Optimizing Stellarators for Turbulent Transport

Mynick, H.E.; Pomphrey, N.; Xanthopoulos, P.

doi:10.1103/physrevlett.105.095004

Cited by 61 publications

(69 citation statements)

References 16 publications

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“…In addition to the issue of the confinement field optimization for decreasing the neoclassical transport, 2 reduction of the anomalous transport, which is caused by the plasma turbulence, is another critical issue for improving confinement properties of helical plasmas. 3,4 Furthermore, the anomalous transport phenomena make significant roles not only in fusion plasmas but also in more general fields, e.g., astrophysics, space physics, and laboratory plasmas, 5 as the turbulence is ubiquitously found in the nature. Nowadays, it has been considered that plasma turbulence is determined by the interaction between the microinstabilities such as ion temperature gradient (ITG) modes and zonal flows.…”

Section: Introductionmentioning

confidence: 99%

Gyrokinetic turbulent transport simulation of a high ion temperature plasma in large helical device experiment

et al. 2012

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Ion temperature gradient turbulent transport in the large helical device (LHD) is investigated by means of gyrokinetic simulations in comparison with the experimental density fluctuation measurements of ion-scale turbulence. The local gyrokinetic Vlasov simulations are carried out incorporating full geometrical effects of the LHD configuration, and reproduce the turbulent transport levels comparable to the experimental results. Reasonable agreements are also found in the poloidal wavenumber spectra of the density fluctuations obtained from the simulation and the experiment. Numerical analysis of the spectra of the turbulent potential fluctuations on the two-dimensional wavenumber space perpendicular to the magnetic field clarifies the spectral transfer into a high radial wavenumber region which correlates with the regulation of the turbulent transport due to the zonal flows. The resultant transport levels at different flux surfaces are expressed in terms of a simple linear relation between the transport coefficient and the ratio of the squared turbulent potential fluctuation to the averaged zonal flow amplitude. V C 2012 American Institute of Physics. [http://dx

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Section: Introductionmentioning

confidence: 99%

Gyrokinetic turbulent transport simulation of a high ion temperature plasma in large helical device experiment

et al. 2012

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“…However, we demonstrated that configurations with very different effective ripple can have similar turbulent heat levels. Separate work [7], complementing this one, addressed the ITG instability, by minimizing regions of bad curvature. There, configurations with drastically reduced turbulent levels were generated, without affecting nc transport.…”

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confidence: 99%

“…In stellarators, there is an intermediate stage of slow (compared with the GAM) damped ZF oscillations, and the RH residual level can be much lower than in tokamaks [3,5]. The details of this behavior depend on the magnetic configuration in question.Recent work [7] sought to reduce turbulence in stellarators by directly targeting the ITG instability. Complementing that study, here we investigate the role of ZF dynamics in regulating turbulent transport.…”

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confidence: 99%

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Zonal Flow Dynamics and Control of Turbulent Transport in Stellarators

