Advanced operation modes relying on core plasma turbulence stabilization in tokamak fusion devices

Abstract

Recent progress of advanced operation modes in tokamaks is addressed focusing upon internal transport barrier (ITB) discharges. These ITB discharges are being considered as one of candidate operation modes in fusion reactors. Here, “internal” means core region of a fusion plasma, and “transport barrier” implies bifurcation of transport phenomena due to suppressing plasma turbulence. Although ITB discharges have been developed since the mid-1990, they have been suffering from harmful plasma instabilities, impurity accumulation, difficulty of feedback control of kinetic plasma profiles such as pressure or current density, and so on. Sustainment of these discharges in long-pulse operations above wall saturation time is another huddle. Recent advances in ITB experiments to overcome the difficulties of ITB discharges are addressed for high βp plasmas in DIII-D, broad ITB without internal kink mode in HL-2A, F-ATB (fast ion-induced anomalous transport barrier) in ASDEX upgrade, ion and electron ITB in LHD, and FIRE (fast ion regulated enhancement) mode in KSTAR. The core-edge integration is discussed in the ITB discharges. The DIII-D high βp plasmas facilitate divertor detachment which weakens the edge transport barrier (ETB) but extends the ITB radius resulting in a net gain in energy confinement. Double transport barriers were observed in KSTAR without edge localized mode (ELM). FIRE modes in KSTAR are equipped with the I-mode-like edge which prevents the ELM burst and raise the fusion performance together with ITB. Finally, long sustainment of ITBs is discussed. EAST established electron ITB mode in long-pulse operations. JET achieved quasi-stationary ITB with active control of the pressure profile. JT-60U obtained 28 s of high βp hybrid mode, and KSTAR sustained stable ITB in conventional ITB mode as well as FIRE mode. These recent outstanding achievements can promise ITB scenarios as a strong candidate for fusion reactors.