Experimental studies of electron transport

Ryter, F.; Angioni, C.; Beurskens, M.; Cirant, S.; Hoang, G. T.; Hogeweij, G. M. D.; Imbeaux, F.; Jacchia, A.; Mantica, P.; Suttrop, W.; Tardini, G.

doi:10.1088/0741-3335/43/12a/325

Cited by 129 publications

(128 citation statements)

References 43 publications

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“…The energy transport in tokamak core plasmas is dominated by turbulence which restricts the normalised temperature gradient ∇T /T near the marginal stability limit as the turbulent transport increases rapidly when the gradient exceeds the critical value [4]. This stiff transport dictates that increasing core heating will not be very effective at increasing the core temperature, but will just increase the turbulent heat flux.…”

Section: Introductionmentioning

confidence: 99%

Non-local effects on pedestal kinetic ballooning mode stability

Saarelma

Martin-Collar

Dickinson

et al. 2017

Plasma Phys. Control. Fusion

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The H-mode pedestal height plays an important role in determining the global confinement of the tokamak plasma. In type I ELMy H-mode the ultimate limit for the pedestal pressure at constant width is set by the ideal MHD peeling-ballooning modes that are thought to be the trigger for the ELMs. However, the peeling-ballooning mode criterion does not uniquely determine the pedestal. Increasing the width of the pedestal, the marginally peeling-ballooning stable pedestal height increases as well. The second criterion for the pedestal is set by the transport processes in the pedestal that limit the gradient between the ELMs.One candidate for driving this transport is the kinetic ballooning mode (KBM) that is driven by the pressure gradient Non-Local Effects on Pedestal Kinetic Ballooning Mode Stability 2 with the pressure gradient clamped near to the stability limit. In the local linear gyrokinetic analysis of experimental MAST and JET plasmas we have found that, like the n=∞ ideal MHD ballooning modes, the KBMs can access locally so called second stability if the magnetic shear becomes low enough [2,3]. However, in the pedestal region the local assumption that the equilibrium can be considered radially constant for the investigated modes is no longer justified. In this paper we revisit the KBM analysis using a global code ORB5 to investigate whether second stability access exists for KBMs.We find that counter to the local analysis, the global KBM stability is not sensitive to the magnetic shear in the pedestal region. At sufficiently high β (but still below the ideal peeling-ballooning limit) the pedestal region becomes KBM unstable regardless of the amount of bootstrap current assumed in the equilibrium reconstruction. However, just as in local analysis, the mode is stabilised by reducing the pressure gradient. This suggests that KBMs can regulate the pedestal pressure gradient during the ELM cycle even when local analysis finds them stable due to high bootstrap current.

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Section: Introductionmentioning

confidence: 99%

Non-local effects on pedestal kinetic ballooning mode stability

Saarelma

Martin-Collar

Dickinson

et al. 2017

Plasma Phys. Control. Fusion

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“…Recently, significant progress has been made in the investigation of the electron heat transport in L-mode plasmas with pure electron heating added to ohmic plasmas [1,2,3,4,5,6,7,8]. In contrast to these cases, in which the electron temperature is larger than the ion temperature (Te > Ti), the transport in plasmas with dominant ion heating (Te ≤ Ti), and particularly in H-mode, has not been thoroughly understood yet.…”

Section: Introductionmentioning

confidence: 99%

Role ofT_e/T_iand ∇v_torin ion heat transport of ASDEX Upgrade H-mode plasmas

et al. 2006

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Experiments in H-mode plasmas have shown that both heat and particle transport are sensitive to the ratio between electron and ion temperature (Te/Ti). While decreasing Te/Ti is beneficial for confinement, an increased electron heating in these so called "hot ion plasmas" deteriorates the confinement. H-mode plasmas with low Te/Ti are often accompanied by high toroidal rotation velocity (v φ ). Its gradient (∇v φ ) can destabilise the ion temperature gradient mode (ITG) through its parallel component in the parallel velocity shear, but it has also stabilising effects since it produces an E ×B shearing rate (ωE×B). In this paper, the effects of electron heating on the ion heat transport is investigated in H-mode plasmas heated by neutral beam injection (NBI) and electron cyclotron heating (ECH). In particular, the correlation on Te/Ti and ∇v φ is studied and compared with calculations made with GLF23 and GS2. Experimentally it is shown that the normalised gradient length of the ions (R/LT i ) is correlated to both Te/Ti and ∇v φ : peaked ion temperature profiles are only obtained with low Te/Ti and high ∇v φ , and vice-versa. When ECH is added, both ion heat and momentum transport are enhanced, leading to a drop of both the Ti and v φ profiles. The effective growth rate γ ef f = γ − ωE×B is calculated, with the mode growth rate γ determined with GS2 and ωE×B with GLF23. The ion transport is enhanced due to the decrease of the ITG R/LT i threshold with increasing Te/Ti. Comparison of the dependence of R/LT i on Te/Ti and ∇v φ between experiments and modelling indicates that the deterioration of confinement cannot be explained by the changes in only Te/Ti or ∇v φ , but by the combined effects of both parameters. The changes in Te/Ti act directly on the ITG threshold, while the ones in ∇v φ modify the ωE×B shearing rate leading to changes in the effective threshold.

