Advances in stellarator gyrokinetics

Helander, P.; Bird, T.; Jenko, F.; Kleiber, R.; Plunk, G. G.; Proll, J. H. E.; Riemann, J.; Xanthopoulos, P.

doi:10.1088/0029-5515/55/5/053030

Cited by 53 publications

(79 citation statements)

References 22 publications

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“…On the other hand, if the electrons are in a ground state, they act to stabilise any ion-driven instability and one would expect that the available energy is relatively small. Gyrokinetic simulations of turbulence in approximately maximum-J stellarator configurations do indeed suggest that the transport under these conditions is up to an order of magnitude smaller than in a typical tokamak (Helander et al 2015).…”

Section: P Helandermentioning

confidence: 86%

Available energy of magnetically confined plasmas

Helander

2020

J. Plasma Phys.

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The concept of the available energy of a collisionless plasma is discussed in the context of magnetic confinement. The available energy quantifies how much of the plasma energy can be converted into fluctuations (including nonlinear ones) and is thus a measure of plasma stability, which can be used to derive linear and nonlinear stability criteria without solving an eigenvalue problem. In a magnetically confined plasma, the available energy is determined by the density and temperature profiles as well as the magnetic geometry. It also depends on what constraints limit the possible forms of plasma motion, such as the conservation of adiabatic invariants and the requirement that the transport be ambipolar. A general method based on Lagrange multipliers is devised to incorporate such constraints in the calculation of the available energy, and several particular cases are discussed for which it can be calculated explicitly. In particular, it is shown that it is impossible to confine a plasma in a Maxwellian ground state relative to perturbations with frequencies exceeding the ion bounce frequency.

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Section: P Helandermentioning

confidence: 86%

Available energy of magnetically confined plasmas

Helander

2020

J. Plasma Phys.

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“…Moreover, in quasi-isodynamic configurations with the socalled 'maximum-J' property (the parallel adiabatic invariant J attains its maximum on the magnetic field axis, while monotonically decreasing towards the edge), it can be analytically shown that collisionless TEMs are stable over a large parameter range [9]. An electron displaced by the density gradient would need to gain energy in order to conserve J (for ∂J/∂r < 0) thus tapping energy from the instability.…”

Section: Turbulence In Advanced Stellaratorsmentioning

confidence: 99%

First steps towards modeling of ion-driven turbulence in Wendelstein 7-X

et al. 2017

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Due to the reduction of neoclassical transport in optimised stellarator configurations, it is expected that turbulence will significantly contribute to the heat and particle transport-at least in the edge region of the plasma-and may thus pose a limit to the achievable confinement and performance of such machines. In order to predict the confinement and develop plasma scenarios for W7-X and beyond for Helias reactors in an efficient way, it is important to develop models which can describe the characteristics of turbulent transport using only a fraction of the computational power which is normally required for gyrokinetic simulations on petaflop scale. In this work we concentrate on the ion-temperature-gradient mode which has so far been well investigated and is considered to strongly contribute to transport. Non-linear full-flux surface gyrokinetic simulation results are compared for W7-X and its predecessor W7-AS. Although the fluctuations are more localised in W7-X, it is intriguing that the normalised heat flux scaling is nearly identical. On this basis experimental results from W7-AS are reviewed which suggest that the critical gradient behaviour of the ITG mode indeed dominates the edge transport. As the experimental and theoretical obtained characteristics are similar a basic ITG critical gradient model is proposed and verified in 1-D transport simulations against the W7-AS experimental results showing good agreement. The model is therefore further applied to W7-X showing, that even with ITG transport the a global confinement enhancement with respect to the ISS04 scaling of τ 1D E /τ ISS04 E = 1.7 is retained. Extrapolation to a Helias reactor scenario shows similar promising results with an confinement improvement factor of 1.35. However, the overall transport is sensitive to the 'stiffness' used in ITG model. In future works it is envisaged to enhance the model by taking into account the density gradient and radial electric field which was so far neglected, but are both considered to reduce the turbulence level of the ITG.

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“…A very interesting question is how neoclassical optimization of ε eff affects turbulent transport. 3-D calculations of ion-temperature-gradient-driven turbulence suggest a critical gradient similar to tokamaks, but with much reduced temperature profile stiffness [20], [21]. An interesting question, which the W7-X results will have to answer, is whether an H-mode transport barrier [22] at the plasma edge will be required to achieve sufficiently good confinement.…”

Section: Development Of Integrated Steady-state Plasmas (Op2)mentioning

confidence: 99%

Wendelstein 7-X Program—Demonstration of a Stellarator Option for Fusion Energy

Wolf

Beidler

Dinklage

et al. 2016

IEEE Trans. Plasma Sci.

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The superconducting stellarator Wendelstein 7-X is currently being commissioned. First plasmas are expected for the second half of 2015. W7-X is designed to overcome the main drawbacks of the stellarator concept and simultaneously demonstrate its intrinsic advantages relative to the tokamaki.e., steady-state operation without the requirement of current drive or stability control. An elaborate optimization procedure was used to avoid excessive neoclassical transport losses at high plasma temperature, while simultaneously achieving satisfactory equilibrium and stability properties at high β in combination with a viable divertor concept. In addition, fastion confinement must be consistent with the requirements of alpha-heating in a power plant. Plasma operation of Wendelstein 7-X follows a staged approach following the successive completion of the in-vessel components. The main objective of Wendelstein 7-X is the demonstration of steady-state plasma at fusion relevant plasma parameters. Wendelstein 7-X will address major questions for the extrapolation of the concept to a power plant. These include divertor operation at high densities, plasma fueling at high central temperatures, avoiding impurity accumulation, and an assessment of the effect of neoclassical optimization on turbulent transport and fast-ion confinement. A power plant concept based on an extrapolation from Wendelstein 7-X, the helical advanced stellarator, has been developed.Index Terms-Fusion power plant, steady-state magnetic confinement, stellarator.

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Advances in stellarator gyrokinetics

Cited by 53 publications

References 22 publications

Available energy of magnetically confined plasmas

Available energy of magnetically confined plasmas

First steps towards modeling of ion-driven turbulence in Wendelstein 7-X

Wendelstein 7-X Program—Demonstration of a Stellarator Option for Fusion Energy

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