Stability of transport barrier in negative shear tokamak discharge

Summary form only given. The inner transport barrier (ITB) in tokamaks is characterized by steep density and temperature gradients and thus large ballooning parameter /spl alpha/. In the JT-60U tokamak operated in negative shear, improved confinement has been achieved with clear formation of ITB having /spl alpha//spl ap/3. The ideal ballooning mode is known to be stable if shear is negative. However, as shown recently, an ion temperature gradient driven kinetic ballooning mode, which has characteristics similar to the mode identified in the MHD second stability regime with positive shear, may still persist. Also, the well known finite /spl beta/ stabilization of the predominantly electrostatic ion temperature gradient (ITG) mode found for positive shear may not be operative when shear is negative. The analysis is based on a kinetic shooting code in which ions and trapped electrons are treated fully kinetically and ion transit effects perturbatively. For ITB parameters similar to those in JT-60U, long wavelength kinetic ballooning and ITG modes have been found to be stable.