Microturbulence in DIII-D tokamak pedestal. I. Electrostatic instabilities

Fulton, Daniel; Zhang, Lin; Holod, I.; Xiao, Yao

doi:10.1063/1.4871387

Cited by 42 publications

(69 citation statements)

References 39 publications

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“…The unconventional eigen modes with θ p = 0 have been recently discovered in the strong gradient parameter regime. Typically, |θ p | or < π/2 have been shown to exist [7,8,13,14]. In this work, we find the most general unconventional eigen mode structures from first principle gyrokinetic simulations.…”

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confidence: 73%

Unconventional ballooning structures for toroidal drift waves

Xie

Xiao

2015

Physics of Plasmas

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With strong gradients in the pedestal of high confinement mode (H-mode) fusion plasmas, gyrokinetic simulations are carried out for the trapped electron and ion temperature gradient modes. A broad class of unconventional mode structures is found to localize at arbitrary poloidal positions or with multiple peaks. It is found that these unconventional ballooning structures are associated with different eigen states for the most unstable mode. At weak gradient (low confinement mode or L-mode), the most unstable mode is usually in the ground eigen state, which corresponds to a conventional ballooning mode structure peaking in the outboard mid-plane of tokamaks. However, at strong gradient (H-mode), the most unstable mode is usually not the ground eigen state and the ballooning mode structure becomes unconventional. This result implies that the pedestal of H-mode could have better confinement than L-mode. Although numerous theoretical models have been suggested [1], a yet unexplained phenomenon in tokamak fusion plasmas, is the transition of low (L) to high (H) confinement states, where H-mode[2] has significant better confinement property than that of the L-mode. Understanding of the H-mode physics is not only important to make controlled fusion more feasible, but also that the existence of and transitions among multi-equilibrium states are important fields of nonlinear physics in laboratory and the Universe. Drift wave turbulence is one of the major causes that leads to the anomalous transport widely observed in fusion and space plasmas [3,4]. In order to control the turbulent transport, it is crucial to understand the underlying transport mechanism, which may vary for different types of instability that drive the turbulence. The correlation time and length are found to be closely related to the mode structure of the turbulence [5]. Therefore, the mode structure of the turbulence has a significant effect on the transport level [6].In this Letter, we show that the linear properties of two major types of electrostatic micro-instabilities [3], namely the trapped electron mode (TEM) and ion temperature gradient (ITG) mode, are completely different in the H-mode (strong gradient) and L-mode (weak gradient) stages. With the conventional weak gradient, the mode structures for drift wave instabilities such as the ITG and TEM are of ballooning type, peaking at the outboard mid-plane of the tokamak (c.f., [7,8]). This type of solution has been intensively studied using the ballooningrepresentation[9, 10] by reducing one two-dimensional (2D) real space eigen mode equation for the drift waves to two one-dimensional (1D) ballooning space eigen mode equations. For the 2D case we solve the eigen equation in the poloidal plane. For the 1D case we solve the eigen equation in the parallel direction. The most unstable solutions in the ballooning space found in the past have usually the ballooning-angle parameter ϑ k = 0 [11], which corresponds to the solution localized at the outside midplane, i.e., θ p = 0 in our notation, where θ p...

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confidence: 73%

Unconventional ballooning structures for toroidal drift waves

Xie

Xiao

2015

Physics of Plasmas

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“…Experimentally, fluctuations have been diagnosed that are consistent with MTM [22,23,25], KBM [22,[24][25][26], and trapped electron modes (TEM) [22], while linear gyrokinetic modeling has identified MTM [19,[27][28][29][30], TEM [30,31], ETG [29,30,32,33], KBM [19,26,27,29,30,34,35], and unidentified drift waves [36]. Due to the large number of possible instabilities, and the complexity of the nonlinear turbulent state (that reflects the underlying instabilities in not so obvious ways), no clear picture has emerged regarding the relative importance of these modes for pedestal transport.…”

