Transport in high-performance plasmas with a transport barrier

In recent tokamak experiments several types of high-performance plasmas were obtained. One is the H (high)-mode triggered by the L (low) to H (high) transition. In this case the transport barrier is generated at the edge region. The obtained high confinement plasmas are characterized by gyro-Bohm-type transport from the scaling law. Another is the negative (central) shear or (enhanced) reverse shear mode with the transport barrier at the inner region close to the minimum point of the safety factor profile. With the internal transport barrier (ITB) the turbulent transport is suppressed significantly; however, the scaling study is primitive for the core plasma. To understand the negative shear configuration with the transport barrier, the high-βp plasmas are useful, since the ion thermal transport is improved without the reversal of magnetic shear. One key ingredient to obtain the above high-performance plasmas is the ExB velocity shear which may suppress the turbulence and associated anomalous transport in general. Shafranov shift is also favourable. However, it is a difficult task to predict by what degree the anomalous transport is suppressed, particularly compared to L-mode plasmas. Some theoretical efforts into the understanding of high-performance plasmas are discussed. (author)