Evolution of global modes and magnetic reconnection in fusion burning plasmas

Plasmas under ignition conditions have relatively high peak pressures that can make them vulnerable to the effects of large scale pressure gradient driven modes of both ideal MHD and reconnecting types. These modes can manifest themselves as large sawtooth oscillations or as triggers of more extensive instabilities. The electron collision frequencies are higher than the diamagnetic frequencies so that the reconnecting modes are collisional. Given the very low thresholds that are found, numerically for ideal MHD, n = 1 modes in realistic configurations, the limits of the relevant linear stability analysis are investigated. For toroidal geometry the excitation of an n = 2 component of the toroidal current density is seen to mark the transition from the linear to the nonlinear stage at very low amplitudes. This and the fact that the relevant layers whose reconnection can take place are extremely narrow lead to identify a range of parameters corresponding to low values of γMHD τA (here, γMHD is the ideal MHD growth rate), where existing analyses have to be replaced by others including, e.g. the effects of an underlying state of microscopic turbulence that can affect the electron momentum transport and involve broader reconnection layers. A second approach is the analysis of sawtooth oscillations that are observed in most of the plasma regimes produced by the Alcator C-Mod machine which cover the range of collisionalities relevant to ignition experiments but are below the threshold for ideal MHD n = 1 instabilities. The main trends of the relevant observations are found to be consistent with the existing theoretical framework for reconnecting modes. The indications of existing theory and experimental analysis are that high magnetic field ignition experiments are present offer the most secure route to achieve their main objectives. (author). 25 refs, 6 figs, 2 tabs