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The effect of impurity ions on the coupling between geodesic acoustic mode (GAM) zonal flow and local turbulent transport has been studied using a Langmuir probe array in HL-2A ohmically heated deuterium plasmas. The experimental results illustrate that both the frequency and amplitude level of the GAM zonal flow significantly reduce with increase in the carbon ion concentration, which is qualitatively consistent with theoretical predictions (Xie et al 2018 Plasma Phys. Control. Fusion 60 025015). Meanwhile less energy is transferred from turbulence to GAM zonal flow due to the reduction in tilting and stretching of the turbulent vortex. Consequently, the impurity ions enhance the turbulence and turbulent transport owing to the reduced GAM zonal flow, as demonstrated by experiment. The experimental results presented here therefore reveal the dual roles played by impurity ions in the dynamics of GAM zonal flow, which could contribute to the understanding of inherent mechanisms governing turbulent transport in the presence of impurity ions.

Purpose This paper aims to investigate the corrosion behavior of zinc in a typical hot and dry atmosphere. It proposes the dynamic corrosion for different exposure periods. Results can provide the basic data and corrosion mechanism of zinc in such environment. Design/methodology/approach In this paper, the authors investigated the corrosion behavior of pure zinc exposed in the typical hot and dry environment in Turpan for one-four years, which has never been studied. Scanning electron microscopy, laser scanning confocal microscopy, electron probe micro-analyzer (EPMA), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) were conducted to measure the corrosion morphology and products of zinc. Finally, combining electrochemical impedance spectroscopy and scanning Kelvin probe techniques, the corrosion mechanism of zinc in Turpan was examined. Findings The thickness loss of the zinc followed an exponential law with respect to exposure time: D = 3.17 t0.61, and both of the rust layer resistance and the charge transfer resistance increased with exposure time. The corrosion products mainly comprised ZnO, Zn(OH)2, Zn5(CO3)2(OH)6, Zn4SO4(OH)6·5H2O and Zn12(SO4)3Cl3(OH)15·5H2O. The Kelvin potentials shifted toward the positive direction from −0.380 to −0.262 V (vs saturated calomel electrode [SCE]) when the exposure time extended from one to four years and the distribution of the corrosion products became more and more uniform. Originality/value The corrosion behavior of pure zinc in the typical hot and dry environment in Turpan has not been studied. The dynamic corrosion for different exposure periods was obtained. The corrosion products were systemically investigated via energy-dispersive X-ray spectroscopy, EPMA, XPS and XRD.

The control of edge localized mode(ELM) in tokamaks has been intensively investigated with different techniques. Recently, ELM control with impurity seeding has also raised to be a hot topic. This paper reports the effect of the laser blow-off(LBO) impurity seeding on the pedestal turbulence and ELMs. It has been found that the seeded impurity could have dual impacts on pedestal instabilities, depending on the quantity of the injected impurity. The impurity enhances the pedestal turbulence and further mitigates the ELMs when its quantity exceeds a lower threshold. Instead, the impurities suppress the pedestal turbulence and suppress ELMs when the quantity of the injected particles reaches a second higher threshold. It has also been observed that the ion toroidal velocity increases due to the impurity seeding while the collision rate is increasing. In addition, the impurities reduce the velocity shear rate by changing the toroidal velocity term rather than by changing the pressure gradient term as observed in ELM mitigation with LHCD. The reduction of the E×B velocity shear rate enhances the pedestal turbulence possibly through the turbulence spectral shift process during ELM mitigation.

A space-resolved extreme ultraviolet (EUV) spectrometer system has been developed in the HL-2A tokamak for impurity monitoring and transport studies. A charge-coupled-device (CCD) detector is mounted on a two-stage wavelength scanning mechanism for measuring the spatial profile of impurity line emissions in the wavelength range of 30−600 A. A good spatial profile of impurity line emissions is observed throughout the lower half of the HL-2A plasma along the vertical direction by installing a space-resolved slit in front of an entrance slit. The vertical observation range is ∼43 cm covering a wide plasma area from the plasma central region to the vicinity of divertor separatrix X-point. In order to make a critical analysis of the impurity transport, in particular, in the plasma edge, an accurate position calibration in the observed vertical impurity line intensity profile is necessary. For the purpose a thin metal bar is installed at the diagnositc port. Then, the vertical position of the impurity profile can be accurately determined throughout the entire wavelength range. Based on the present method, the local emissivity profile of impurity line emissions is well reconstructed with good accuracy through Abel inversion technique. Resultant CIV and CVI local emissivity profiles show a clear difference in the carbon transport between ohmic and NBI heating phases.

