Y. Nam

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.

Microwave imaging reflectometry for density fluctuation measurement on KSTAR

A dual-frequency microwave imaging reflectometry system was commissioned to measure both coherent and turbulent electron density fluctuations in KSTAR plasmas. Imaging of the density fluctuations is achieved with an array of 16 vertically aligned detectors and two X-mode probe beam frequencies (tunable over 78–92 GHz between plasma discharges). The system provides the capability of fluctuation measurements with poloidal wavenumbers (kθ) up to ~3 cm−1 at the maximum sampling rate of 2 MHz. Following extensive laboratory tests, the system was further tested with known coherent density fluctuations during the precursor oscillation of the m/n = 1/1 internal kink mode. The phase information of the reflected beam was compared with the precursor oscillation of the electron temperature measured by an electron cyclotron emission (ECE) radiometer. Density fluctuation levels (δne/ne) at two radial positions separated by the inversion radius (inside and outside) were comparable to temperature fluctuation levels (δTe/Te) from ECE signals. Subsequently, two correlation analysis methods were applied to turbulent fluctuation measurements in a neutral beam heated L-mode plasma to determine the mean poloidal rotation velocities of density fluctuations at two radial positions. The measured mean poloidal velocities were ~8.4 km s−1 at r/a ~ 0.6 and ~5 km s−1 at r/a ~ 0.7 in the clockwise direction, which differed by 1–2 km s−1 with the projected poloidal velocities from the toroidal rotation velocity measured by charge exchange recombination spectroscopy.

Dynamics of multiple flux tubes in sawtoothing KSTAR plasmas heated by electron cyclotron waves: I. Experimental analysis of the tube structure

Multiple (two or more) flux tubes are commonly observed inside and/or near the q = 1 flux surface in KSTAR tokamak plasmas with localized electron cyclotron resonance heating and current drive (ECH/CD). Detailed 2D and quasi-3D images of the flux tubes obtained by an advanced imaging diagnostic system showed that the flux tubes are m/n = 1/1 field-aligned structures co-rotating around the magnetic axis. The flux tubes typically merge together and become like the internal kink mode of the usual sawtooth, which then collapses like a usual sawtooth crash. A systematic scan of ECH/CD beam position showed a strong correlation with the number of flux tubes. In the presence of multiple flux tubes close to the q = 1 surface, the radially outward heat transport was enhanced, which explains naturally temporal changes of electron temperature. We emphasize that the multiple flux tubes are a universal feature distinct from the internal kink instability and play a critical role in the control of sawteeth using ECH/CD.

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.

Post calibration of the two-dimensional electron cyclotron emission imaging instrument with electron temperature characteristics of the magnetohydrodynamic instabilities

The electron cyclotron emission imaging (ECEI) instrument is widely used to study the local electron temperature (Te) fluctuations by measuring the ECE intensity IECE ∝ Te in tokamak plasmas. The ECEI measurement is often processed in a normalized fluctuation quantity against the time averaged value due to complication in absolute calibration. In this paper, the ECEI channels are relatively calibrated using the flat Te assumption of the sawtooth crash or the tearing mode island and a proper extrapolation. The 2-D relatively calibrated electron temperature (Te,rel) images are reconstructed and the displacement amplitude of the magnetohydrodynamic modes can be measured for the accurate quantitative growth analysis.

Dynamics of multiple flux tubes in sawtoothing KSTAR plasmas heated by electron cyclotron waves: II. Theoretical and numerical analysis

The dynamics of multiple closed flux tubes in the core of a sawtoothing tokamak plasma are studied using nonlinear simulations. This is motivated by recent observations of long-lived hot spots in the electron cyclotron emission (ECE) images of KSTAR plasmas with electron cyclotron heating (ECH) (Yun et al 2012 Phys. Rev. Lett. 109 145003). Using an empirical source term in a reduced set of MHD equations, it is shown that flux tubes with helicity h?=?1 are easily produced and survive for the observed time intervals only if the safety factor is close to unity (|q???1|???0.5%) and the magnetic shear is small (|s|???1). This suggests that sawteeth in KSTAR leave behind wide regions where q???1. On the basis of the relevant time scales, we discuss how this magnetic geometry and the spatial localization of the EC resonance may allow ECH to actively induce the formation of flux tubes. Using simulations with q profiles that possess a wide q?=?1 region inside the sawtooth inversion radius, we examine how the flux tubes merge and annihilate, and how their dynamics depend on the strength of the drive. The phenomena seen in the simulations and experiments lead us to conclude that, during the sawtooth ramp phase, there is a dynamic competition between sources and sinks of thermal and magnetic energy, where the flux tubes may play an important role; both as carriers of and channels for energy. The development of self-consistent simulation models is motivated and directions for future experiments are given.

Design of the reflective optics for Tore Supra ECEI system.

A 2D electron cyclotron emission (ECE) imaging system for Tore Supra is under design for studying the MHD physics of the magnetically confined plasma such as sawteeth, tearing modes, and turbulent fluctuations. Complex beam path due to the tight access in Tore Supra led to the design of reflective optics made of 6 or more large cylindrical∕flat mirrors. The total path length of the ECE beam is about 11 m, including almost 4 m inside the vacuum vessel. The imaging property of the optics has been estimated using the Gaussian beam simulation and ray transfer analysis. The possible setups for the optical alignment of the diagnostic and the operation scenarios with single- or dual-array measurement system are discussed.

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.

New compact and efficient local oscillator optic system for the KSTAR electron cyclotron emission imaging system

Electron cyclotron emission imaging (ECEI) diagnostic on Korean Superconducting Tokamak Advanced Research utilizes quasi-optical heterodyne-detection method to measure 2D (vertical and radial) Te fluctuations from two toroidally separated poloidal cross section of the plasma. A cylindrical lens local oscillator (LO) optics with optical path length (OPL) 2-2.5 m has been used in the current ECEI system to couple the LO source to the 24 vertically aligned array of ECE detectors. For efficient and compact LO optics employing the Powell lens is proposed so that the OPL of the LO source is significantly reduced from ∼2.0 m to 0.4 m with new optics. The coupling efficiency of the LO source is expected to be improved especially at the edge channels. Results from the optical simulation together with the laboratory test of the prototype optics will be discussed in this paper.

Comparative study between the reflective optics and lens based system for microwave imaging system on KSTAR

Recently, two-dimensional microwave imaging diagnostics such as the electron cyclotron emission imaging (ECEI) system and microwave imaging reflectometry (MIR) have been developed to study magnetohydrodynamics instabilities and turbulence in magnetically confined plasmas. These imaging systems utilize large optics to collect passive emission or reflected radiation. The design of this optics can be classified into two different types: reflective or refractive optical systems. For instance, an ECEI/MIR system on the TEXTOR tokamak [Park et al., Rev. Sci. Instrum. 75, 3787 (2004)] employed the reflective optics which consisted of two large mirrors, while the TEXTOR ECEI upgrade [B. Tobias et al., Rev. Sci. Instrum. 80, 093502 (2009)] and systems on DIII-D, ASDEX-U, and KSTAR adopted refractive systems. Each system has advantages and disadvantages in the standing wave problem and optical aberrations. In this paper, a comparative study between the two optical systems has been performed in order to design a MIR system for KSTAR.