[Show abstract][Hide abstract] ABSTRACT: This paper overviews recent progress on the experimental identification and physics interpretation of 3D effects of magnetic field geometry on divertor transport. The 3D effects are elucidated as a consequence of competition between transports parallel (||) and perpendicular (⊥) to magnetic field, in open field lines cut by divertor plates, or in magnetic islands. The competition has strong impacts on divertor functions, such as determination of density regime, impurity screening, and detachment control. The effects of magnetic perturbation on the edge electric field and turbulent transport are also discussed. Based on the experiments and numerical simulations, key parameters governing the 3D transport physics for the individual divertor functions, e.g. pumping efficiency through divertor density regime, impurity screening and detachment control, are discussed.
[Show abstract][Hide abstract] ABSTRACT: Abstract Two-dimensional (2-D) distribution of impurity line emissions has been measured with 2-D extreme ultraviolet (EUV) spectroscopy in Large Helical Device (LHD) for studying the edge impurity transport in stochastic magnetic field layer with three-dimensional (3-D) structure. The impurity behavior in the vicinity of two X-points at inboard and outboard sides of the toroidal plasma can be separately examined with the 2-D measurement. As a result, it is found that the carbon location changes from inboard to outboard X-points when the plasma axis is shifted from Rax = 3.6 m to 3.75 m. A 3-D simulation with EMC3-EIRENE code agrees with the result at Rax = 3.75 m but disagreed with the result at Rax = 3.60 m. The discrepancy between the measurement and simulation at Rax = 3.60 m is considerably reduced when an effect of neutral hydrogen localized in the inboard side is taken into account, which can modify the density gradient and friction force along the magnetic field.
[Show abstract][Hide abstract] ABSTRACT: In this study, corrosion behavior of 2B06 aluminum alloy was investigated after exposure to a tropical marine atmosphere for up to 4 years. After 6 months, the specimen showed exfoliation corrosion as well as rapid increase in thickness loss and corrosion rate. Exfoliation corrosion was found to initiate from hydrogen-assisted intergranular cracks and propagate extensively due to the wedge effect of the corrosion products. During the exposure test, corrosion on the groundward surface was considerably more severe than that on the skyward surface, which could be attributed to the different exposure conditions on the two surfaces.
Journal of Materials Engineering and Performance 01/2015; 24(1). DOI:10.1007/s11665-014-1258-3 · 1.00 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: In recent experiments at the HL-2A tokamak, dynamic features across the low─intermediate─high (L─I─H) confinement transition have been investigated in detail. Experimental evidence shows two types of opposite limit cycles (dubbed type-Y and type-J) between the radial electric field (Er) and turbulence evolution during the intermediate I-phase. Whereas for type-Y the turbulence grows prior to the change in Er, for type-J the oscillation in Er leads turbulence. It has been found that the type-Y usually appears first after an L─I transition, followed by type-J before the transition to the H-mode phase. Possible roles played by zonal flows and the enhanced pressure-gradient-induced flow shear in suppressing turbulence, respectively, in the type-Y and type-J periods have been identified. In addition, during the I-phase of the L─I─H discharges a kink-type MHD mode routinely occurs and crashes rapidly just prior to the I → H transition. The mode crash evokes substantial energy release from the core to plasma boundary and further increases the edge pressure gradient and Er shear, which eventually results in confinement improvement into the H-mode.
[Show abstract][Hide abstract] ABSTRACT: Two groups of frequency sweeping modes are observed and interpreted in the HL-2 A plasmas with q min ∼ 1. The tokamak simulation code calculations indicate the presence of a reversed shear q-profile during the existence of these modes. The mode frequencies lie in between TAE and BAE frequencies, i.e. ω BAE ω ω TAE, and these modes are highly localized near q min, i.e. r/a ∼ 0.25. A group of modes characterized by down-sweeping frequency with q min decrease due to q min > 1 and nq min − m > 0, and another group of modes characterized by up-sweeping frequency with q min drop, owing to q min nq min − m q-profile measurements.
