Comparison of internal transport barriers in JT-60U and DIII-D NCS discharges

Negative central shear discharges in JT-60U revealed an internal transport barrier (ITB) which is located at larger minor radii (rho less than or equal to 0.8) than those in other devices and is accompanied by a clear reduction in chi(e). In order to investigate this origin of the unique and favourable feature for future reactors, the discharge parameters were compared with those in DIII-D, where ITB was at smaller minor radii (rho less than or equal to 0.55) and less reduction in chi(c) was observed. Strong negative magnetic shear was found to be related to the reduction in chi(c). The thin skin depth in JT-60U originating from high T(e), together with the continuous decrease in q(95), was effective in moving the ITB location to larger minor radii and in sustaining the strong negative shear.