Electromagnetic radiation at the plasma frequency and/or its second harmonic, the so-called plasma emission, is widely accepted as the fundamental process responsible for solar type II and III radio bursts. There have also been occasional observations of higher-harmonic plasma emissions in the solar-terrestrial environment. This paper presents the first demonstration of multiple harmonic emission by means of twodimensional electromagnetic particle-in-cell simulation. This finding indicates that under certain circumstances the traditional mechanism of fundamental-harmonic pair emission might also be accompanied by higher-harmonic components. Consequently, the present findings are highly relevant to in situ observations of third-and/or higher-harmonic plasma emission in astrophysical and solar-terrestrial environments.

Abstract. Global, heat flux-driven ITG gyrokinetic simulations which manifest the formation of macroscopic, mean toroidal flow profiles with peak thermal Mach number 0.05, are reported. Both a particle-in-cell (XGC1p) and a semi-Lagrangian (Gysela) approach are utilized without a priori assumptions of scale-separation between turbulence and mean fields. Flux-driven ITG simulations with different edge flow boundary conditions show in both approaches the development of net unidirectional intrinsic rotation in the co-current direction. Intrinsic torque is shown to scale approximately linearly with the inverse scale length of the ion temperature gradient. External momentum input is shown to effectively cancel the intrinsic rotation profile, thus confirming the existence of a local residual stress and intrinsic torque. Fluctuation intensity, intrinsic torque and mean flow are demonstrated to develop inwards from the boundary. The measured correlations between residual stress and two fluctuation spectrum symmetry breakers, namely E × B shear and intensity gradient, are similar. Avalanches of (positive) heat flux, which propagate either outwards or inwards, are correlated with avalanches of (negative) parallel momentum flux, so that outward transport of heat and inward transport of parallel momentum are correlated and mediated by avalanches. The probability distribution functions of the outward heat flux and the inward momentum flux show strong structural similarity.

We report the status of hybrid scenario experiments in Korea Superconducting Tokamak Advanced Research (KSTAR). The hybrid scenario is defined as stationary discharges with β N ⩾ 2.4 and H 89 ⩾ 2.0 at q 95 < 6.5 without or with very mild sawtooth activities in KSTAR. It is being developed towards reactor-relevant conditions. High performance of β N ≲ 3.0, H 89 ≲ 2.4 and G-factor (≡ β N H 89 /q 2 95 ) ≲ 0.46 has been achieved and sustained for ≳ 40τ E at n e /n GW ~0.7 with heating power of ≲5 MW. Some KSTAR hybrid discharges exhibit a unique feature of a slow transition from conventional H-mode to hybrid mode after the third neutral beam injection. The reason for the confinement enhancement is extensively studied in this transition period of a representative discharge exhibiting a common feature of KSTAR hybrid scenarios. 0D performance analysis with magnetohydrodynamic activities, 1D kinetic profile dynamics, power balance analysis, linear gyro-kinetic analysis and edge pedestal stability analysis were conducted. The enhancement is thought to be from both the core and the pedestal. The improvement in the core region of the ion energy channel is observed from the linear gyro-kinetic analysis considering the electromagnetic, the fast ion, the Shafranov shift, ω E×B , and the magnetic shear effect. The electromagnetic finite β stabilisation plays a role in the inner core region at ρ tor ∼ 0.35 together with the fast ion effect. The alpha stabilisation effect is also found at ρ tor ∼ 0.5. ω E×B , which could reduce the linear growth of the ion temperature gradient mode in the outer core region at ρ tor ∼ 0.5 − 0.7 with the highest contribution from the toroidal rotation. Regarding the improvement in the pedestal, Shafranov shift broadens the stability boundary of the pedestal in support of the diamagnetic effect. The pedestal height and width could be reproduced by the EPED model, while a realistic current profile is used to calculate the internal inductance for Shafranov shift. Based on these findings, a comprehensive confinement enhancement mechanism has been proposed by considering the core-edge interplay.

A study of intrinsic rotation and hysteresis in ion thermal internal transport barrier (ITB) is presented. Global flux-driven gyrofluid simulations are performed. It is found that significant co-current intrinsic rotation (0.1 ≲ M th ≲ 0.2, where M th is the thermal Mach number) can be produced in ITB plasmas. Exploration of the relationship between the intrinsic rotation and the ITB temperature gradient leads to a novel scaling of intrinsic rotation in ITB plasmas. Long time power ramp simulations with self-consistently evolving profiles clearly demonstrate the existence of hysteresis in reversed shear ITBs. It is shown that intrinsic rotation plays an important role in ITB dynamics and is responsible for determining unique properties of ITB hysteresis. A negative feedback mechanism based on destruction of E × B shear prevails in barrier back transition, triggered by an outward momentum transport event during the power ramp down.

One of key objectives of tokamak experiments is the exploration of enhanced confinement regimes, and the access of the internal transport barrier (ITB) formation is dealt with an important physics issue in the most of major tokamaks. Also, the advanced tokamak scenario with ITB is expected to lead to a continuous reactor with high fusion power density. From that point of view, the formation of the ITB in KSTAR which is designed for long pulse operation capability is very important although its heating and current drive systems are not fully equipped yet. We have therefore assumed that an early injection of the full NBI power (∼5.5 MW) during the current ramp-up would give a chance to form an internal barrier if the plasma could stay in the L-mode. To avoid the H-mode transition, we have produced inboard limited plasmas with detaching from the both upper and lower divertors. Using this approach, an ITB formation during L-mode has been observed which shows improved core confinement. Ion and electron temperature profiles show the barrier clearly in the temperature, and it was sustained for about 7 s in the dedicated experiment. This is the first stationary ITB observed in a full superconducting tokamak. This operation scenario with the ITB could be an alternative way to achieve a high performance regime in KSTAR, and the length of the ITB discharge could be extended even longer. In this paper, we present the formation of the ITB using measured and simulated characteristic profiles.