[Show abstract][Hide abstract] ABSTRACT: AC corrosion of the X80 and X100 steels in 0.1 M NaCl solution were studied by the AC voltammetry technique. Corrosion electrochemical kinetics and solid/solution interface structure changes under the influence of AC voltage were characterized. Results illustrate that corrosion potential of the two steels shift negatively with the increase of AC amplitude and decrease of AC frequency. The anodic processes are under charge-transfer control and the anodic Tafel slopes increase with the increase of AC magnitude. The cathodic processes are under diffusion control at low AC amplitudes, while they become increasingly under charge-transfer control with higher AC amplitudes.
[Show abstract][Hide abstract] ABSTRACT: Subject to Gleeble processing, microstructures of weld heat affected zone were simulated in X80 steel. Corrosion behavior of the simulated specimen with a microstructure gradient was studied by polarization, local electrochemical impedance spectroscopy and scanning vibrating electrode technique. Microstructure of granular bainite mixed with ferrite (region B) showed the highest charge transfer resistance and the most positive current density value. Acicular ferrite base metal displayed the lowest charge transfer resistance and the most negative current density. Both the positive and negative peak current densities increased at the first few hours of immersion followed by a decrease.
[Show abstract][Hide abstract] ABSTRACT: Corrosion behaviour of AZ31 magnesium during the initial six exposure periods in a tropical marine atmosphere is investigated. The results reveal that corrosion process of magnesium is dominated by pitting corrosion which consists of initiation of new pits, propagation of small scale pits and coalescence of neighbouring pits. There exists a critical depth above which the pits cease to grow down, resulting in the fluctuation of the mean pit depth. Different exposure conditions are found to be crucial for the different pit characters. Pits on the skyward surface are in deep-hole shape, while the groundward surface is covered with shallow dish pits.
[Show abstract][Hide abstract] ABSTRACT: Linear regularization has been applied to the HL-2A infrared imaging bolometer to reconstruct local plasma emission with one-dimensional (1D) and three-dimensional (3D) modeling under the assumption of toroidal symmetry. In the 3D modeling, a new method to calculate the detector point response function is introduced. This method can be adapted to an arbitrarily shaped pinhole. With the full 3D treatment of the detector geometry, up to 50% of the mean-squared error is reduced compared with the 1D modeling. This is attributed to the effects of finite detector size being taken into account in the 3D modeling. Meanwhile, the number of the bolometer pixels has been optimized to 20 × 20 by making a trade-off between the number of bolometer pixels and the sensitivity of the system. The plasma radiated power density distributions have been calculated as a demonstration using 1D modeling and 3D modeling, respectively.
[Show abstract][Hide abstract] ABSTRACT: The frequency modulated continuous wave reflectometer was developed for the first time on the HL-2A tokamak. The system utilizes a voltage controlled oscillator and an active multiplier for broadband coverage and detects as heterodyne mode. Three reflectometers have been installed and operated in extraordinary mode polarization on HL-2A to measure density profiles at low field side, covering the Q-band (33-50 GHz), V-band (50-75 GHz), and W-band (75-110 GHz). For density profile reconstruction from the phase shift of the probing wave, a corrected phase unwrapping method is introduced in this article. The effectiveness of the method is demonstrated. The density profile behavior of a fast plasma event is presented and it demonstrates the capability of the reflectometer. These diagnostics will be contributed to the routine density profile measurements and the plasma physics study on HL-2A.
The Review of scientific instruments 02/2014; 85(1):013507. DOI:10.1063/1.4861918 · 1.61 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Geodesic acoustic mode (GAM) and low-frequency zonal flow (LFZF) are
both observed through Langmuir probe arrays during electron cyclotron
resonance heating (ECRH) on the HL-2A tokamak edge. The radial
distributions of the amplitude and peak frequency of GAM in floating
potential fluctuations are investigated through rake probe arrays under
different ECRH powers. It is observed that the GAM frequency would
decrease and the intensity of carbon line emission would increase as the
ECRH power exceeds a certain threshold. The analyses suggest that the
impurity ions may play an important role in the GAM frequency at the
edge region. It is also found that during the ECRH phase besides the
mean flow, both GAM and LFZF are strengthened. The total fluctuation
power and the fraction of that power associated with zonal flows both
increase with the ECRH power, consistent with a predator-prey
model. The auto- and cross-bicoherence analyses show the coupling
between GAM and its second harmonic during the ECRH phase. Moreover, the
results also suggest that the couplings between GAM and the components
with multiple GAM frequency are strengthened. These couplings may be
important for GAM saturation during the ECRH phase.
