Assessment of vertical instability and passive stabilization in SPARC-like plasmas

Plasma shaping greatly improves fusion power density in tokamaks, but some parameters, like elongation, make the plasma column vertically unstable to small perturbations, leading to a vertical displacement event (VDE) [1]. The vertical instability growth rate is a measure of the timescale of a VDE in the absence of a control system and depends on both the vacuum vessel and plasma column geometry. For high-field tokamaks like SPARC and ARC, it is imperative to determine the set of parameters that the vertical instability growth rate is sensitive to, as to remain within the operational limits of the control system. A parameter scan of SPARC double-null, flat-top, H-mode equilibria was conducted using MEQ [2] for rigid and non-rigid plasma perturbations. We found that changing the elongation via the midplane gap had the greatest impact on vertical stability. A simple eddy current response model was used alongside MEQ to determine which passive structures are important for vertical stability. This work is relevant for maximizing passive vertical stabilization in ARC-class devices.

[1] G. Laval, R. Pellat, J. S. Soule; Phys. Fluids 1 April 1974; 17 (4): 835–845.

[2] F. Carpanese, EPFL PhD thesis, 2021.