Observation of Reduced Heat Transport inside the Magnetic Island O Point in the Large Helical Device

Inagaki, S.; Tamura, N.; Ida, K.; Nagayama, Y.; Kawahata, K.; Sudo, S.; Morisaki, T.; Tanaka, Kaoru; Tokuzawa, T.

doi:10.1103/physrevlett.92.055002

Cited by 92 publications

(73 citation statements)

References 17 publications

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“…The cold pulse is produced at ϭ0.75 by injecting the TESPEL to the X-point of the magnetic island. 36 The cold pulse propagation starts at the boundary of the magnetic island ͑O-point͒ and the cold pulse propagates both toward the plasma center ͑outside of magnetic island͒ and towards the plasma edge ͑inside the magnetic island͒. The details of the topology of the magnetic island and of the TESPEL injection and ECE measurements are described in a previous paper.…”

Section: Transport Near the Rational Surfacementioning

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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Section: Transport Near the Rational Surfacementioning

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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“…The electron heat diffusivity evaluated with power balance calculations inside the magnetic island shows that electron transport inside the island was comparable to the transport in the ambient plasma [10]. In contrast, perturbation experiments inside the magnetic island using cold pulse or heat propagation show that the transport inside the magnetic island is strongly reduced with respect to the surrounding plasma [11,12]. There is a discrepancy in the thermal diffusivity evaluated by pulse propagation and power balance inside the magnetic island in tokamak plasma [12].…”

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Reduction of Ion Thermal Diffusivity Inside a Magnetic Island in JT-60U Tokamak Plasma

et al. 2012

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A peaked ion temperature profile is observed inside the magnetic island during mode locking after the back transition from H mode to L mode in JT-60U. The thermal diffusivity evaluated inside the magnetic island is 0:1 m 2 =s, which is much smaller than that outside the magnetic island by an order of magnitude. The present experiment gives clear evidence that ion heat transport inside a magnetic island can bifurcate and the transport level can be suppressed to the very low level associated with the strong flow shear at the boundary. Heat transport at a rational surface and/or magnetic island is important in understanding the role of rational surfaces and magnetic islands in the formation of an internal transport barrier [1][2][3]. This is because there is a strong link between E Â B flow shear to the location of rational surfaces and magnetic islands [4,5]. In particle transport, a good confinement in impurity and bulk particle transport inside a magnetic island has been observed as very peaked soft x-ray profiles at the O point of the magnetic island in tokamaks [6][7][8]; however, the temperature profile is usually only slightly peaked at the O point of the magnetic island [9]. The electron heat diffusivity evaluated with power balance calculations inside the magnetic island shows that electron transport inside the island was comparable to the transport in the ambient plasma [10]. In contrast, perturbation experiments inside the magnetic island using cold pulse or heat propagation show that the transport inside the magnetic island is strongly reduced with respect to the surrounding plasma [11,12]. There is a discrepancy in the thermal diffusivity evaluated by pulse propagation and power balance inside the magnetic island in tokamak plasma [12]. It is not clear whether this discrepancy is due to the transient and steady-state heat diffusivity or the difference in the level of temperature gradient (for example, below or above the critical temperature gradient). Although the transport inside the magnetic island is a crucial issue, the characteristics are still open to question.In general, the temperature gradient inside the magnetic island is small enough to assume a flat temperature profile within the magnetic island. However, this is because the heat flux perpendicular to the magnetic flux surface bypass as the magnetic island and flows through the X point of the magnetic island. Therefore, only the power deposited inside the magnetic island can contribute to the heat flux inside the magnetic island, which is much smaller than the heat flux in the ambient plasma. In order to evaluate the ion heat transport inside the magnetic island, the precise measurement of temperature profiles with high spatial resolution is required. In this Letter, the first observation of a clear peaked ion temperature profile at the O point of a magnetic island and a significant reduction of ion thermal diffusivity inside the magnetic island are described. In JT-60U, the radial profile of ion temperature is measured with the modulation ...

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“…In this simulation, the perturbed heat transport equation based on a simple diffusion model is solved numerically ͑see Ref. 27 for details͒. The heat diffusivity used in the simulation, which has a radial dependence, is obtained by power balance analysis.…”

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Observation of core electron temperature rise in response to an edge cooling in toroidal helical plasmas

et al. 2005

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The first observation of a significant rise of core electron temperature in response to edge cooling in a helical plasma has been made on the Large Helical Device ͓O. Motojima et al., Phys. Plasmas 6, 1843 ͑1999͔͒. When the phenomenon occurs, the electron heat diffusivity in the core region is reduced abruptly without changing local parameters in the region of interest. Therefore the phenomenon can be regarded as a so-called "nonlocal" electron temperature rise observed so far only in many tokamaks. © 2005 American Institute of Physics. ͓DOI: 10.1063/1.2131047͔The clarification of electron heat transport in magnetically confined plasmas is still an important issue, since the performance of a probable fusion reactor should be determined by electron heating as a result of the interaction between electrons and alpha particles as a fusion reaction product. In order to promote a better understanding of the electron heat transport, the electron heat transport analysis for both transient and steady state has been carried out diligently in many tokamaks 1-3 and helical systems. [4][5][6] One of the significant issues found in these studies is a "nonlocal transport phenomenon" observed in perturbation experiments on many tokamaks 7-12 and a few helical systems. 4 In particular, a rise of the core electron temperature T e invoked by the rapid cooling of the edge plasma has been observed in various tokamaks with both ohmically heated plasmas and plasmas with an auxiliary heating, such as electron cyclotron heating ͑ECH͒, at a sufficiently low density ͑e.g., Ref. 13͒. The amplitude reversal of the cold pulse propagation in the core plasma cannot be explained even by the model based on the assumption that heat flux has a strong nonlinear dependence on temperature and its gradient. In addition there seem to be no changes in the thermodynamic forces, such as those due to the temperature gradient and/or the density gradient, in the core plasma at the onset of the core T e rise. Consequently, the core T e rise invoked by the edge cooling is considered to result from a nonlocality in the electron heat transport. On the contrary, such a core T e rise in response to the edge cooling has not been observed so far in helical systems.14 Recently, to rationalize the so-called "nonlocal" T e rise, some physics-based transport models including a critical gradient scale length, such as the ion temperature gradient ͑ITG͒ model, have been set up and tested.2,15 It should be noted that despite being dependent on local variables, the ITG-based model shows a nonlocal response to small changes in the profiles as a result of a slight deviation from near marginality. The ITG-based model with strongest stiffness 16 can reproduce some of the same qualitative characteristics observed in carbon laser blow-off experiments in the Texas Experimental Tokamak ͑TEXT͒.13 The magnitude and response time of the core T e rise, however, are still in quantitative disagreement with those predicted by the strongest stiff ITG-based model. 15 Moreover, it is an open ...

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Observation of Reduced Heat Transport inside the Magnetic Island O Point in the Large Helical Device

Cited by 92 publications

References 17 publications

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

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

Reduction of Ion Thermal Diffusivity Inside a Magnetic Island in JT-60U Tokamak Plasma

Observation of core electron temperature rise in response to an edge cooling in toroidal helical plasmas

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