Spatial structure of internal and edge transport barriers
T. Fujita
Japan Atomic Energy Research Institute, Naka Fusion Research Establishment Naka-machi, Naka-gun, Ibaraki-ken 311-0193 Japan
Abstract.
This paper treats of spatial structure or radial profiles of internal and edge transport barriers. The spatial structure is closely related to the global stability, confinement and hence the plasma performance. It also reflects the mechanism of transport barrier formation and hence serves to clarification of the formation mechanism together with the time evolution of transport barrier formation and destruction.
For the edge transport barrier or H-mode pedestal, the pressure at the top of pedestal (pedestal pressure), the pressure gradient in the pedestal, and the pedestal width have been intensively studied in H-mode with type I ELMs since they affect the global confinement and stability significantly. For the pedestal width, some kinds of scaling depending on poloidal ion Larmor radius, poloidal beta or magnetic shear have been proposed. Improvement of pedestal pressure has been observed with increasing the triangularity. The H-mode regime with small ELMs, where a heat load to the divertor target is reduced, has been explored in several tokamaks. The pedestal parameters in this regime are also under investigation.
For the internal transport barriers (ITBs), the radial profile has larger variety and the situation is more complicated. The location of ITB and even the existence of ITB can differ in ne, Te and Ti profiles. One of the most important parameter is the radial location of ITB foot, ρfoot, where the pressure gradient starts to increase toward the center. The profiles inside ρfoot are roughly classified into two groups; a parabolic type and a box type. In a parabolic type, ITB a large pressure gradient continues to the center. In a box type ITB, the gradient becomes small again at some location (ρshoulder), which results in a localized steep gradient zone (ITB layer). The origin of this enhanced transport near the axis has not been identified yet. A large ρfoot, ultimately merging with the edge pedestal, is favorable for the global stability and confinement. As for the profile inside ρfoot, a parabolic type ITB is favorable for the stability while high confinement is often obtained in a box type ITB.
The ExB shear is considered one of the major causes for the formation of transport barrier or reduction of anomalous transport. Modification of ExB shear profile by changing the toroidal momentum input has been carried out to investigate its effects on radial structure of ITB. The q profile is also related to the location of ITB foot and it is often observed that ρfoot is close to the location with q = integer or qmin. In a high beta plasma with a large bootstrap current fraction, both of ExB shear and current profiles are strongly related to the ITB structure, which in turn depends on ExB shear and current profiles. The structure formation in a plasma where the pressure and current profiles are thus strongly linked will be discussed.