We report a discovery of a fusion plasma regime suitable for commercial fusion reactor where the ion temperature was sustained above 100 million degree about 20 s for the rst time. Nuclear fusion as a promising technology for replacing carbon-dependent energy sources has currently many issues to be resolved to enable its large-scale use as a sustainable energy source. State-of-the-art fusion reactors cannot yet achieve the high levels of fusion performance, high temperature, and absence of instabilities required for steady-state operation for a long period of time on the order of hundreds of seconds. This is a pressing challenge within the eld, as the development of methods that would enable such capabilities is essential for the successful construction of commercial fusion reactor. Here, a new plasma con nement regime called fast ion roled enhancement (FIRE) mode is presented. This mode is realized at Korea Superconducting Tokamak Advanced Research (KSTAR) and subsequently characterized to show that it meets most of the requirements for fusion reactor commercialization. Through a comparison to other well-known plasma con nement regimes, the favourable properties of FIRE mode are further elucidated and concluded that the novelty lies in the high fraction of fast ions, which acts to stabilize turbulence and achieve steady-state operation for up to 20 s by self-organization. We propose this mode as a promising path towards commercial fusion reactors.

The accumulation of tungsten impurities measured in a KSTAR experiment was analyzed theoretically using a drift-kinetic code, NEO, to determine the contribution of neoclassical transport. According to the NEO simulation results, there is a certain value of impurity toroidal rotation speed maximizing the neoclassical inward convection. The inward convection decreases or the outward convection increases as the rotation increases only beyond the speed value. The non-monotonic dependency of the neoclassical convection on the rotation is analyzed by the several coefficients for many profile effects, including ion and electron profiles. The dependency of the coefficients for the main ion density gradient on the rotation is different from that for the temperature gradient, so it results in the amplification of the temperature screening beyond the certain value of the rotation. In the KSTAR case with high toroidal rotation of the tungsten (around Mach number 4.5), only in the mid-radius does the rotation reduce the inward impurity particle convection or change the inward convection to the outward convection. Thus, the rotation is a useful tool to control the impurity accumulation conditionally. The favorable condition occurs only for high rotation, which significantly depends on the radius and the collisionality due to the complicated non-monotonic dependency of the convection on the rotation speed.

The first measurement of the impurity density profile via charge exchange spectroscopy (CES) has been successfully achieved in KSTAR. Since the neutral beam density profile is essential for the measurement, the neutral beam penetration code that was originally developed for the Alcator C-mod tokamak has been optimized for the KSTAR experimental environment. The method of the impurity density measurement by the KSTAR CES system is introduced and the sensitivity analysis of various physical parameters, such as the effective charge in the estimation of the impurity density, is performed to examine the validation of the method. This method has been applied to measure the C6+ density profile affected by the resonant magnetic perturbations (RMPs), which is mainly used to suppress the edge localized modes (ELMs) in KSTAR. The dynamics of the C6+ density profile represent that the C6+ density decreases immediately after the application of RMPs but recovers soon during the ELM-suppressed phase.

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