Simultaneous Attainment of High Electron and Ion Temperatures in Discharges with Internal Transport Barriers in ASDEX Upgrade

Günter, S.; Wolf, R. C.; Leuterer, F.; Gruber, O.; Kaufmann, Michael; Lackner, K.; Maraschek, M.; Carthy, P. J. Mc; Meister, H.; Peeters, A. G.; Pereverzev, G.; Salzmann, H.; Schade, S.; Schweinzer, J.; Suttrop, W.

doi:10.1103/physrevlett.84.3097

Cited by 54 publications

(26 citation statements)

References 13 publications

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“…(d) The constraint (7) depends on τ e = T e /T i via the coefficient D in such a manner that the range of validity of the assumption w l,m < ∆ l,m , and therefore of the expression (6a) of the growth rate, extends to higher poloidal mode numbers if T e /T i increases. This favourable trend (we note that there is a rapid increase of the growth rate when the condition w l,m < ∆ l,m is no longer fulfilled) explains why transport barriers, first observed in plasmas with predominantly ion heating and characterized by τ e < 1, are maintained when τ e ∼ 1 [6]. This result is important for reactors with predominant heating by thermonuclear α particles.…”

Section: Initiation Of Ion Itbs In Ncs Dischargesmentioning

confidence: 67%

“…Analysis of negative central shear (NCS) tokamak discharges has shown that an ion internal transport barrier (ITB) may develop near the position where the safety factor q reaches its minimum [1][2][3][4][5][6]. There is also evidence that, in addition to the NCS q profile, E × B flow shear in the core plays an important role in stabilizing turbulence [7][8][9].…”

Section: Introductionmentioning

confidence: 99%

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First principles model of internal transport barriers in negative central shear discharges

Rogister¹

2001

Nucl. Fusion

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Abstract. At and around minima or maxima of the safety factor q, overlapping, and therefore toroidicity induced coupling of drift eigenmodes centred on neighbouring rational surfaces r l,m defined by q(r l,m ) = m/l and r l,m±1 defined by q(r l,m±1 ) = (m ± 1)/l, is generally negligible; l and m(m ± 1) are the toroidal and (main) poloidal mode numbers, respectively. Under these conditions, a careful analysis shows that the growth rate of the ion temperature gradient (ITG) mode is proportional to the absolute value of the magnetic shear parameter; its radial width and frequency (in the E × B rotating frame) are larger in the axisymmetric torus than in the plasma slab or cylinder, especially at small values of k θ ai (k θ is the poloidal mode number and ai the characteristic ion Larmor radius). These results provide a straightforward theoretical interpretation of the origin of internal ion transport barriers (ITBs) in discharges with negative central shear if the ITG instability is indeed the main channel for ion energy transport. In agreement with recent experiments, the mechanism does not degrade for Te ∼ Ti, an important result for extrapolation to reactors. Once initiated, an ITB will locally increase the curvature of the ion temperature profile and, consequently, the gradients of the radial electric field and E × B rotation profiles (until neoclassical (or sub-neoclassical) transport comes into play).The damping associated with velocity shear yields an upper bound on the unstable mode number range, whereas Landau damping is expected to introduce a lower bound. In cases where toroidal effects are subdominant, the ratio of the damping rate associated with velocity shear to the growth rate associated with magnetic shear leads naturally to the Hamaguchi-Horton parameter (divided by the factor ηi − 2/3 which characterizes the departure from the instability threshold (ηi is the ratio of the density and temperature length scales)) when introducing the most unfavourable mode number. Toroidal effects are always dominant for sufficiently weak magnetic shear or sufficiently large values of |LN |/R (where LN is the density gradient length), and therefore before steepening of the profiles has occurred near the zero magnetic shear layer; the counterpart of the Hamaguchi-Horton characteristic parameter appropriate to that case is also obtained, considering again the most unfavourable mode number.

