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Nonlinear Saturation of Ballooning Modes in Stellarators

Date: Thursday, January 22nd

Time: 1:00 pm

Place: Sterling Hall B343

Speaker: Xu Chu , PPPL

Abstract: The nonlinear evolution and saturated states of ballooning modes – in particular, metastable states – are critical for understanding explosive MHD events in magnetic confinement fusion (MCF) devices, such as edge-localized modes (ELMs) in tokamaks and core density collapses (CDCs) in LHD. In this work, ballooning mode saturation is investigated in realistic stellarator configurations using the flux tube approach of Ham et. al. [1]. The method is adapted to account for the lack of exact force balance in stellarator equilibrium solvers that assume existence of nested flux surfaces. A variational approach for calculating flux tube energy is developed to overcome this force error problem in stellarator numerical equilibria. Saturated (equilibrium) flux tube states that cross 10-20% of the plasma minor radius are shown to exist for linearly ballooning unstable profiles. It is shown that several features of the displaced flux tube structure in a full nonlinear MHD simulation of Wendelstein 7X are reproduced by our model. Saturated states are found in a compact stellarator equilibrium close but below the marginal ballooning linear instability, i.e. the unperturbed equilibrium is metastable. This suggests that Edge-Localized-Mode-like explosive MHD behavior may be possible in stellarators.

[1] Ham C J, Cowley S C, Brochard G and Wilson H R 2018 Plasma Physics and Controlled Fusion 60 075017

Host: Tony Qian

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