Implications of Vertical Stability Control on the SPARC Tokamak
arxiv(2024)
Abstract
To achieve its performance goals, SPARC plans to operate in equilibrium
configurations with a strong elongation of κ_areal∼1.75,
destabilizing the n=0 vertical instability. However, SPARC also features a
relatively thick conducting wall that is designed to withstand disruption
forces, leading to lower vertical instability growth rates than usually
encountered. In this work, we use the TokSyS framework to survey families of
accessible shapes near the SPARC baseline configuration, finding maximum growth
rates in the range of γ≲100s^-1. The addition of steel
vertical stability plates has only a modest (∼25%) effect on reducing the
vertical growth rate and almost no effect on the plasma controllability when
the full vertical stability system is taken into account, providing flexibility
in the plate conductivity in the SPARC design. Analysis of the maximum
controllable displacement on SPARC is used to inform the power supply voltage
and current limit requirements needed to control an initial vertical
displacement of 5% of the minor radius. From the expected spectra of plasma
disturbances and diagnostic noise, requirements for filter latency and vertical
stability coil heating tolerances are also obtained. Small modifications to the
outboard limiter location are suggested to allow for an unmitigated vertical
disturbance as large as 5% of the minor radius without allowing the plasma
to become limited. Further, investigations with the 3D COMSOL code reveal that
strategic inclusion of insulating structures within the VSC supports are needed
to maintain sufficient magnetic response. The workflows presented here help to
establish a model for the integrated predictive design for future devices by
coupling engineering decisions with physics needs.
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