Resilience of Quasi-Isodynamic Stellarators against Trapped-Particle Instabilities

Proll, J. H. E.; Helander, P.; Connor, J. W.; Plunk, G. G.

doi:10.1103/physrevlett.108.245002

Cited by 73 publications

(123 citation statements)

References 16 publications

Supporting

Mentioning

112

Contrasting

Order By: Relevance

“…Since it still holds for the electrons, these act stabilising and any instability must draw its energy from the ions. This result has earlier been found through linear stability analysis by Proll et al (2012), but is here found to be true even for large perturbations.…”

Section: Discussionsupporting

confidence: 86%

Available energy and ground states of collisionless plasmas

Helander

2017

J. Plasma Phys.

View full text Add to dashboard Cite

The energy budget of a collisionless plasma subject to electrostatic fluctuations is considered, and the excess of thermal energy over the minimum accessible to it under various constraints that limit the possible forms of plasma motion is calculated. This excess measures how much thermal energy is 'available' for conversion into plasma instabilities, and therefore constitutes a nonlinear measure of plasma stability. A distribution function with zero available energy defines a 'ground state' in the sense that its energy cannot decrease by any linear or nonlinear plasma motion. In a Vlasov plasma with small density and temperature fluctuations, the available energy is proportional to the mean square of these quantities, and exceeds the corresponding energy in ideal or resistive magnetohydrodynamics. If the first or second adiabatic invariant is conserved, ground states generally have inhomogeneous density and temperature. Magnetically confined plasmas are usually not in any ground state, but certain types of stellarator plasmas are so with respect to fluctuations that conserve both these adiabatic invariants, making the plasma linearly and nonlinearly stable to such fluctuations. Similar stability properties can also be enjoyed by plasmas confined by a dipole magnetic field.

show abstract

Section: Discussionsupporting

confidence: 86%

Available energy and ground states of collisionless plasmas

Helander

2017

J. Plasma Phys.

View full text Add to dashboard Cite

show abstract

“…As we have just seen, the simplest form of the TEM is not present in such configurations, and in Ref. [61] it is shown from basic energy considerations that any particle species with k v T a ≫ ω and 0 < η a < 2/3 exerts a stabilising influence on arbitrary electrostatic, collisionless instabilities. Physically, the point is that, because J is an adiabatic invariant, if an instability with ωτ b ≪ 1 results in the radial movement ∆ψ of a particle, then…”

Section: Analytical Considerationsmentioning

confidence: 99%

Stellarator and tokamak plasmas: a comparison

Helander

Beidler

Bird

et al. 2012

Plasma Phys. Control. Fusion

135

181

View full text Add to dashboard Cite

An overview is given of physics differences between stellarators and tokamaks, including magnetohydrodynamic equilibrium, stability, fast-ion physics, plasma rotation, neoclassical and turbulent transport, and edge physics. Regarding microinstabilities, it is shown that the ordinary, collisionless trapped-electron mode is stable in large parts of parameter space in stellarators that have been designed so that the parallel adiabatic invariant decreases with radius. Also, the first global, electromagnetic, gyrokinetic stability calculations done for Wendelstein 7-X suggest that kinetic ballooning modes are more stable than in a typical tokamak.

show abstract

“…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.…”

mentioning

confidence: 99%

Controlling Turbulence in Present and Future Stellarators

et al. 2014

Self Cite

View full text Add to dashboard Cite

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...

show abstract

Resilience of Quasi-Isodynamic Stellarators against Trapped-Particle Instabilities

Cited by 73 publications

References 16 publications

Available energy and ground states of collisionless plasmas

Available energy and ground states of collisionless plasmas

Stellarator and tokamak plasmas: a comparison

Controlling Turbulence in Present and Future Stellarators

Contact Info

Product

Resources

About