Beta limits in beam-heated tokamaks

Consideration is given to some practical features associated with beta optimization in neutral-beam-heated tokamaks. Previous ideal MHD stability calculations of beta limits have employed ad hoc pressure and current density profiles. A coupled 1-1/2-D transport-ballooning eigenmode model was used to match observable experimental parameters and to simulate those not directly measured (such as the beam deposition and heating profiles), including neutral gas and impurity effects. With this model it was found that: (1) beta values exceeding the ideal MHD limits have been obtained, and (2) strong effects of direct beam contributions to the pressure, or indirect effects of strong local beam heating, can outweigh the relatively remote vacuum field shaping in well-penetrated discharges. Since beam-induced currents can produce significant changes in transport by changing the location and width of islands produced by saturated resistive tearing modes, these have been calculated self-consistently. (author)