Turbulence mitigation in maximum-J stellarators with electron-density gradient

Proll, J. H. E.; Plunk, G. G.; Faber, B. J.; Görler, T.; Helander, P.; McKinney, I. J.; Pueschel, M. J.; Smith, H. M.; Xanthopoulos, P.

doi:10.1017/s002237782200006x

Cited by 20 publications

(25 citation statements)

References 45 publications

(62 reference statements)

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“…(2012) and Proll et al. (2022), which consider a plasma instability which moves a particle radially by . The energy required to do so, , can be found through the conservation of both and the cumulative energy of the particle and instability () Because (see (1.3)), we see that will be negative when .…”

Section: Introductionmentioning

confidence: 80%

Constructing precisely quasi-isodynamic magnetic fields

Goodman,

Camacho Mata,

Henneberg

et al. 2023

J. Plasma Phys.

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We present a novel method for numerically finding quasi-isodynamic stellarator magnetic fields with excellent fast-particle confinement and extremely small neoclassical transport. The method works particularly well in configurations with only one field period. We examine the properties of these newfound quasi-isodynamic configurations, including their transport coefficients, particle confinement and available energy for trapped-electron-instability-driven turbulence, as well as the degree to which they change when a finite pressure profile is added. We finally discuss the differences between the magnetic axes of the optimized solutions and their respective initial conditions, and conclude with the prospects for future quasi-isodynamic optimization.

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Section: Introductionmentioning

confidence: 80%

Constructing precisely quasi-isodynamic magnetic fields

Goodman,

Camacho Mata,

Henneberg

et al. 2023

J. Plasma Phys.

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“…As usual, the temperature gradient scale length is measured relative to the minor radius, a/L T = −(a/T )dT /dr. To study the most unstable ITG mode conditions, we neglect certain stabilizing factors such as the density gradient [22,23] and plasma beta (electromagnetic effects) [24].…”

mentioning

confidence: 99%

Coarse-grained gyrokinetics for the critical ion temperature gradient in stellarators

Roberg-Clark

Plunk

Xanthopoulos

2022

Phys. Rev. Research

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We present a modified gyrokinetic theory to predict the critical gradient that determines the linear onset of the ion temperature gradient (ITG) mode in stellarator plasmas. A coarse-graining technique is applied to the drift curvature, entering the standard gyrokinetic equations, around local minima. Thanks to its simplicity, this novel formalism yields an estimate for the critical gradient with a computational cost low enough for application to stellarator optimization. On comparing against a gyrokinetic solver, our results show good agreement for an assortment of stellarator designs. Insight gained here into the physics of the onset of the ITG driven instability enables us to devise a compact configuration, similar to the Wendelstein 7-X device, but with almost twice the ITG linear critical gradient, an improved nonlinear critical gradient, and reduced ITG mode transport above the nonlinear critical gradient.

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“…This, however, grants none of the properties beneficial to TEM stability possessed by locally maximum-J configurations. Interestingly, it has been found that HSX performs well in nonlinear simulations of turbulent transport, despite the less favourable stability properties (McKinney et al 2019;Proll et al 2022).…”

Section: Stellarator Configurations Consideredmentioning

confidence: 99%

“…To investigate how the linear universal modes identified at in the high-mirror configuration of W7-X in § 4 translate to nonlinear simulations, we examine the nonlinear W7-X high-mirror simulations recently published by Proll et al. (2022). These simulations were carried out at a density gradient of , which facilitates comparison with the linear results in § 4 while also being in the experimentally relevant density-gradient range.…”

Section: Evidence Of Universal Modes In Nonlinear Simulations In W7-xmentioning

confidence: 99%

The universal instability in optimised stellarators

Costello,

Proll,

Plunk

et al. 2023

J. Plasma Phys.

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In tokamaks and neoclassically optimised stellarators, like Wendelstein 7-X (W7-X) and the Helically Symmetric Experiment, turbulent transport is expected to be the dominant transport mechanism. Among the electrostatic instabilities that drive turbulence, the trapped-electron mode (TEM) has been shown both analytically and in simulations to be absent over large ranges of parameter space in quasi-isodynamic stellarator configurations with the maximum- $J$ property. It has been proposed that the reduction of the linear TEM growth rate in such configurations may lead to the passing-electron-driven universal instability, which is often subdominant to the TEM, becoming the fastest-growing instability over some range of parameter space. Here, we show through gyrokinetic simulations using the Gene code, that the universal instability is dominant in a variety of stellarator geometries over a range of parameter space typically occupied by the TEM, but most consequentially in devices which possess beneficial TEM stability properties like W7-X, which locally satisfies the maximum- $J$ property for deeply trapped particles in the regions of worst curvature. We find that the universal instability exists at long perpendicular wavelengths and, as a result, dominates the potential fluctuation amplitude in nonlinear simulations. In W7-X, universal modes are found to differ in parallel mode structure from trapped-particle modes, which may impact turbulence localisation in experiments.

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Turbulence mitigation in maximum-J stellarators with electron-density gradient

Cited by 20 publications

References 45 publications

Constructing precisely quasi-isodynamic magnetic fields

Constructing precisely quasi-isodynamic magnetic fields

Coarse-grained gyrokinetics for the critical ion temperature gradient in stellarators

The universal instability in optimised stellarators

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