231 research outputs found
Controlling Tokamak Geometry with 3D Magnetic Perturbations
It is shown that small externally applied magnetic perturbations can
significantly alter important geometric properties of magnetic flux surfaces in
tokamaks. Through 3D shaping, experimentally relevant perturbation levels are
large enough to influence turbulent transport and MHD stability in the pedestal
region. It is shown that the dominant pitch-resonant flux surface deformations
are primarily induced by non-resonant 3D fields, particularly in the presence
of significant axisymmetric shaping. The spectral content of the applied 3D
field can be used to control these effects
A model for microinstability destabilization and enhanced transport in the presence of shielded 3-D magnetic perturbations
A mechanism is presented that suggests shielded 3-D magnetic perturbations
can destabilize microinstabilities and enhance the associated anomalous
transport. Using local 3-D equilibrium theory, shaped tokamak equilibria with
small 3-D deformations are constructed. In the vicinity of rational magnetic
surfaces, the infinite-n ideal MHD ballooning stability boundary is strongly
perturbed by the 3-D modulations of the local magnetic shear associated with
the presence of nearresonant Pfirsch-Schluter currents. These currents are
driven by 3-D components of the magnetic field spectrum even when there is no
resonant radial component. The infinite-n ideal ballooning stability boundary
is often used as a proxy for the onset of virulent kinetic ballooning modes
(KBM) and associated stiff transport. These results suggest that the achievable
pressure gradient may be lowered in the vicinity of low order rational surfaces
when 3-D magnetic perturbations are applied. This mechanism may provide an
explanation for the observed reduction in the peak pressure gradient at the top
of the edge pedestal during experiments where edge localized modes have been
completely suppressed by applied 3-D magnetic fields
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Computational modeling of neoclassical and resistive MHD tearing modes in tokamaks
Numerical studies of the nonlinear evolution of MHD-type tearing modes in three-dimensional toroidal geometry with neoclassical effects are presented. The inclusion of neoclassical physics introduces an additional free-energy source for the nonlinear formation of magnetic islands through the effects of a bootstrap current in Ohm`s law. The neoclassical tearing mode is demonstrated to be destabilized in plasmas which are otherwise {Delta}` stable, albeit once an island width threshold is exceeded. The plasma pressure dynamics and neoclassical tearing growth is shown to be sensitive to the choice of the ratio of the parallel to perpendicular diffusivity ({Chi}{parallel}/{Chi}{perpendicular}). The study is completed with a demonstration and theoretical comparison of the threshold for single helicity neoclassical MHD tearing modes, which is described based on parameter scans of the local pressure gradient, the ratio of perpendicular to parallel pressure diffusivities {Chi}{perpendicular}/{Chi}{parallel}, and the magnitude of an initial seed magnetic perturbation
Bounce-averaged drifts: Equivalent definitions, numerical implementations, and example cases
In this article we provide various analytical and numerical methods for
calculating the average drift of magnetically trapped particles across field
lines in complex geometries, and we compare these methods against each other.
To evaluate bounce-integrals, we introduce a generalisation of the trapezoidal
rule which is able to circumvent integrable singularities. We contrast this
method with more standard quadrature methods in a parabolic magnetic well and
find that the computational cost is significantly lower for the trapezoidal
method, though at the cost of accuracy. With numerical routines in place, we
next investigate conditions on particles which cross the computational
boundary, and we find that important differences arise for particles affected
by this boundary, which can depend on the specific implementation of the
calculation. Finally, we investigate the bounce-averaged drifts in the
optimized stellarator NCSX. From investigating the drifts, one can readily
deduce important properties, such as what subset of particles can drive
trapped-particle modes, and in what regions radial drifts are most deleterious
to the stability of such modes.Comment: 12 pages, 6 figure
Enhanced Transport at High Plasma Pressure and Subthreshold Kinetic Ballooning Modes in Wendelstein 7-X
High-performance fusion plasmas, requiring high pressure β, are not well understood in stellarator-type experiments. Here, the effect of β on ion-temperature-gradient-driven (ITG) turbulence is studied in Wendelstein 7-X (W7-X), showing that subdominant kinetic ballooning modes (KBMs) are unstable well below the ideal MHD threshold and get strongly excited in the turbulence. By zonal-flow erosion, these subthreshold KBMs (stKBMs) affect ITG saturation and enable higher heat fluxes. Controlling stKBMs will be essential to allow W7-X and future stellarators to achieve maximum performance.</p
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