Collisionless microinstabilities in stellarators. II. Numerical simulations

Proll, J. H. E.; Xanthopoulos, P.; Helander, P.

doi:10.1063/1.4846835

Cited by 39 publications

(79 citation statements)

References 34 publications

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“…Instead, the simulations reveal the presence of another density-gradientdriven instability, which, in the words of Proll et al (2013), "evades standard classification". The mode amplitude peaks in magnetic wells, as expected for TEMs, but the instability propagates in the ion diamagentic direction and draws its energy from the ions rather than the electrons.…”

Section: Introductionmentioning

confidence: 92%

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Collisionless microinstabilities in stellarators. Part 4. The ion-driven trapped-electron mode

2017

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Optimised stellarators and other magnetic-confinement devices having the property that the average magnetic curvature is favourable for all particle orbits are called maximum-J devices, and have recently been shown to be immune to trapped-particle instabilities driven by the density gradient. Gyrokinetic simulations reveal, however, that another instability can arise, which is also associated with particle trapping but causes less transport than typical trapped-electron modes. The nature of this instability is clarified here. It is shown to be similar to the "ubiquitous mode" in tokamaks, and is driven by ion free energy but requires trapped electrons to exist.

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Section: Introductionmentioning

confidence: 92%

“…The latter feature, as we will show, can be traced to favourable spatial averaging of the ion drive. This paper is the fourth part in a series on microinstabilities in stellarators ; Proll et al (2013); Plunk et al (2014)), and can be considered as a logical continuation of the first part.…”

Section: Introductionmentioning

confidence: 99%

Collisionless microinstabilities in stellarators. Part 4. The ion-driven trapped-electron mode

2017

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“…Recent measurements made in the Madison symmetric torus (MST) reversed field pinch have shown a similar QC signature whereas gyrokinetic simulations predicted turbulence to be TEM-dominated [29]. These findings on QC-TEM may also help to investigate whether TEM can play a role in stellarators [30] and if they are stabilized in optimized stellarators [31].…”

Section: Fusion Devices and Diagnosticsmentioning

confidence: 97%

Identification of trapped electron modes in frequency fluctuation spectra

Arnichand

Citrin

Hacquin

et al. 2015

Plasma Phys. Control. Fusion

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Abstract.Ion temperature gradient (ITG) and trapped electron modes (TEM) are two important micro-instabilities in the plasma core region of fusion devices (r/a ≤ 0.9). They usually coexist in the same range of spatial scale (around 0.1 < k ⊥ ρ i < 1), which makes their discrimination difficult. To investigate them, one can perform gyrokinetic simulations, transport analysis and phase velocity estimations. In Tore Supra, the identification of trapped electron modes (TEM) is made possible due to measured frequency fluctuation spectra. Indeed, turbulent spectra generally expected to be broad-band can become narrow in case of TEM turbulence, inducing "quasi-coherent" (QC) modes named QC-TEM. Therefore the analysis of frequency fluctuation spectra becomes a possible tool to differentiate TEM from ITG. We have found indications that the TEM can have a QC signature by comparing frequency fluctuation spectra from reflectometry measurements, gyrokinetic simulations and synthetic diagnostic results. Then the scope of the analysis of QC-TEM are discussed and an application is shown, namely transitions between TEM turbulence and MHD fluctuations.

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“…If the electrons are also treated gyrokinetically, recent analytical theory [27][28][29] and numerical results [30] show that densitygradient-driven instabilities are much more stable in many stellarators than in tokamaks. The reason is that these modes are caused by electrons being magnetically trapped in regions of bad curvature.…”

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confidence: 99%

Controlling Turbulence in Present and Future Stellarators

et al. 2014

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Turbulence is widely expected to limit the confinement, and thus the overall performance, of modern neoclassically-optimized stellarators. We employ novel petaflop-scale gyrokinetic simulations to predict the distribution of turbulence fluctuations and the related transport scaling on entire stellarator magnetic surfaces, and reveal striking differences to tokamaks. Using a stochastic global-search optimization method, we derive the first turbulence-optimized stellarator configuration stemming from an existing quasi-omnigenous design.Throughout the history of magnetic fusion, a recurrent theme has been the surprising sensitivity of plasma performance to the details of the magnetic field. For instance, it has long been known that the confinement of alpha particles can be spoiled by small ripples in the magnetic field. More recently, magnetic perturbations have been found to dramatically influence instabilities of the plasma edge [1]. In both stellarators and tokamaks, experiments show that the level of turbulence may be reduced by modifying the magnetic field. As notable examples, the confinement time in the TCV tokamak is doubled by reversing the triangularity of the poloidal cross section of the flux surfaces [2], and in the LHD stellarator the turbulent transport can be reduced significantly by adjusting the coil currents so as to shrink the circumference of the torus by pushing it radially inwards [3].Stellarators typically possess 40-50 degrees of freedom in the shaping of the magnetic field, an order of magnitude more than for tokamaks [4]. This enhanced flexibility can be used to optimize various plasma properties [5], and the latest demonstration of the power of such optimization is expected to be realized in the superconducting stellarator experiment Wendelstein 7-X (W7-X), in Greifswald, Germany [6]. A tantalizing possibility for fusion researchers is to try to exploit any leeway in the magnetic geometry to design configurations with better confinement. In W7-X, this has already been done for the collisional (so-called "neoclassical") transport, but no device built so far is optimized with respect to turbulence.In order to understand how energy transport depends on the magnetic-field geometry, it is helpful to numerically simulate the turbulence in a large portion of the plasma. In tokamaks, thanks to axisymmetry, restricting the computational domain to a "flux tube", a slender volume encompassing a magnetic-field line [7], suffices to calculate the transport at a radial location. In a stellarator, however, different flux tubes on a magnetic surface are not geometrically equivalent, thus it appears necessary to simulate the entire magnetic surface. Much has been learned from the flux-tube approach which, except for inspiring efforts [8], has characterized stellarator turbulence simulations to date [9][10][11][12] . Nevertheless, all these simulations have a major inherent drawback: the transport cannot be reliably determined, as the turbulence strength generally varies between different flux tubes on th...

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Collisionless microinstabilities in stellarators. II. Numerical simulations

Cited by 39 publications

References 34 publications

Collisionless microinstabilities in stellarators. Part 4. The ion-driven trapped-electron mode

Collisionless microinstabilities in stellarators. Part 4. The ion-driven trapped-electron mode

Identification of trapped electron modes in frequency fluctuation spectra

Controlling Turbulence in Present and Future Stellarators

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