Long-pulse hybrid scenario development in JT-60U

The performance and sustained duration of long-pulse discharges for the 'ITER hybrid scenario' have been improved in JT-60U. The modification of power supply systems for three perpendicular neutral beam (NB) injections provides a long period of central NB heating up to 30 s, which is important for keeping the internal transport barrier (ITB). The peaked density profile in the core plasma can be maintained even when the density at the pedestal increased in the latter phase of the discharge due to the increase in the divertor recycling. Then, the peaked pressure profile attributed to the ITB can be kept constant through the discharge with the peaked power deposition profile. In these long-pulse discharges, MHD activity with toroidal mode number n = 1 is observed even when neoclassical tearing modes (NTMs) are suppressed. When the amplitude of the mode in the peripheral region becomes large, the pedestal pressure is degraded. The mode amplitude is sensitive to the toroidal magnetic field (or edge safety factor) and heating power. After the adjustment of the toroidal magnetic field so as to reduce the mode amplitude, a high normalized beta (β_N) of 2.6 and a high thermal confinement enhancement factor (H_H98(y,2) > 1) are sustained for 25 s (∼14τ_R, where τ_R is the current diffusion time) under the ITER relevant small toroidal rotation condition. The peaked pressure profile in low safety factor plasma (safety factor at 95% flux surface q₉₅ ∼ 3.2) is stable against NTMs up to β_N ∼ 3. A high β_NH_H98(y,2) of 2.6 gives a high G-factor ( ) of 0.25 and a peaked pressure profile gives a large bootstrap current fraction (f_BS > 0.43).