Quasi 3D ECE imaging system for study of MHD instabilities in KSTAR

A second electron cyclotron emission imaging (ECEI) system has been installed on the KSTAR tokamak, toroidally separated by 1/16th of the torus from the first ECEI system. For the first time, the dynamical evolutions of MHD instabilities from the plasma core to the edge have been visualized in quasi-3D for a wide range of the KSTAR operation (B0 = 1.7???3.5 T). This flexible diagnostic capability has been realized by substantial improvements in large-aperture quasi-optical microwave components including the development of broad-band polarization rotators for imaging of the fundamental ordinary ECE as well as the usual 2nd harmonic extraordinary ECE.open1

Mesoscopic transport in KSTAR plasmas: avalanches and the E×BE \times B staircase

The self-organization is one of the most interesting phenomena in the non-equilibrium complex system, generating ordered structures of different sizes and durations. In tokamak plasmas, various self-organized phenomena have been reported, and two of them, coexisting in the near-marginal (interaction dominant) regime, are avalanches and the $E \times B$ staircase. Avalanches mean the ballistic flux propagation event through successive interactions as it propagates, and the $E \times B$ staircase means a globally ordered pattern of self-organized zonal flow layers. Various models have been suggested to understand their characteristics and relation, but experimental researches have been mostly limited to the demonstration of their existence. Here we report detailed analyses of their dynamics and statistics and explain their relation. Avalanches influence the formation and the width distribution of the $E \times B$ staircase, while the $E \times B$ staircase confines avalanches within its mesoscopic width until dissipated or penetrated. Our perspective to consider them the self-organization phenomena enhances our fundamental understanding of them as well as links our findings with the self-organization of mesoscopic structures in various complex systems

Characteristics of turbulence spreading and zonal flow near magnetic island in electrostatic gyrokinetic simulations

We present characteristics of turbulence spreading and zonal flow near magnetic island in electrostatic simulations of a gyro-kinetic code, XGC1. This work was motivated by recent ECEI measurements in dedicated KSTAR experiment employing resonant magnetic field perturbation(RMP). From the experiment, it was found that magnetic island induced by RMP can impact fluctuations and flows, and consequently electron thermal transport around the island[1]. A subsequent simulation study showed that 3D RMP field can enhance equilibrium ExB flow strong enough to suppress ambient micro-instabilities[2]. In this talk, we present more comprehensive nonlinear simulations with all relevant neoclassical and turbulence physics in the KSTAR experimental condition. In the simulations, we explore turbulence evolution around and inside (2,1) magnetic island. Unlike linear analysis results, fluctuation penetrates the strong ExB shearing layer around the island and the electron temperature profile inside has a finite gradient. We also investigate how flow evolution around the island affect ambient transport

Improved accuracy in the estimation of the tearing mode stability parameters (Delta ' and w(c)) using 2D ECEI data in KSTAR

The accuracy in estimation of two important tearing mode stability parameters (Delta' and w(c)) is improved by employing two-dimensional (2D) ECE imaging data which help one to overcome the resolution limit of conventional one-dimensional data. The experimentally measured 2D images are directly compared with synthetic ones from a tearing mode T-e model to estimate the parameters and an excellent agreement is achieved. The results imply that the observed tearing mode is classically stable but has non-negligible bootstrap current drive.close2

Experimental observation of the non-diffusive avalanche-like electron heat transport events and their dynamical interaction with the shear flow structure

We present experimental observations suggesting that non-diffusive avalanche-like transport events are a prevalent and universal process in the electron heat transport of tokamak plasmas. They are observed in the low confinement mode and the weak internal transport barrier plasmas in the absence of magnetohydrodynamic instabilities. In addition, the electron temperature profile corrugation, which indicates the existence of the E x B shear flow layers, is clearly demonstrated as well as their dynamical interaction with the avalanche-like events. The measured width of the profile corrugation is around 45(rho i), implying the mesoscale nature of the structure

Overview of recent progress in 3D field physics in KSTAR

Various 3D field physics challenges of magnetically confined plasmas arise when the driving source comes from either externally applied non-axisymmetric 3D magnetic perturbations or plasma instabilities inside the plasma. Recently, several key outstanding topics of 3D field physics have been extensively studied in the Korean Superconducting Tokamak Advanced Research (KSTAR), such as edge-localized-mode (ELM) control by resonant magnetic perturbation (RMP), error field (EF) control, 3D field effects on rotation and transport, and RMP-induced alteration of divertor heat flux and detachment. KSTAR has a few physically unique features (i.e., high rotation and long-pulse plasmas with a low intrinsic EF) and machine/diagnostic capabilities (i.e., 3-row in-vessel control coil and state-of-the-art 2D/3D imaging diagnostics), which have been taken advantage of until now to address critical 3D field physics issues relevant to ITER and K-DEMO. Among many remarkable achievements are the robust access to and control of n = 1 RMP ELM suppression, along with a development of its physics basis tools, parameter expansion, optimization, and long-pulse control techniques. Nonetheless, a series of unresolved 3D physics themes, as well as limited coverage of 3D field operating regimes, have also been identified as future works for the 3D field research in KSTAR. In this paper, we provide an overview about the recent progress of KSTAR 3D field physics and present future plans of KSTAR 3D research toward a future fusion reactor