Global stability and operational regimes of Ignitor, ITER, and Alcator C-Mod

Summary form only given. One of the primary requirements for an ignition experiment is the macroscopic stability of the plasma column. For two proposed ignition experiments (Ignitor and ITER) we analyze numerically (using both the PEST and DCON codes, involving the ideal MHD approximation) the stability for different plasma pressure and current profiles. Since in both cases the aspect ratio is tight (2.8 and 2.9 respectively), and q(/spl Psi/)<1 within r/spl ap/a/2, n=1 modes, for which the m=1 and m=2 poloidal harmonics prevail, involve a major part of the plasma column (q(/spl Psi/)=2 for r/sub 2/=3a/4). We have also considered conditions corresponding to the reversed shear configuration, with reduced plasma currents (q/sub a//spl cong/6). In this case, stability against ballooning modes becomes a concern. The excitation of collisional n=1 modes involving a magnetic reconnection is also considered: we have analyzed a set of discharges obtained by Alcator C-Mod with and without RF heating, all exhibiting sawtooth oscillations while ideal MHD stability is verified.