et al. 2011

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The relation between magnetic geometry and the level of ion-temperature-gradient (ITG) driven turbulence in stellarators is explored through gyrokinetic theory and direct linear and nonlinear simulations. It is found that the ITG radial heat flux is sensitive to details of the magnetic configuration that can be understood in terms of the linear behavior of zonal flows. The results throw light on the question of how the optimization of neoclassical confinement is related to the reduction of turbulence.Understanding the turbulence present in tokamaks and stellarators is one of the most important challenges in plasma physics. A particularly interesting question in this context is how the magnetic geometry affects the nature and amplitude of the turbulence. In plasmas where the ion-temperature-gradient (ITG) instability is significant, so-called zonal flows (ZFs) have a favorable effect on the confinement [1]. However, the complexity introduced by nonaxisymmetry had rendered, only until very recently, the study of ZFs in stellarator configurations intractable. The dynamical character of the ZF response in a stellarator was first predicted analytically in Refs. [2,3] and was later confirmed by linear gyrokinetic simulations [4,5]. In a tokamak, the linear response to an imposed ZF perturbation consists of geodesic acoustic mode (GAM) oscillations followed by a steady state so-called Rosenbluth-Hinton (RH) residual level [6]. In stellarators, there is an intermediate stage of slow (compared with the GAM) damped ZF oscillations, and the RH residual level can be much lower than in tokamaks [3,5]. The details of this behavior depend on the magnetic configuration in question.Recent work [7] sought to reduce turbulence in stellarators by directly targeting the ITG instability. Complementing that study, here we investigate the role of ZF dynamics in regulating turbulent transport. As will become apparent, the situation is different from that in tokamaks, since the residual level is not always reached before turbulence saturates (see Figs. 4,6), and in these cases, the RH level alone cannot account for differences in the heat flux (as sometimes claimed in the tokamak literature, see e.g., Ref.[8]). Furthermore, we identify the geodesic curvature as an important ingredient affecting the ZF oscillations, viz., minimizing the geodesic curvature proves to have a significant favorable effect on the confinement. Finally, we revisit the relationship between neoclassical (nc) optimization and turbulence reduction. Although these two concepts turn out to be correlated (see also Refs. [4,9]), here we show that turbulence must be treated in addition to nc optimization. It should be remembered that turbulent transport is important in optimized stellarators, especially at low temperatures [10].As a preparatory step, to gain confidence in the numerical treatment, we present (the first published) linear and nonlinear benchmarks in stellarator geometry. Most of the presented simulations are performed with the GENE code (see, e.g., Refs. [11...

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“…These functions are embedded into the χ 2 function that STELLOPT seeks to minimize. Since direct simulations require a prohibitively large amount of time for an iterative scheme, target functions are constructed using simpler "proxies", involving either geometric quantities (such as the curvature of the magnetic field or the distance between adjacent flux surfaces) or more sophisticated expressions based on mixing-length estimates for the heat flux [25,26]. This methodology has been applied so far to QUASAR [27] -the results from that paper are shown in Figure 5 -as well as W7-X [28] suggesting that both these configurations can be modified to reduce turbulent transport, also without compromising the neoclassical optimization.…”

Section: Turbulent Transport Studiesmentioning

confidence: 99%

Recent advances in stellarator optimization

et al. 2017

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Computational optimization has revolutionized the field of stellarator design. To date, optimizations have focused primarily on optimization of neoclassical confinement and ideal MHD stability, although limited optimization of other parameters has also been performed.The purpose of this paper is to outline a select set of new concepts for stellarator optimization that, when taken as a group, present a significant step forward in the stellarator concept.One of the criticisms that has been leveled at existing methods of design is the complexity of the resultant field coils. Recently, a new coil optimization code -COILOPT++, which uses a spline instead of a Fourier representation of the coils, -was written and included in the STELLOPT suite of codes. The advantage of this method is that it allows the addition of real space constraints on the locations of the coils. The code has been tested by generating coil designs for optimized quasi-axisymmetric stellarator plasma configurations of different aspect ratios. As an initial exercise, a constraint that the windings be vertical 1 was placed on large major radius half of the non-planar coils. Further constraints were also imposed that guaranteed that sector blanket modules could be removed from between the coils, enabling a sector maintenance scheme. Results of this exercise will be presented.New ideas on methods for the optimization of turbulent transport have garnered much attention since these methods have led to design concepts that are calculated to have reduced turbulent heat loss. We have explored possibilities for generating an experimental database to test whether the reduction in transport that is predicted is consistent with experimental observations. To this end, a series of equilibria that can be made in the now latent QUASAR experiment have been identified that will test the predicted transport scalings. Fast particle confinement studies aimed at developing a generalized optimization algorithm are also discussed. A new algorithm developed for the design of the scraper element on W7-X is presented along with ideas for automating the optimization approach.

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Optimizing Stellarators for Turbulent Transport

Cited by 61 publications

References 16 publications

Gyrokinetic turbulent transport simulation of a high ion temperature plasma in large helical device experiment

Gyrokinetic turbulent transport simulation of a high ion temperature plasma in large helical device experiment

Zonal Flow Dynamics and Control of Turbulent Transport in Stellarators

Recent advances in stellarator optimization

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