A slowly growing kink that exploits the finite resistivity of the vacuum vessel wall to bypass what should be a hard stability limit.
A resistive wall mode (RWM) is a global MHD instability that would be stabilized by a perfectly conducting wall but grows on the resistive timescale of the real wall surrounding the plasma. In ideal MHD a close-fitting superconducting shell can raise the pressure limit (the so-called "wall-stabilized" beta limit) well above the no-wall value. A real wall, however, has finite resistivity: eddy currents induced by the mode decay on a timescale τw = μ0σdδ, where σ is the wall conductivity, d its thickness, and δ its effective distance from the plasma. The RWM grows on roughly this timescale—seconds in a large tokamak—making it slow compared with ideal kinks but still fast enough to terminate a discharge.1
Advanced tokamak scenarios depend on operating above the no-wall beta limit to achieve high bootstrap-current fractions and economically attractive fusion power density. The RWM is the principal obstacle: unless it is actively controlled, the plasma cannot access the regime between the no-wall and ideal-wall stability boundaries.2
Two broad strategies exist. Passive stabilization relies on plasma rotation: a toroidally rotating plasma interacts with the resistive wall through a rotationally induced phase shift that provides effective dissipation, converting the growing mode into a damped one. Experiments on DIII-D showed that a critical rotation threshold (typically a few percent of the Alfvén speed) is sufficient.3
Active feedback control uses external saddle coils driven by real-time magnetic sensors to apply a correction field that opposes the RWM perturbation. This approach has been demonstrated on DIII-D, NSTX, JT-60U, and KSTAR, and is the baseline strategy for devices where neutral-beam torque may be insufficient to maintain rotation.4
Kinetic effects—resonances between the slow RWM and thermal-ion or energetic-particle drift motions—provide additional damping and can raise the rotation threshold or even eliminate it, a finding with important implications for reactor designs where rotation drive is limited.