Section: Pacs Numbersmentioning

confidence: 99%

Microtearing turbulence limiting the JET-ILW pedestal

et al. 2016

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Gyrokinetic simulations identify microtearing modes (MTM) to be the dominant microinstabilities in the JET-ILW (ITER like wall) pedestal. Nonlinear simulations show that MTM-driven turbulence produces the bulk of the transport in the steep gradient region, demonstrating that MTM may be the principal mechanism limiting JET-ILW pedestal evolution. The combination of MTM, electron temperature gradient (ETG), and neoclassical transport reproduces experimental power balance across most of the pedestal. Kinetic ballooning modes are significant only in the local limit and only at low β, far below the experimental operating point. PACS numbers:The tokamak H-mode [1] is defined by a narrow insulating region-the pedestal-at the plasma edge, where turbulence is suppressed and sharp pressure gradients develop. The pedestal is at the center of the most pressing issues facing fusion energy. ITER, for example, must reach a sufficient temperature at the pedestal's inner boundary in order to achieve its fusion power targets [2]. This work reports the results of, perhaps, the very first, first-principles simulations of the H-mode pedestal dynamics that reproduce experimentally observed transport levels. In addition to providing unprecedented insight into the dynamics of the existing H-mode pedestals, such simulations are likely to advance our capabilities towards predictive modeling of future burning plasma devices.This study targets the JET-ILW (ITER-like wall) pedestal [4][5][6], which approaches ITER conditions in two important ways: 1) as the largest tokamak in operation, it most-closely approximates plasma parameters that are dependent on machine size (like ρ * , the ratio of the gyroradius to minor radius), 2) to approximate ITER conditions even better, JET has recently installed an ITERlike wall (ILW) (composed of a tungsten divertor and beryllium chamber). This modification decreases the global performance of discharges by 20-30%, attributable largely to changes in pedestal structure. In addition to the performance loss, certain observed key properties of the ILW pedestal are inconsistent with predictions of the leading pedestal model (EPED [7,8]).In this work, we elucidate possible mechanisms limiting profile evolution in the JET-ILW discharges. We demonstrate, through simulations using the gyrokinetic code Gene [9,10], that the microtearing mode (MTM) [11][12][13][14] is the dominant instability in the pedestal. Interestingly, the simulations do not find the kinetic ballooning mode (KBM), a basic component of the EPED model, except locally in a narrow region near the separatrix. Most importantly, we determine via nonlinear gyrokinetic simulations that a combination of MTM and electron temperature gradient (ETG) [9,[15][16][17][18] driven turbulence plus the neoclassical flux, produces transport levels closely matching experimental power balance across most of the pedestal, demonstrating the capacity of these mechanisms to limit JET-ILW pedestal evolution.The JET-ILW Pedestal-JET pulse 82585 (described in Ref. [4]) is pa...

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“…[28][29][30] Gyrokinetic codes such as GTC or GYRO were also successfully applied to simulate microinstabilities in the edge of tokamak plasmas. 7,31,32 The validity of the gyrokinetic ordering (q=L < 1) at the plasma edge, where the profile scale lengths (L) may be close to the ion gyroradius (q i ), was further assessed for the values used in this numerical study. Table I shows the ratios (q i =L ptot ) calculated from the experimental results, with q i the ion gyroradius and L ptot the total pressure gradient scale length.…”

Section: Introductionmentioning

confidence: 99%

Linear gyrokinetic simulations of microinstabilities within the pedestal region of H-mode NSTX discharges in a highly shaped geometry

et al. 2016

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This version is available at https://strathprints.strath.ac.uk/57449/ Strathprints is designed to allow users to access the research output of the University of Strathclyde. Unless otherwise explicitly stated on the manuscript, Copyright © and Moral Rights for the papers on this site are retained by the individual authors and/or other copyright owners. Please check the manuscript for details of any other licences that may have been applied. You may not engage in further distribution of the material for any profitmaking activities or any commercial gain. You may freely distribute both the url (https://strathprints.strath.ac.uk/) and the content of this paper for research or private study, educational, or not-for-profit purposes without prior permission or charge.Any correspondence concerning this service should be sent to the Strathprints administrator: strathprints@strath.ac.ukThe Strathprints institutional repository (https://strathprints.strath.ac.uk) is a digital archive of University of Strathclyde research outputs. It has been developed to disseminate open access research outputs, expose data about those outputs, and enable the management and persistent access to Strathclyde's intellectual output.Linear gyrokinetic simulations of microinstabilities within the pedestal region of H-mode NSTX discharges in a highly shaped geometry Linear (local) gyrokinetic predictions of edge microinstabilities in highly shaped, lithiated and non-lithiated NSTX discharges are reported using the gyrokinetic code GS2. Microtearing modes dominate the non-lithiated pedestal top. The stabilization of these modes at the lithiated pedestal top enables the electron temperature pedestal to extend further inwards, as observed experimentally. Kinetic ballooning modes are found to be unstable mainly at the mid-pedestal of both types of discharges, with unstable trapped electron modes nearer the separatrix region. At electron wavelengths, electron temperature gradient (ETG) modes are found to be unstable from mid-pedestal outwards for g e; exp $ 2:2, with higher growth rates for the lithiated discharge. Near the separatrix, the critical temperature gradient for driving ETG modes is reduced in the presence of lithium, reflecting the reduction of the lithiated density gradients observed experimentally. A preliminary linear study in the edge of non-lithiated discharges shows that the equilibrium shaping alters the electrostatic modes stability, which was found more unstable at high plasma shaping. Published by AIP Publishing.

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Microturbulence in DIII-D tokamak pedestal. I. Electrostatic instabilities

Cited by 42 publications

References 39 publications

Unconventional ballooning structures for toroidal drift waves

Unconventional ballooning structures for toroidal drift waves

Microtearing turbulence limiting the JET-ILW pedestal

Linear gyrokinetic simulations of microinstabilities within the pedestal region of H-mode NSTX discharges in a highly shaped geometry

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