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    Microstability of β ~ 1 tokamak equilibria
    (Cambridge University Press, 2023-03-01) Gaur, Rahul; Abel, Ian G.; Dickinson, David; Dorland, William D.
    High-power-density tokamaks offer a potential solution to design cost-effective fusion devices. One way to achieve high power density is to operate at a high β value (the ratio of thermal to magnetic pressure), i.e. β∼1. However, a β∼1 state may be unstable to various pressure- and current-driven instabilities or have unfavourable microstability properties. To explore these possibilities, we generate β∼1 equilibria and investigate their stability. First, we demonstrate the generation of high- β equilibria with the computer code VMEC. We then analyse these equilibria to determine their stability against the infinite- n ideal-ballooning mode. We follow that by engaging in a detailed microstability study using the GS2 code, beginning with assessments of electrostatic ion-temperature-gradient and trapped election mode instabilities. We observe interesting behaviour for the high- β equilibria – stabilization of these modes through two distinct mechanisms – large negative local shear and reversal of electron precession drift. Finally, we perform electromagnetic gyrokinetic simulations and observe enhanced stability in the outer core of high- β equilibria and absence of kinetic ballooning modes in the negative-triangularity, high- β equilibria. The enhanced outer-core stability of high- β equilibria is different from their lower- β counterparts and offers an alternative, potentially favourable regime of tokamak operation.
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    Mapping the space of quasisymmetric stellarators using optimized near-axis expansion
    (Cambridge University Press, 2022-12-23) Landreman, Matt
    A method is demonstrated to rapidly calculate the shapes and properties of quasi-axisymmetric and quasi-helically symmetric stellarators. In this approach, optimization is applied to the equations of magnetohydrodynamic equilibrium and quasisymmetry, expanded in the small distance from the magnetic axis, as formulated by Garren & Boozer [Phys. Fluids B, vol. 3, 1991, p. 2805]. Due to the reduction of the equations by the expansion, the computational cost is significantly reduced, to times of the order of 1 cpu second, enabling wide and high-resolution scans over parameter space. In contrast to traditional stellarator optimization, here, the cost function serves to maximize the volume in which the expansion is accurate. A key term in the cost function is ∥∇B∥ , the norm of the magnetic field gradient, to maximize scale lengths in the field. Using this method, a database of 5×105 optimized configurations is calculated and presented. Quasisymmetric configurations are observed to exist in continuous bands, varying in the ratio of the magnetic axis length to average major radius. Several qualitatively new types of configuration are found, including quasi-helically symmetric fields in which the number of field periods is two or more than six.