Current ramp experiments on the scaling of energy confinement

The dependence of global and local confinement upon plasma current and poloidal field is studied in an analysis of JET data from a series of current ramp experiments. In such experiments the current profile is transiently decorrelated from its steady state shape over a period lasting more than ten confinement times. Global confinement is shown to depend not only on total current, but also on some measure of the current distribution, for example the internal inductance. The origin of this dependence is found in the local analysis to be the dependence of diffusivity on the poloidal field. A weak dependence of diffusivity on shear is observed in the outer half of the plasma. Predictive transport calculations are made in which the electron-ion thermal diffusivities depend on shear; the calculations reproduce the measured temperature profiles to within the measurement errors. Simple mathematical expressions linking global confinement are used to demonstrate that the results from the global and local analysis are fully consistent.