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

In Hybrid plasma operation in JET with its ITER-like wall (JET-ILW) it is found that n>1 tearing activity can significantly enhance the rate of on-axis peaking of tungsten impurities, which in turn significantly degrades discharge performance. Core n=1 instabilities can be beneficial in removing tungsten impurities from the plasma core (e.g. sawteeth or fishbones), but can conversely also degrade core confinement (particularly in combination with simultaneous n=3 activity). The nature of MHD instabilities in JET Hybrid discharges, with both its previous Carbon wall and subsequent JET-ILW, is surveyed statistically and the character of the instabilities is examined. Possible qualitative models for how the n>1 islands can enhance on-axis tungsten transport accumulation processes are presented.

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“…Flattening of the ion temperature inside an island has been observed in other devices (e.g. Fig 2a of Ref [37]).…”

Section: Fig 15supporting

confidence: 62%

The role of MHD in causing impurity peaking in JET hybrid plasmas

et al. 2016

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“…The effect of the Spitzer ohmic current perturbation has been extensively studied, and it is believed to have provided a significant stabilizing effect in a number of experiments [9,26,45,[48][49][50]. There is experimental evidence of strongly reduced transport in the interior of islands [51][52][53][54], and the associated increase in the temperature perturbation will enhance both effects.…”

Section: Pacs Numbersmentioning

confidence: 99%

Suppression of Tearing Modes by Radio Frequency Current Condensation

Reiman

Fisch

2018

Phys. Rev. Lett.

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Currents driven by rf (radio frequency) waves in the interior of magnetic islands can stabilize deleterious tearing modes in tokamaks. Present analyses of stabilization assume that the local electron acceleration is unaffected by the presence of the island. However, the power deposition and electron acceleration are sensitive to the perturbation of the temperature. The nonlinear feedback on the power deposition in the island increases the temperature perturbation, and can lead to a bifurcation of the solution to the steady-state heat diffusion equation. The combination of the nonlinearly enhanced temperature perturbation with the rf current drive sensitivity to the temperature leads to an rf current condensation effect, which can increase the efficiency of rf current drive stabilization and reduce its sensitivity to radial misalignment of the ray trajectories. The threshold for the effect is in a regime that has been encountered in experiments, and will likely be encountered in ITER. PACS numbers:Introduction: A study of the root causes of disruptions in the JET tokamak found that neoclassical tearing modes (NTMs) were the single most common cause [1,2]. Theoretical calculations in the early 1980's showed the feasibility of using rf current drive to stabilize tearing modes [3,4]. The recognition in the late 1990's that bootstrap currents were driving NTMs in hot, collisionless tokamak plasmas [5][6][7][8], led to a resurgence of theoretical work in this area [9][10][11][12][13], to experimental demonstrations of stabilization [14][15][16][17][18][19][20], and to continuing intensive attention [21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36][37][38][39][40]. A variety of rf waves are used to drive current [41], but, for stabilizing the NTM, the most studied methods are electron cyclotron current drive (ECCD) [42] and lower hybrid current drive (LHCD) [43]. ITER is designed with an NTM ECCD stabilization capability, with continued effort to model and improve this capability [25,29,30,44]. We identify here an rf current condensation effect, previously overlooked, which can significantly facilitate island stabilization.Calculations of rf stabilization of magnetic islands assume, at present, that the local acceleration of electrons is unaffected by the presence of the island. However, the local deposition is sensitive to small changes in the temperature, and these changes can be significantly affected by the presence of an island. The effect on the local deposition becomes significant when the fractional temperature perturbation exceeds about 5% for electron cyclotron waves and 2.5% for lower hybrid waves. Temperature perturbations as high as 20% have been measured in islands in rf stabilization experiments [45].In the conventional picture of rf current drive stabilization of a rotating island, a geometric effect associated with the equilibration of the rf driven current density within the flux surfaces of the island leads to a higher current density near the center of the island than near its peripher...

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“…We note that in order to do quantitative analysis of the actual saturated island widths we should use measured profiles and actual impurity radiation concentrations. There is another important non-linearity associated with the order-of-magnitude reduction in cross-field diffusivity inside an island first reported in reference [18], which will strongly enhance the insulation quality of the islands and will increase the radiation island drive effect.…”

Section: Radiation Termmentioning

confidence: 99%

“…Where we have made use of the experimental observation that the cross field diffusivity inside and island is approximately and order of magnitude lower than in the bulk plasma [18].…”

Section: Radiation Termmentioning

confidence: 99%

Physics of radiation-driven islands near the tokamak density limit

Gates¹,

Delgado‐Aparicio²,

White³

2013

Nucl. Fusion

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In previous work [1], the onset criterion for radiation driven islands [2] in combination with a simple cylindrical model of tokamak current channel behavior was shown to be consistent with the empirical scaling of the tokamak density limit [3]. A number of the unexplained phenomena at the density limit are consistent with this novel physics mechanism. In this work, a more formal theoretical underpinning, consistent with cylindrical tearing mode theory, is developed for the onset criteria of these modes. The appropriate derivation of the radiation-driven addition to the modified Rutherford equation is discussed.Additionally, the ordering of the terms in the MRE is examined in a regime near the density limit. It is hoped that given the apparent success of this simple model in explaining the observed global scalings will lead to a more comprehensive analysis of the possibility that radiation driven islands are the physics mechanism responsible for the density limit.In particular, with modern diagnostic capabilities detailed measurements of current densities, electron densities and impurity concentrations at rational surfaces should be possible, enabling verification of the concepts described above.

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

Cited by 49 publications

References 18 publications

The role of MHD in causing impurity peaking in JET hybrid plasmas

The role of MHD in causing impurity peaking in JET hybrid plasmas

Suppression of Tearing Modes by Radio Frequency Current Condensation

Physics of radiation-driven islands near the tokamak density limit

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