Formation of Internal Transport Barriers and its Impact on the JT-60U Plasmas
Shunsuke Ide and the JT-60 Team
Naka Fusion Research Establishment, Japan Atomic Energy Research Institute, Naka-machi, Naka-gun, Ibaraki-ken, 311-0193, Japan
Abstract.
Confinement improvement in JT-60U plasmas is characterized by transport barriers, where thermal and particle diffusion drops significantly. Especially one that is formed in a core plasma region, an internal transport barrier (ITB), can be a key to access high performance which is relevant to advanced tokamak operation. On JT-60U, ITBs are found in both the high bp and the reversed magnetic shear (RS) plasmas. In the high bp plasma, a large reduction in the ion transport is observed, while such a noticeable reduction in the electron transport is not always observed. Moreover the ITBs stay at positive magnetic shear region. On the other hand, in the RS plasma, a drastic reduction in both the ion and electron transport is usually found. The position of the ITBs stays at negative or nearly zero magnetic shear region. Furthermore the region of the reduced transport can be as narrow as the ion poloidal gyro radius. Due to the steep pressure gradient, also to the sharp change in toroidal rotation profile, a large spatial variation of the radial electric field (Er), that is the Er shear, is confirmed to be formed at the ITB. From the linear theories, the shear can be expected to contribute to the ITB formation and sustainment. Active modification of ITB structure was demonstrated by momentum injection by neutral beams. The formation of ITBs is beneficial not only to global confinement but also to steady state operation based on an advanced tokamak scenario. Since large fraction of the bootstrap current (IBS), which helps to reduce requirement of external non-inductive current drive for a steady state operation, can be generated at the ITB. The growth of IBS can modify the ITB structure via modification in the total current profile. That is, the current, pressure and also Er profiles are strongly coupled each other to organize the ITB structure in a plasma.
In this paper, the global characteristics of the ITBs in the JT-60U high bp and RS plasmas are described from a view point of both confinement improvement and steady state operation.