[Show abstract][Hide abstract] ABSTRACT: A 2D electron cyclotron emission imaging (ECEI) system has been developed for measurement of electron temperature fluctuations in the HL-2A tokamak. It is comprised of a front-end 24 channel heterodyne imaging array with a tunable RF range spanning 75-110 GHz, and a set of back-end ECEI electronics that together generate 24 × 8 = 192 channel images of the 2nd harmonic X-mode electron cyclotron emission from the HL-2A plasma. The simulated performance of the local oscillator (LO) optics and radio frequency (RF) optics is presented, together with the laboratory characterization results. The Gaussian beams from the LO optics are observed to properly cover the entire detector array. The ECE signals from the plasma are mixed with the LO signal in the array box, then delivered to the electronics system by low-loss microwave cables, and finally to the digitizers. The ECEI system can achieve temporal resolutions of ∼μs, and spatial resolutions of 1 cm (radially) and 2 cm (poloidally).
The Review of scientific instruments 11/2013; 84(11):113501. DOI:10.1063/1.4828671 · 1.61 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: For the first time supersonic molecular beam injection (SMBI) and cluster jet injection (CJI) were applied to mitigate edge-localized modes (ELMs) in HL-2A successfully. The ELM frequency increased by a factor of 2-3 and the heat flux on the divertor target plates decreased by 50% on average after SMBI or CJI. Energetic particle induced modes were observed in different frequency ranges with high-power electron cyclotron resonance heating (ECRH). The high frequency (200-350 kHz) of the modes with a relatively small amplitude was close to the gap frequency of the toroidicity-induced Alfven eigenmode. The coexistent multi-mode magnetic structures in the high-temperature and low-collision plasma could affect the plasma transport dramatically. Long-lived saturated ideal magnetohydrodynamic instabilities during strong neutral beam injection heating could be suppressed by high-power ECRH. The absolute rate of nonlinear energy transfer between turbulence and zonal flows was measured and the secondary mode competition between low-frequency (LF) zonal flows (ZFs) and geodesic acoustic modes (GAMs) was identified, which demonstrated that ZFs played an important role in the L-H transition. The spontaneously generated E x B shear flow was identified to be responsible for the generation of a large-scale coherent structure (LSCS), which provided unambiguous experimental evidence for the LSCS generation mechanism. New meso-scale electric potential fluctuations (MSEFs) at frequency f similar to 10.5 kHz with two components of n = 0 and m/n = 6/2 were also identified in the edge plasmas for the first time. The MSEFs coexisted and interacted with magnetic islands of m/n = 6/2, turbulence and LF ZFs.
[Show abstract][Hide abstract] ABSTRACT: Edge impurity transport is studied in electron cyclotron resonance heating (ECRH) L-mode plasmas of the HL-2A tokamak based on space-resolved vacuum ultraviolet spectroscopy with which radial profiles of impurity line emissions are measured from the core region inside the last closed flux surface (LCFS) and the edge region in the scrape-off layer, simultaneously. The radial profile of carbon emissions of C V (2271 Å: 1s2s 3S–1s2p 3P) reconstructed into the local emissivity profile is analysed with a one-dimensional impurity transport code, and the diffusion coefficient and convective velocity of impurity ions are determined in the core region of the HL-2A tokamak. The impurity source is also determined with the measured absolute emissivity profiles of C IV (1548 Å: 1s22s 2S–1s22p 2P) located at the LCFS. The ratio of C V to C IV can therefore be used as an index to characterize the core impurity transport between the LCFS and the radial region of the C V emission at a normalized radius of about ρ = 0.6. The ratio measured from ohmic discharges shows a gradual decrease with electron density. However, the ratio suddenly decreases by a factor of three when the ECRH focused in the plasma centre is switched on, suggesting a strong enhancement of the impurity transport. The analysis with the transport code indicates a change in the convective term. The convective velocity of C4+ ions changes from inward to outward direction during the ECRH phase, while an inward velocity usually exists in the ohmic phase. Possible mechanisms for the reversal of the convective velocity are discussed.