A Shear-Flow Instability Relevant to Advanced Tokamak Operation

An ideal magnetohydrodynamic instability which is driven by sheared flow and which may be relevant to reversed-shear advanced tokamak operation is described. If there is flow shear at the qmin surface, relatively weak velocity shear can drive this instability, with a time scale of the flow. The flows may be significantly slower than the ambient poloidal Alfvén velocity, and no inflection point is needed. Thus the time scale of the instability may be significantly longer than that of the poloidal Alfvén transit time, and it might account for disruption of reversed-shear discharges recently observed in JT-60U.