Stabilization of a magnetic island by localized heating in a tokamak with stiff temperature profile

Maget, P.; Widmer, Fabien; Février, O.; Garbet, X.; Lütjens, H.

doi:10.1063/1.5021759

Cited by 7 publications

(23 citation statements)

References 19 publications

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“…A more relevant model would instead accommodate a finite level of transport in the turbulent regime. There exist a number of models that do so [41][42][43] ; but, here we will consider the so-called critical gradient transport model 41 .…”

Section: Condensation With Relaxed Stiffness: Road To Double Bifmentioning

confidence: 99%

“…With these large temperature gradients, the perpendicular heat flux may increase and no longer be simply proportional to the gradient. In fact, large temperature gradients are a source of free energy which can trigger various instabilities, which then increase heat transport, leading to what are called stiff temperature profiles [41][42][43] . Thus, it can be expected that the increase in heat transport can limit the temperature gradient near an island periphery, thereby also reducing the temperature.…”

Section: Introductionmentioning

confidence: 99%

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RF current condensation in the presence of turbulent enhanced transport

Reiman

Fisch

2020

Physics of Plasmas

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Sharp temperature gradients in a magnetically confined plasma can lead to turbulent motion of the plasma. This turbulence in turn enhances the transport of heat across magnetic field lines. The enhanced transport impacts the temperature differential that can be sustained in magnetic islands between the island centre and its periphery. It is shown here that, by limiting this temperature differential, this enhanced transport can have a profound influence on the extent to which the RF current condensation effect stabilises the island growth. Interestingly, because the heat transport is no longer simply linear in the temperature gradient, the RF current condensation effect also exhibits entirely new hysteresis phenomena.

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Section: Condensation With Relaxed Stiffness: Road To Double Bifmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

RF current condensation in the presence of turbulent enhanced transport

Reiman

Fisch

2020

Physics of Plasmas

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“…Sensitivity of Current Density to Temperature: Both the ohmic current and the rf driven current are affected by the temperature, which is peaked at the O-line because of the well known effect of the thermal insulation in the island. 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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“…The implication is that the contribution of power density to the island stabilization can be much larger than anticipated with a non-stiff transport model when the RF power is moderate compared to the power flowing through the island position [Maget et al, 2018a]. But on the other hand, even a limited background heating, present before the RF system is used, can damp the control capability by localized heating [Maget et al, 2018b]. The RF heating contribution is therefore very much case dependent, and one of the goals of the present study is to evaluate it in an ITER-like configuration.…”

Section: Introductionmentioning

confidence: 93%

“…In tokamaks, the use of RF waves at the Electron Cyclotron (EC) frequency has proved to be very efficient in this respect because of the narrow deposition width that can be obtained, as demonstrated in a large number of experiments [Maraschek, 2012], and verified in numerical simulations [Yu and Günter, 1998, Popov et al, 2002, Yu et al, 2000, Yu et al, 2004, Février et al, 2017. Although the best results are obtained when the RF beam is toroidally oriented to drive a current, the favorable effect of pure heating has also been demonstrated experimentally [Westerhof et al, 2007] and numerically [Kurita et al, 1994, Lazzari and Westerhof, 2009, Lazzari and Westerhof, 2010, Kim et al, 2016, Maget et al, 2018b.…”

Section: Introductionmentioning

confidence: 99%

Non-linear simulations of neoclassical tearing mode control by externally driven RF current and heating, with application to ITER

et al. 2019

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Neoclassical Tearing Modes (NTM) must be controlled or suppressed to prevent a degradation of the energy confinement in tokamak plasmas. This can be done applying RF-current via Electron Cyclotron Current Drive (ECCD) and-heating (ECRH) at the rational surface where the instability appears. Both the current and heating generated by the RF waves are known to provide a stabilizing effect on the magnetic island. In the present work, we address the issue of Neoclassical Tearing Mode stabilization by Heating and Current Drive in an ITER-like configuration, using a stiff transport model. From a revised Generalized Rutherford Equation, we revisit the criterion on the RF current and power required to stabilize an NTM, showing that the level of plasma background heating (residual heat sources) in ITER significantly lowers the benefit of the RF heating contribution. Nonlinear MagnetoHydroDynamic simulations with the XTOR code, where a stiff transport model as well as RF-power and-current drive are implemented, are performed to compute the NTM stabilization efficiency. The stabilization efficiency due to the RF current contribution is found to be less than theoretically predicted in the case of continuous application, but consistent with theory in the modulated control scheme, suggesting an enhanced destabilization at the X-point. The role of RF heating for continuous application is found to be moderate for the range of power envisaged in ITER, essentially because of the detrimental effect of residual heat sources. This numerical work confirms the capability of the ITER RF system to control the (3, 2) NTM, with a larger confidence for the modulated control scheme.

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Stabilization of a magnetic island by localized heating in a tokamak with stiff temperature profile

Cited by 7 publications

References 19 publications

RF current condensation in the presence of turbulent enhanced transport

RF current condensation in the presence of turbulent enhanced transport

Suppression of Tearing Modes by Radio Frequency Current Condensation

Non-linear simulations of neoclassical tearing mode control by externally driven RF current and heating, with application to ITER

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