The source of the vertical instability in tokamaks is relatively well understood, and stabilizing controllers have been successfully implemented in numerous tokamaks. Usually these controllers are designed based on a plasma model that assumes the plasma has zero mass. In reality the plasma mass is small but positive, so that some of the controller designs yield an unstable closed loop.
In this work we expand on [1], further investigating the discrepancy between the massless plasma model and the plasma with mass model. It turns out that conclusions about closed loop behavior depend on the controller’s asymptotic behavior at infinite frequency. Simulations on models of KSTAR and ITER are performed with varying presence of passive stabilizing structures, including the possibility of superconductive control coils. It is shown that erroneous conclusions regarding asymptotic stabilization only result from non proper controllers. The results confirm that neither pure velocity feedback nor a proper or strictly proper form of velocity feedback can asymptotically stabilize the vertical instability.