Microstability of β 1 tokamak equilibria
Abstract
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 unfavorable microstability properties. To explore these possibilities, we generate β 1 equilibria and investigate their stability. Initially, we study an analytical technique that was used in the past to generate β 1 equilibria and outline its limitations. Hence, we demonstrate the generation of high-β equilibria with the computer code VMEC. We then analyze these equilibria to determine their stability against the infinite-n ideal ballooning mode. We follow that by engaging in a detailed microstability study, beginning with assessments of electrostatic ITG and TEM instabilities. We observe interesting behavior for the high-β equilibria -- stabilization of these modes through two distinct mechanisms. Finally, we perform electromagnetic gyrokinetic simulations and again observe stabilizing trends in the equilibria at high β. These trends are different from their lower β counterparts and offer an alternative, potentially favorable regime of tokamak operation.
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