Active Control of Internal Transport Barrier and Confinement Database in JT-60 Reversed Shear Plasma
Y. Sakamoto, H. Shirai, T. Fujita, Y. Kamada, S. Ide, T. Takizuka
Naka Fusion Research Establishment, Japan Atomic Energy Research Institute Naka-machi, Naka-gun, Ibaraki-ken 311-0193, Japan
Abstract.
Active controls of internal transport barrier (ITB) in reversed shear plasmas have been demonstrated. In JT-60U reversed shear plasmas, thermal and perticle diffusivities quickly reduced to the level of neoclassical transport within the narrow region, where the steep gradients in density and temperature profiles and the notched profile in toroidal rotation were formed. Strong Er shear was generated at the ITB, since the radial force balance equation for each ion species in toroidal plasmas indicates that is a connection between the Er and pressure gradient, plasma rotation. Therefore two scenarios for the controlling the ITB can be considered; namely the toroidal rotation and the pressure gradient can modify the Er shear respectivery. Various injection directions of neutral beams (NBs) are simultaneously available in JT-60U. Tangential NBs can directly control the toroidal rotation and also the pressure gradient. Perpendicular NBs can control the pressure gradient without the toroidal momentum. Modifications of the Er shear profile using the combinations of such NBs successfully controlled the ITB strength in JT-60U reversed shear plasmas. The changes in Er shear play an important role. Unidirectional momentum injection (co- or ctr-current direction) made the toroidal rotation vary from the notched to monotonic, which caused mainly by toroidal momentum source profiles. Then Er shear profile changed gradually because the monotonic rotation profile canceled the pressure gradient term in Er shear partially. As a result, ITB was degraded by unidirectional injection. On the other hand, balanced injection enhanced the notched profile in the toroidal rotation, and then ITB was retrieved. In both ITB degradation and ITB retrieve phase, some time delays from changing the NB combinations were observed, which may be related to the momentum transport.