23 research outputs found
Critical gradient turbulence optimization toward a compact stellarator reactor concept
Integrating turbulence into stellarator optimization is shown by targeting
the onset for the ion-temperature-gradient mode, highlighting effects of
parallel connection length, local magnetic shear, and flux surface expansion.
The result is a compact quasihelically symmetric stellarator configuration,
admitting a set of uncomplicated coils, with significantly reduced turbulent
heat fluxes compared to a known stellarator. The new configuration combines low
values of neoclassical transport, good alpha particle confinement, and Mercier
stability at a plasma beta of almost 2.Comment: 5 pages, 5 figures. Phys. Rev. Research 5, L032030 (2023
Assessing global ion thermal confinement in critical-gradient-optimized stellarators
We investigate the confinement properties of two recently devised
quasi-helically symmetric stellarator configurations, HSK and QSTK. Both have
been optimized for large critical gradients of the ion temperature gradient
mode, which is an important driver of turbulent transport in magnetic
confinement fusion devices. To predict the resulting core plasma profiles, we
utilize an advanced theoretical framework based on the gyrokinetic codes GENE
and GENE-3D, coupled to the transport code TANGO. Compared to the HSX
stellarator, both HSK and QSTK achieve significantly higher core-to-edge
temperature ratios, partly thanks to their smaller aspect ratios, with the
other part due to more detailed shaping of the magnetic geometry achieved
during optimization. The computed confinement time, however, is less sensitive
to core temperature than edge temperature, simply due to the disproportionate
influence the edge has on stored plasma energy. We therefore emphasize the
possible benefits of further optimizing turbulence in the outer core region,
and the need to include accurate modelling of confinement in the edge region in
order to assess overall plasma performance of turbulence optimized
stellarators
Experimental confirmation of efficient island divertor operation and successful neoclassical transport optimization in Wendelstein 7-X
We present recent highlights from the most recent operation phases of Wendelstein 7-X, the most advanced stellarator in the world. Stable detachment with good particle exhaust, low impurity content, and energy confinement times exceeding 100 ms, have been maintained for tens of seconds. Pellet fueling allows for plasma phases with reduced ion-temperature-gradient turbulence, and during such phases, the overall confinement is so good (energy confinement times often exceeding 200 ms) that the attained density and temperature profiles would not have been possible in less optimized devices, since they would have had neoclassical transport losses exceeding the heating applied in W7-X. This provides proof that the reduction of neoclassical transport through magnetic field optimization is successful. W7-X plasmas generally show good impurity screening and high plasma purity, but there is evidence of longer impurity confinement times during turbulence-suppressed phases.EC/H2020/633053/EU/Implementation of activities described in the Roadmap to Fusion during Horizon 2020 through a Joint programme of the members of the EUROfusion consortium/ EUROfusio
Coarse-grained gyrokinetics for the critical ion temperature gradient in stellarators
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. When 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