J. Leem

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

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.

Poloidal rotation velocity measurement with an MIR system on KSTAR

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.

Evaluation of the imaging properties of Microwave Imaging Reflectometry

Microwave Imaging Reflectometry (MIR) has been developed for unambiguous measurement of electron density fluctuations in fusion plasmas. The loss of phase information limiting the use of conventional reflectometry can be minimized by a large aperture imaging optics and an array of detectors in the MIR embodiment. The evaluation of the optical system is critical for precise reconstruction of the fluctuations. The optical systems of the prototype TEXTOR MIR [2] and newly-designed KSTAR MIR [5] systems have been tested with a corrugated target simulating density fluctuations at the cut-off surface. The reconstructed phase from the MIR system has been compared to the directly measured phase of corrugations taking into account the rotational speed of the target. The effects of optical aberrations and interference between lenses on the phase reconstruction have been investigated by the 2D amplitude measurement of the reflected waves and the diffraction-based optical simulations. (CODE V) A preliminary design of the KSTAR MIR optics has been suggested which can minimize the aberration and interference effects.

Design of a collective scattering system for small scale turbulence study in Korea Superconducting Tokamak Advanced Research

The design characteristics of a multi-channel collective (or coherent) scattering system for small scale turbulence study in Korea Superconducting Tokamak Advanced Research (KSTAR), which is planned to be installed in 2017, are given in this paper. A few critical issues are discussed in depth such as the Faraday and Cotton-Mouton effects on the beam polarization, radial spatial resolution, probe beam frequency, polarization, and power. A proper and feasible optics with the 300 GHz probe beam, which was designed based on these issues, provides a simultaneous measurement of electron density fluctuations at four discrete poloidal wavenumbers up to 24 cm(-1). The upper limit corresponds to the normalized wavenumber kθρe of ∼0.15 in nominal KSTAR plasmas. To detect the scattered beam power and extract phase information, a quadrature detection system consisting of four-channel antenna/detector array and electronics will be employed.

Ion gyroscale fluctuation measurement with microwave imaging reflectometer on KSTAR.

Ion gyroscale turbulent fluctuations with the poloidal wavenumber kθ ∼ 3 cm-1 have been measured in the core region of the neutral beam (NB) injected low confinement (L-mode) plasmas on Korea superconducting tokamak advanced research. The turbulence poloidal wavenumbers are deduced from the frequencies and poloidal rotation velocities in the laboratory frame, measured by the multichannel microwave imaging reflectometer. Linear and nonlinear gyrokinetic simulations also predict the unstable modes with the normalized wavenumber kθρs ∼ 0.4, consistent with the measurement. Comparison of the measured frequencies with the intrinsic mode frequencies from the linear simulations indicates that the measured ones are primarily due to the E × B flow velocity in the NB-injected fast rotating plasmas.

Development of fast RF spectrometer system for MHD detection

During the 2010 KSTAR plasma experiment, unidentified spiky RF pulses ( < 3 GHz, ~ 10 μs) were detected by an electron cyclotron emission imaging (ECEI) system at the occurrence of large MHD instabilities such as edge localized modes (ELMs). A fast RF spectrometer system has been constructed for direct detection of the RF radiation in the range of 0.5 ~ 3.0 GHz to study the spectral evolution of the RF spikes in more detail.

Estimation of the radial size and density fluctuation amplitude of edge localized modes using microwave interferometer array

A novel technique to estimate the range of radial size and density fluctuation amplitude of edge localized modes (ELMs) in the KSTAR tokamak plasma is presented. A microwave imaging reflectometry (MIR) system is reconfigured as a multi-channel microwave interferometer array (MIA) to measure the density fluctuations associated with ELMs, while electron cyclotron emission imaging (ECEI) system is used as a reference diagnostics to confirm the MIA observation. Two dimensional full-wave (FWR2D) simulations integrated with optics simulation are performed to investigate the Gaussian beam propagation and reflection through the plasma as well as the MIA optical components and obtain the interferometric phase undulations of individual channels at the detector plane due to ELM perturbation. The simulation results show that the amplitude of the phase undulation depends linearly on both radial size and density perturbation amplitude of ELM. For a typical discharge with ELMs, it is estimated that the ELM structure observed by the MIA system has density perturbation amplitude in the range ~ 7 % to 14 % while radial size in the range ~ 1 to 3 cm.

Microwave imaging reflectometry for KSTAR

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 cut-off 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.