Are there disruption forces in stellarators?*

One of the most well-known advantages of stellarators is the elimination of the problematic issues due to current-driven disruptions, specifically large induced currents and runaway electron beams, which are problematic in tokamaks with their large current parallel to the magnetic field.  In the absence of large J_‖, the MHD equilibrium equation, ▽p = J × B, stipulates the necessity for a large perpendicular current density, J_⊥= B ×▽p / B², for a magnetic confinement device with a strong pressure gradient, as expected in a stellarator fusion reactor. If an event occurs for which the plasma pressure is lost rapidly, this diamagnetic current, J_⊥, will quench on the same timescale, inducing an electric field per Lenz’s law.  Since this electric field is perpendicular to the field lines, it cannot accelerate electrons to form relativistic runaway beams.  But it will induce perpendicular current in the vacuum vessel, with resulting I_⊥× B force trying to compress the minor radius of the vessel.  The magnitude of these effects depends on the timescale in which plasma thermal energy collapses, as well as the magnitude of the drop in plasma pressure.  For fast collapses the resulting forces can be substantial and must be taken into account in machine design.  This presentation will assess such scenarios in terms of likelihood and avoidance of such events, the relevant time scales, and potential consequences for the design of a stellarator reactor.