Negative magnetic shear is found to suppress electron turbulence and improve electron thermal transport for plasmas in the National Spherical Torus Experiment (NSTX). Sufficiently negative magnetic shear results in a transition out of a stiff profile regime. Density fluctuation measurements from high-k microwave scattering are verified to be the electron temperature gradient (ETG) mode by matching measured rest frequency and linear growth rate to gyrokinetic calculations. Fluctuation suppression under negligible E×B shear conditions confirm that negative magnetic shear alone is sufficient for ETG suppression. Measured electron temperature gradients can significantly exceed ETG critical gradients with ETG mode activity reduced to intermittent bursts, while electron thermal diffusivity improves to below 0.1 electron gyro-Bohms.

Dual (or sometimes multiple) flux tubes (DFTs) have been observed in the core of sawtoothing KSTAR tokamak plasmas with electron cyclotron resonance heating. The time evolution of the flux tubes visualized by a 2D electron cyclotron emission imaging diagnostic typically consists of four distinctive phases: (1) growth of one flux tube out of multiple small flux tubes during the initial buildup period following a sawtooth crash, resulting in a single dominant flux tube along the m/n=1/1 helical magnetic field lines, (2) sudden rapid growth of another flux tube via a fast heat transfer from the first one, resulting in approximately identical DFTs, (3) coalescence of the two flux tubes into a single m/n=1/1 flux tube resembling the internal kink mode in the normal sawteeth, which is explained by a model of two current-carrying wires confined on a flux surface, and (4) fast localized crash of the merged flux tube similar to the standard sawtooth crash. The dynamics of the DFTs implies that the internal kink mode is not a unique prerequisite to the sawtooth crash, providing a new insight on the control of the sawtooth.

A multi-channel microwave imaging reflectometry (MIR) system has been commissioned in the 2012 and 2013 KSTAR campaigns for the measurement of semi 2D (16 poloidal and 2 radial channels) electron density fluctuations for transport study on KSTAR. A time delayed cross correlation analysis among 16 poloidal channels has been applied to obtain the poloidal rotation velocities of the measured turbulent density fluctuations. The measured poloidal rotation directions for an 170 GHz ECH assisted ohmic plasma was in opposite direction to that of a neutral beam (NB) heated L-mode plasma. This is due to the fact that the intrinsic toroidal rotation in ohmic plasma (counter-clockwise) is in opposite direction to the NB heated plasma (clockwise) with respect to the plasma current direction.

A microwave imaging reflectometry (MIR) system with two probing frequencies is being developed for 2D measurement of electron density fluctuations for KSTAR plasmas. The two-frequency probe beam enables simultaneous measurement of density fluctuations at two cutoff layers. Laboratory test of two approaches (reflective and refractive optics) has suggested an extreme care is needed in designing the optics considering problems such as aberration issues and standing waves. Currently, an optics based on refractive element is being designed with the minimum number of lens. The detector system consists of array detectors and electronics for the fluctuation phase reconstruction. The two-frequency MIR system will be installed for the 2012 KSTAR campaign and will be expanded to five frequencies by 2014.

The design of an electron cyclotron emission imaging (ECEI) system for two-dimensional (2D) observation of the magnetohydrodynamical modes in high temperature ITER (from 'International Thermonuclear Experimental Reactor') H-mode-like plasmas (5.3 T and 25 keV) based on fundamental ordinary mode (O1-mode) and second-harmonic extraordinary mode (X2-mode) measurements is explored conceptually. For studying the spatial resolution in high temperature plasmas, the relativistic broadening and inward shift of the emission layer in the mid-plane are calculated. The radial spatial resolution is significantly degraded in the range R < 5.1 m for the O1-mode and in the range R < 6.9 m for the X2-mode. The region with R < 6.5 m is inaccessible for X2-mode study. The emission layer width is enlarged in a narrow region of the pedestal due to the magnetic field being modified by the large pressure gradient. The broadening and shift in the poloidal plane are also calculated, to investigate their effects on 2D measurements. The frequency range of electron cyclotron emission measurements is selected to protect the system from stray radiations of the 170 GHz electron cyclotron resonance heating source and to avoid harmonic overlap. The frequency ranges of 115-160 GHz for the O1-mode and 230-320 GHz for the X2-mode provide radial coverage of 5.9 < R < 8.2 m or −0.15 < r/a < 1. The ECEI system utilizes a dual-array detection technique which provides a simultaneous measurement at two radial positions, and each array has 8 by 16 (radial by vertical) channels. The radial image size with 8 channels is ∼41-76 cm for the O1-mode and ∼19-36 cm for the X2-mode, with sufficient resolution. The front-end optics, which focuses the electron cyclotron emission to the low loss corrugated transmission waveguides, is designed with two flat mirrors and two focusing mini-lens arrays. The vertical image size with 16 channels is ∼150 cm and the spot size of each channel is 8-15 cm in the plasma region, taking into account the sensitivity pattern of the waveguide. The refraction effect due to inhomogeneous plasma enlarges the vertical image size up to 20% and 5% for the O1-mode and X2-mode cases, respectively. The horizontal distortion due to the relativistic inwards shift is reduced by the increased toroidal field in the core region.