Extreme low-edge safety factor tokamak scenarios via active control of three dimensional magnetic fields on RFX and DIII-D

High current, stable tokamak plasmas with edge safety factor below or around 2 are attractive for magnetic fusion due to favourable high fusion gain and higher confinement. But they have long been considered inaccessible in modern devices owing to the unforgiving MHD instabilities. Even in devices with a resistive wall, the onset of an n=1 resistive wall mode (RWM) leads to a disruptive limit at edge safety factor qedge=2 (for limiter plasmas) and q95 2 (diverted plasmas). Recently long leaps have been made by DIII-D and RFX-mod through active control of MHD stability by the application of three-dimensional magnetic field [1,2,3]. This paper describes how for the first time two very different tokamaks, a MA-class shaped device like DIII-D and a high aspect ratio circular experiment like RFX-mod (operated as a tokamak), have robustly overcome the edge safety factor = 2 limit by active control of plasma stability and demonstrated that operation below 2 is possible for hundreds of resistive wall times. In addition these experiments have revealed a new tool to control sawtooth frequency and amplitude, a result that has the potential of extending the tokamak operating space even further by avoiding the deleterious effects of long-period sawteeth. Comparison with theory and numerical modelling of stability and of the effect of externally applied 3D fields in low edge q plasmas is presented.