G. Turri

Overview of recent experimental results on MAST

Note: Proc. 19th IAEA Fusion Energy Conference, Lyon, France, October 2002, IAEA-CN-94/EX/OV2-3 Reference CRPP-CONF-2002-068 Record created on 2008-05-13, modified on 2017-05-12

MAST and the impact of low aspect ratio on tokamak physics

Low aspect ratio plasmas in devices such as the mega ampere spherical tokamak (MAST) are characterized by strong toroidicity, strong shaping and self fields, low magnetic field, high beta, large plasma flow and high intrinsic E × B flow shear. These characteristics have important effects on plasma behaviour, provide a stringent test of theories and scaling laws and offer new insight into underlying physical processes, often through the amplification of effects present in conventional tokamaks (e.g. impact of fuelling source and magnetic geometry on H-mode access). The enhancement of neoclassical effects makes MAST ideal for the study of particle pinch processes and neoclassical resistivity corrections, which can be assessed with unique accuracy. MAST data have an important influence on scaling laws for confinement and H-mode threshold power, exerting strong leverage on the form of these scaling laws (e.g. scaling with aspect ratio, beta, magnetic field, etc). The high intrinsic flow shear is conducive to transport barrier formation by turbulence suppression. Internal transport barriers are readily formed in MAST with both co- and counter-NBI, and electron and ion thermal diffusivities have been reduced to the ion neoclassical level. The strong variation in toroidal field (~ × 5 in MAST) between the inboard and outboard plasma edges, provides a useful test of edge models prompting, for example, a comparison of inboard and outboard scrape-off-layer transport to highlight magnetic field effects. Low aspect ratio plasmas are also an ideal testing ground for plasma instabilities, such as neoclassical tearing modes, edge localized modes (ELMs) and Alfven eigenmodes, which are readily generated due to the supra-Alfvenic ion population. Examples of how MAST is providing new insights into such instabilities (e.g. ELM structure) are described.

Recent electron cyclotron emission results on TCV

Electron cyclotron emission (ECE) diagnostics on Tokamak à Configuration Variable (TCV) allow study of the electron temperature evolution in time with good spatial and temporal resolution at the high field side and low field side at various lines of sight. That is why ECE is being widely used to obtain both qualitative and quantitative information on heat transport, magnetohydrodynamics (MHD) phenomena, and fast electron dynamics. In this paper, a new regime on TCV with regular oscillations of the electron temperature in electron cyclotron current drive (ECCD) driven fully noninductive discharges and in discharges with a combination of ohmic/ECCD driven current is discussed. These oscillations are reminiscent of the oscillations of the central electron temperature (O-regime) seen on Tore Supra in fully noninductive lower hybrid current drive plasmas. A link between evolutions of the electron temperature, the MHD modes, and the current density profile on TCV is considered. In order to yield information on the properties of microturbulence of electrostatic and magnetic origin on TCV, a correlation ECE radiometer is currently under development. A technical description of the diagnostic is presented in this paper.