Status of global energy confinement studies

Empirical scaling expressions, reflecting the parametric dependence of the L-mode energy confinement time, have been used not only as benchmarks for tokamak operation and theories of energy transport, but for predicting the performance of proposed tokamak devices. Several scaling expressions based on data from small and medium sized devices have done well in predicting performance in larger devices, although great uncertainty exists in extrapolating yet further, into the ignition regime. Several approaches exist for developing higher confidence scaling expressions. These include reducing the statistical uncertainty by identifying and filling in gaps in the present database, making use of more sophisticated statistical techniques, and developing scalings for confinement regimes within which future devices will operate. Confidence in the scaling expressions will be increased still further if the expressions can be more directly tied to transport physics theory. This can be done through the use of dimensionless parameters, by better describing the edge and core confinement regimes separately, and by incorporating transport models directly into the scaling expressions.