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Section: Initiation Of Ion Itbs In Ncs Dischargesmentioning

confidence: 67%

Section: Introductionmentioning

confidence: 99%

First principles model of internal transport barriers in negative central shear discharges

Rogister¹

2001

Nucl. Fusion

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“…[1]) and ASDEX Upgrade (Ref. [2]) tokamaks. Theoretically, there has been an increasing number of studies on the equilibrium and stability properties of flowing plasmas (see, for example, Refs.…”

Section: Introductionmentioning

confidence: 99%

Magnetohydrodynamic counter-rotating vortices and synergetic stabilizing effects of magnetic field and plasma flow

Throumoulopoulos

Tasso

2010

Physics of Plasmas

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A nonlinear two-dimensional steady state solution in the framework of hydrodynamics describing a row of periodic counter-rotating vortices is extended to the magnetohydrodynamic (MHD) equilibrium equation with incompressible flow of arbitrary direction. The extended solution covers a variety of equilibria because four surface quantities remain free. Similar to the case of the MHD cat-eyes equilibrium [Throumoulopoulos et al., J. Phys. A: Math. Theor. 42, 335501 (2009)] and unlike linear equilibria, the flow has a strong impact on isobaric surfaces by forming pressure islands located within the counter-rotating vortices even for values of β (defined as the ratio of the thermal pressure over the external axial magnetic-field pressure) on the order of 0.01. Also, the axial current density is appreciably modified by the flow. Furthermore, a magnetic-field-aligned flow of experimental fusion relevance, i.e for Alfvén Mach numbers of the order of 0.01, and the flow shear in combination with the variation of the magnetic field perpendicular to the magnetic surfaces have significant stabilizing effects potentially related to the equilibrium nonlinearity. The stable region is enhanced by an external axial magnetic field.

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“…[1][2][3][4][5][6] In these experiments, the radial profiles of rotational transform ͑safety factor͒ are measured or calculated and the role of magnetic shear in the formation of the electron internal transport barrier is discussed and the role of radial electric field E r shear on the electron and ion transport barrier has been studied in tokamak plasma. 7,8 In stellarator plasmas, both the electron and ion ripple losses are sensitive to the radial electric field and electron and ion radial fluxes are functions of the radial electric field in the plasma.…”

Section: Introductionmentioning

confidence: 99%

Characteristics of transport in electron internal transport barriers and in the vicinity of rational surfaces in the Large Helical Device

et al. 2004

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Characteristics of transport in electron internal transport barriers ͑ITB͒ and in the vicinity of a rational surface with a magnetic island are studied with transient transport analysis as well as with steady state transport analysis. Associated with the transition of the radial electric field from a small negative value ͑ion-root͒ to a large positive value ͑electron-root͒, an electron ITB appears in the Large Helical Device ͓M. Fujiwara et al., Nucl. Fusion 41, 1355 ͑2001͔͒, when the heating power of the electron cyclotron heating exceeds a power threshold. Transport analysis shows that both the standard electron thermal diffusivity, e , and the incremental electron thermal diffusivity, e inc ͑the derivative of normalized heat flux to temperature gradient, equivalent to heat pulse e ), are reduced significantly ͑a factor 5-10͒ in the ITB. The e inc is much lower than the e by a factor of 3 just after the transition, while e inc is comparable to or even higher than e before the transition, which results in the improvement of electron transport with increasing power in the ITB, in contrast to its degradation outside the ITB. In other experiments without an ITB, a significant reduction ͑by one order of magnitude͒ of e inc is observed at the O-point of the magnetic island produced near the plasma edge using error field coils. This observation gives significant insight into the mechanism of transport improvement near the rational surface and implies that the magnetic island serves as a poloidally asymmetric transport barrier. Therefore the radial heat flux near the rational surface is focused at the X-point region, and that may be the mechanism to induce an ITB near a rational surface.

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Simultaneous Attainment of High Electron and Ion Temperatures in Discharges with Internal Transport Barriers in ASDEX Upgrade

Cited by 54 publications

References 13 publications

First principles model of internal transport barriers in negative central shear discharges

First principles model of internal transport barriers in negative central shear discharges

Magnetohydrodynamic counter-rotating vortices and synergetic stabilizing effects of magnetic field and plasma flow

Characteristics of transport in electron internal transport barriers and in the vicinity of rational surfaces in the Large Helical Device

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