Aspect ratio effects on neoclassical tearing modes from comparison between DIII-D and National Spherical Torus Experiment

Haye, R.J. La; Buttery, R. J.; Gerhardt, S.; Sabbagh, S.A.; Brennan, D. P.

doi:10.1063/1.4729658

Cited by 22 publications

(32 citation statements)

References 43 publications

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“…This kind of behaviour at w ∼ ρ ϑi is the direct result of the inclusion of the drift islands in our model and is in agreement (qualitative and quantitative) with experimentally observed self-healing of small magnetic islands below the threshold (e.g. [33]). A straight line fit shown in the inset of figure 8.…”

Section: Neoclassical Drive For Ntmssupporting

confidence: 88%

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Drift kinetic theory of neoclassical tearing modes in a low collisionality tokamak plasma: magnetic island threshold physics

Dudkovskaia

Connor

Dickinson

et al. 2021

Plasma Phys. Control. Fusion

101

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A new drift kinetic theory for the plasma response to the neoclassical tearing mode (NTM) magnetic perturbation is presented. Small magnetic islands of width, w ≪ a (a is the tokamak minor radius) are assumed, retaining the limit w ∼ ρ bi (ρ bi is the ion banana orbit width) to include finite orbit width effects. When collisions are small, the ions/electrons follow streamlines in phase space; for passing particles, these lie in surfaces that reproduce the magnetic island structure but have a radial shift by an amount, proportional to ρ ϑ i / e , where ρ ϑ i / e is the ion/electron poloidal Larmor radius. This shift is associated with the curvature and ∇B drifts and is found to be in opposite directions for V ∥ ≶ 0 , where V ∥ is the component of velocity parallel to the magnetic field. The particle distribution function is then found to be flattened across these shifted or drift islands rather than the magnetic island. This results in the pressure gradient being sustained across the magnetic island for w ∼ ρ ϑ i and hence reduces the neoclassical drive for NTMs when w is small. This provides a physics basis for the NTM threshold, which is quantified. In Imada et al (2019 Nucl. Fusion 59 046016, and references therein), a 4D drift kinetic non-linear code has been applied to describe these modes. In the present paper, the drift island formalism is employed. Valid at low collisionality, it allows a dimensionality reduction to a 3D problem, simplifying the numerical task and efficiently resolving the collisional boundary layer across the trapped-passing boundary. An improved model is adopted for the magnetic drift frequency. This decreases the NTM threshold, compared to the results shown in Imada et al (2019 Nucl. Fusion 59 046016, and references therein), making it in quantitative agreement with experimental observations, with w c = 0.45 ρ ϑ i , where w c is the threshold magnetic island half-width, or 2.85ρ bi for the full threshold island width, predicted for our equilibrium.

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Section: Neoclassical Drive For Ntmssupporting

confidence: 88%

“…However, in [32] it has been shown that the marginal island width below which the NTM is removed, i.e. dw/dt < 0, is about 2ρ bi in both ECCD and beta rampdown discharges and is about 3ρ bi in [33]. This is exactly the region where the existing theory breaks down.…”

Section: Introductionmentioning

confidence: 94%

Drift kinetic theory of neoclassical tearing modes in a low collisionality tokamak plasma: magnetic island threshold physics

Dudkovskaia

Connor

Dickinson

et al. 2021

Plasma Phys. Control. Fusion

101

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“…Recently, a comparison experiment between NSTX and DIII-D indeed showed the magnetic curvature stabilizing term to be important for NSTX parameters. 112 Overall, NTMs in ST plasmas therefore appear to be more stable than in conventional tokamaks. Another important feature is the projected high f BS ST reactor q profile [shown, for example, in Fig.…”

Section: Neo-classical Tearing Modesmentioning

confidence: 99%

Recent progress on spherical torus research

Ono

Kaita

2015

Physics of Plasmas

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The spherical torus or spherical tokamak (ST) is a member of the tokamak family with its aspect ratio (A = R0/a) reduced to A ∼ 1.5, well below the normal tokamak operating range of A ≥ 2.5. As the aspect ratio is reduced, the ideal tokamak beta β (radio of plasma to magnetic pressure) stability limit increases rapidly, approximately as β ∼ 1/A. The plasma current it can sustain for a given edge safety factor q-95 also increases rapidly. Because of the above, as well as the natural elongation κ, which makes its plasma shape appear spherical, the ST configuration can yield exceptionally high tokamak performance in a compact geometry. Due to its compactness and high performance, the ST configuration has various near term applications, including a compact fusion neutron source with low tritium consumption, in addition to its longer term goal of an attractive fusion energy power source. Since the start of the two mega-ampere class ST facilities in 2000, the National Spherical Torus Experiment in the United States and Mega Ampere Spherical Tokamak in UK, active ST research has been conducted worldwide. More than 16 ST research facilities operating during this period have achieved remarkable advances in all fusion science areas, involving fundamental fusion energy science as well as innovation. These results suggest exciting future prospects for ST research both near term and longer term. The present paper reviews the scientific progress made by the worldwide ST research community during this new mega-ampere-ST era.

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“…[73,74] studied a class of m/n=2/1 magnetic islands, and showed that they had many characteristics of 2/1 neoclassical tearing mode. Ref.…”

Section: : Onset Of Core N=1 Activitymentioning

confidence: 99%

Disruptions, Disruptivity, and Safer Operating Windows in the High-β Spherical Torus NSTX

Gerhardt¹,

Diallo²,

Gates³

et al. 2012

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Aspect ratio effects on neoclassical tearing modes from comparison between DIII-D and National Spherical Torus Experiment

Cited by 22 publications

References 43 publications

Drift kinetic theory of neoclassical tearing modes in a low collisionality tokamak plasma: magnetic island threshold physics

Drift kinetic theory of neoclassical tearing modes in a low collisionality tokamak plasma: magnetic island threshold physics

Recent progress on spherical torus research

Disruptions, Disruptivity, and Safer Operating Windows in the High-β Spherical Torus NSTX

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