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“…A quantitative comparison of electron thermal diffusivity e [ Q=n e =rT e ] and an incremental electron thermal diffusivity inc e [ dQ=n e =drT e ] has not been done in ITB plasma in helical devices in spite of its importance in understanding the transport in toroidal devices [12]. In the L-mode plasma, the instability, such as electron temperature gradient mode [13,14], often results in the sharp increase of the thermal diffusivity above the critical electron temperature gradients and determines the upper limit of the electron temperature for the available heating power [15]. Therefore, the dependence of the thermal diffusivity on the temperature gradient is an extremely important issue to have a prospect of the plasma performance with an electron ITB in the high temperature regime required for nuclear fusion.…”

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confidence: 99%

Characteristics of Electron Heat Transport of Plasma with an Electron Internal-Transport Barrier in the Large Helical Device

et al. 2003

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Associated with the transition from ion root to electron root, an electron internal transport barrier (ITB) appears in the large helical device, when the heating power of electron cyclotron resonance heating exceeds the threshold power. The incremental thermal diffusivity of electron heat transport inc e in the ITB plasma is much lower than that in the plasma with the heating power below the threshold, and the thermal diffusivity e decreases with increasing of heating power [ Recently, the electron thermal transport barrier has been observed with dominant electron cyclotron heating in the plasma with a negative magnetic field shear in many tokamaks [1][2][3][4][5][6]. In these experiments, the radial profiles of the rotational transform (safety factor) are measured or calculated and the role of the magnetic shear in the formation of electron internal transport barrier is discussed and the role of the radial electric field E r shear on the electron and ion transport barrier has been studied in tokamak plasma [7,8]. On the other hand, in a stellarator, where the magnetic shear is negative, the electron internal transport barrier (ITB) has been observed associated with the transition from ion root (large neoclassical flux with a small E r ) to electron root (small neoclassical flux with a large positive E r ), when the collisionality becomes low enough for the transition [9][10][11]. Although the mechanism of ITB formation associated with the transition from ion root to electron root has been studied [10,11], the characteristics of thermal transport, i.e., the relation between electron temperature gradient rT e and heat flux normalized by electron density Q=n e , have not been studied quantitatively. A quantitative comparison of electron thermal diffusivity e [ Q=n e =rT e ] and an incremental electron thermal diffusivity inc e [ dQ=n e =drT e ] has not been done in ITB plasma in helical devices in spite of its importance in understanding the transport in toroidal devices [12]. In the L-mode plasma, the instability, such as electron temperature gradient mode [13,14], often results in the sharp increase of the thermal diffusivity above the critical electron temperature gradients and determines the upper limit of the electron temperature for the available heating power [15]. Therefore, the dependence of the thermal diffusivity on the temperature gradient is an extremely important issue to have a prospect of the plasma performance with an electron ITB in the high temperature regime required for nuclear fusion. Once the ITB is achieved, it is more important how the thermal diffusivity changes as the heating power is increased than how much is the reduction of thermal diffusivity at the transition from the L-mode plasma to the ITB plasma. The incremental thermal diffusivity inc e is a key parameter in this study, because the dependence of thermal diffusivity on heating power P (sign of d e =dP) is determined by the ratio of two thermal diffusivities ( inc e = e < 1 or > 1). In this Letter, the relation between the T e g...

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Experimental studies of electron transport

Cited by 129 publications

References 43 publications

Non-local effects on pedestal kinetic ballooning mode stability

Non-local effects on pedestal kinetic ballooning mode stability

Role ofT_e/T_iand ∇v_torin ion heat transport of ASDEX Upgrade H-mode plasmas

Characteristics of Electron Heat Transport of Plasma with an Electron Internal-Transport Barrier in the Large Helical Device

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Experimental studies of electron transport

Cited by 129 publications

References 43 publications

Non-local effects on pedestal kinetic ballooning mode stability

Non-local effects on pedestal kinetic ballooning mode stability

Role ofTe/Tiand ∇vtorin ion heat transport of ASDEX Upgrade H-mode plasmas

Characteristics of Electron Heat Transport of Plasma with an Electron Internal-Transport Barrier in the Large Helical Device

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Role ofT_e/T_iand ∇v_torin ion heat transport of ASDEX Upgrade H-mode plasmas