmetry parameter, investigation of the turbulent particle flux as a function of the density gradient shows that the plasma can undergo a bifurcation into a better-confined state with a peaked density.

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Model for the Transition to the Radiatively Improved Mode in a Tokamak

Cited by 94 publications

References 23 publications

Nature of the Isotope Effect on Transport in Tokamaks

Nature of the Isotope Effect on Transport in Tokamaks

Quasistationary High Confinement Discharges with trans-Greenwald Density on TEXTOR-94

Asymmetric radiation-induced toroidal flow and improved confinement in tokamaks

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