Robust stellarator optimization via flat mirror magnetic fields

We present new stellarator equilibria that have been optimized for reduced turbulent transport using nonlinear gyrokinetic simulations within the optimization loop. The optimization routine involves coupling the pseudo-spectral GPU-native gyrokinetic code GX with the stellarator equilibrium and optimization code DESC. Since using GX allows for fast nonlinear simulations, we directly optimize for reduced nonlinear heat fluxes. To handle the noisy heat flux traces returned by these simulations, we employ the simultaneous perturbation stochastic approximation (SPSA) method that only uses two objective function evaluations for a simple estimate of the gradient. We show several examples that optimize for both reduced heat fluxes and good quasi-symmetry as a proxy for low neoclassical transport. Finally, we run full transport simulations using the T3D stellarator transport code to evaluate the changes in the macroscopic profiles.

Section: Turbulence Optimizationmentioning

confidence: 99%

Optimization of nonlinear turbulence in stellarators

Kim,

Buller,

Conlin

et al. 2024

“…Many authors have investigated the behaviour of this quantity (White & Chance 1984;Roach, Connor & Janjua 1995), and analytic expressions exist for large-aspect ratio tokamaks with circular cross-sections and small non-axisymmetric perturbations (Connor, Hastie & Martin 1983;Hegna 2015). For general omnigeneous stellarators (and even more so, upon relaxing omnigeneity), expressions for ω α rarely allow for analytical calculation (Velasco et al 2023). This ends up impeding the dissection of the underlying physics and effects.…”

Section: Introductionmentioning

confidence: 99%

“…For general omnigeneous stellarators (and even more so, upon relaxing omnigeneity), expressions for rarely allow for analytical calculation (Velasco et al. 2023). This ends up impeding the dissection of the underlying physics and effects.…”

Section: Introductionmentioning

confidence: 99%

Trapped-particle precession and modes in quasisymmetric stellarators and tokamaks: a near-axis perspective

Rodríguez,

Mackenbach

2023

This paper presents the calculation of the bounce-averaged drift of trapped particles in a near-axis framework for axisymmetric and quasisymmetric magnetic fields that possess up-down and stellarator symmetry, respectively. This analytic consideration provides important insight on the dependence of the bounce-averaged drift on the geometry and stability properties of the field. In particular, we show that although the maximum- $\mathcal {J}$ property is unattainable in quasisymmetric stellarators, one may approach it through increased plasma $\beta$ and triangular shaping, albeit going through a reduced precession scenario with potentially higher particle losses. The description of trapped particles allows us to calculate the available energy of trapped electrons analytically in two asymptotic regimes, providing insight into the behaviour of this measure of turbulence. It is shown that the available energy is intimately related to magnetohydrodynamics (MHD) stability, providing a potential synergy between this measure of gyrokinetic turbulence and MHD stability.

The maximum-J property in quasi-isodynamic stellarators

Rodríguez,

Helander,

Goodman

2024

Some stellarators tend to benefit from favourable average magnetic curvature for trapped particles when the plasma pressure is sufficiently high. This so-called maximum- $J$ property has several positive implications, such as good fast-particle confinement, magnetohydrodynamic stability and suppression of certain trapped-particle instabilities. This property cannot be attained in quasisymmetric stellarators, in which deeply trapped particles experience average bad curvature and therefore precess in the diamagnetic direction close to the magnetic axis. However, quasi-isodynamic stellarators offer greater flexibility and allow the average curvature to be favourable and the precession to be reversed. We find that it is possible to design such stellarators so that the maximum- $J$ condition is satisfied for the great majority of all particles, even when the plasma pressure vanishes. The qualitative properties of such a stellarator field can be derived analytically by examining the most deeply and the most shallowly trapped particles, although some small fraction of the latter will inevitably not behave as desired. However, through numerical optimisation, we construct a vacuum field in which 99.6 % of all trapped particles satisfy the maximum- $J$ condition.