The External Kink Mode and the Role of q 95 in Diverted Tokamaks

The disruptive instability in diverted tokamaks when EUR q at the 95% poloidal flux surface, EUR q 95 , is driven below 2.0 is shown to be a resistive kink . The mode is a resistive counterpart to the current driven ideal mode that traditionally explained the disru ption in limited cross sections when q a , the safety factor at the outermost closed flux surface, lies just below a rati onal value . D iverted plasmas, in which q a is formally infinite in the MHD model, have presented a longstanding difficulty since the theory would predict stability, yet, the disruptive limit occurs in practice when q 95 , reaches 2. N umerical calculations show the resistive kink mode is linearly destabilized by the rapidly increasing resistivity at the plasma edge when q 95 < 2, but q a >> 2. The resistive kink also occurs for limiter plasmas but quickly transforms to the ideal mode when the rational surface exits the plasma; this also explains an observed small discrepancy in onset conditions.