Macroscopic Stability of High Beta MAST Plasmas

Full text: The high-beta capability of the spherical tokamak geometry, coupled with a suite of world-leading diagnostics on MAST, has facilitated significant improvements in the understanding of performance-limiting core instabilities in high performance plasmas. For instance, the newly installed Motional Stark Effect diagnostic, with radial resolution < 25 mm, has enabled detailed study of saturated long-lived modes (LLMs) in advanced tokamak scenarios. These modes significantly degrade confinement, damp the core rotation and enhance fast ion losses, so detailed understanding is required for their amelioration. The LLM is diagnosed as an ideal internal mode growing unstable as qmin approaches one and the role of rotation, fast ions and ion diamagnetic effects in determining the marginal mode stability are also discussed. Similarly, the upgraded Thomson Scattering system, with radial resolution < 10 mm and the possibility of temporal resolution of 1microsecond, has allowed detailed analysis of the density and temperature profiles in and around a Neo-classical Tearing Mode (NTM), permitting tests of models for the critical NTM island width. High resolution Charge Exchange Recombination Spectroscopy provided detailed measurement of rotation braking induced by both applied magnetic fields and by magnetohydrodynamic (MHD) instabilities, allowing tests of Neoclassical Toroidal Viscosity theory predictions. Finally, MAST is also equipped with internal and external coils that allow non-axisymmetric fields to be applied for active MHD spectroscopy of instabilities near the no-wall beta limit. The enhanced understanding of the physical mechanisms driving deleterious core MHD activity given by these leading-edge capabilities has provided guidance to optimise operating scenarios for improved plasma performance. This work was funded by the United Kingdom Engineering and Physical Sciences Research Council under grant EP/G003955 and the European Communities under the contract of Association between EURATOM and CCFE. The views and opinions expressed herein do not necessarily reflect those of the European Commission. (author)