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 R ax = 3.6 m to 3.75 m. A 3-D simulation with EMC3-EIRENE code agrees with the result at R ax = 3.75 m but disagreed with the result at R ax = 3.60 m. The discrepancy between the measurement and simulation at R ax = 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.

Tungsten ion densities are determined by measuring radial profiles of line emissions in LHD. Zn-(W44+) and Cu-like (W45+) ion densities are determined from line emission at 60.9 Å and 127.0 Å, respectively. Ion densities of W24+, W25+ and W26+ are also determined from tungsten pseudo-continuum (a so-called unresolved transition array (UTA)) at 29-33 Å. Observations of magnetic dipole (M1) forbidden lines at 3377Å enable evaluation of the ion density of W27+. In EAST tokamak the tungsten concentration is estimated by a combination of the UTA and total radiation loss. In HL-2A the tungsten influx is evaluated from line emission of W6+ ions at 216.2 Å and 261.4 Å, which are measured during the tungsten reappearance phase after tungsten laser blow-off. All these results demonstrate a good approach towards quantitative tungsten analysis.

In HL-2A, an inverse sawtooth oscillation is observed with a long-lasting m/n = 1/1 mode during ECRH phase with power deposition inside sawtooth inversion radius (inner-deposited ECRH), while a normal sawtooth instead appears when the ECRH power is deposited outside sawtooth inversion radius (outer-deposited ECRH). Aluminum is then injected as a trace impurity with laser blow-off (LBO) method into the inner- and outer-deposited ECRH phases of HL-2A discharges to investigate the effect of ECRH on impurity transport. Temporal behavior of soft x-ray (SXR) array signals is analyzed with a 1D impurity transport code, and radial structures of impurity transport coefficients are obtained. The result shows that the radial transport of Al ions is strongly enhanced during the inner-deposited ECRH phase. In particular, an outward convection velocity is developed with positive values of 0 ≤ V(ρ) ≤ 3.8 m s−1 in ρ ≤ 0.5, while the convection velocity is inward in ρ ≥ 0.6. In the outer-deposited ECRH discharge, on the other hand, the convection velocity takes a big negative value in ρ ≤ 0.4 and close to zero at ρ ~ 0.6. In ohmic discharges, an inward V(ρ) always appears in the whole plasma radii and gradually increases toward the plasma edge (−3.2 m s−1 at ρ = 1). The simulation result also indicates that centrally-peaked Al ion density profiles presented in the outer-deposited ECRH discharge can be flattened by the inner-deposited ECRH. Modification of impurity transport is discussed in the presence of long-lasting m/n = 1/1 MHD mode.

In this paper, the method of time-dependent fundamental solutions is extended to solving transient heat conduction problems in inhomogeneous media. The solution of the problem under investigation is split into two parts, namely the particular and homogenous solutions. The novelty of the proposed approach lies in that an approximation of the particular solution is derived by using the fundamental solutions of the associated eigenvalue equations. Numerical results for one- and two-dimensional geometries are presented to verify the efficacy of the proposed method. The effects of the numbers of source and collocation points, the eigenvalues and the parameter T on the accuracy of the numerical solution are also investigated.

Recently, Hon and Wei proposed a method of fundamental solutions for solving isotropic inverse heat conduction problems (IHCP). It provides an efficient global approximation scheme in both spatial and time domains. In this paper, we try to extend the inherently meshless and integration-free method to solve 2D IHCP in an anisotropic medium. First, we acquire the fundamental solution of the governing equation through variables transformation. Then the truncated singular value decomposition and the L-curve criterion are applied to solve the resulting matrix equation which is highly ill-conditioned. Results for several numerical examples are presented to demonstrate the efficiency of the method proposed. The relationship between the accuracy of the numerical solutions and the value of the parameter T is also investigated.

Excimer laser surface treatment was found to be an effective method for improving the pitting corrosion resistance of the aluminum alloy 7075. The results of the TEM study showed that laser surface melting of the alloy at an intensity of 10.3 J/cm2 resulted in the elimination of coarse second-phase particles in the laser-melted zone. More importantly, two compact layers containing aluminum oxide were formed on top of the laser-melted surface. Potentiodynamic polarization tests showed that as a result of the laser treatment, the corrosion current can be reduced by as much as six times, and a passive region was obtained. Besides, the analysis of the electrochemical impedance measurements showed that at an open-circuit potential (OCP), the polarization resistance and double-layer capacitance of the film/electrolyte interface of the laser-treated specimen were one order of magnitude higher and six times lower than that of the untreated specimen, respectively. Furthermore, when tested at OCP+50 mV, the untreated specimen suffered serious pitting corrosion, while a passive film had formed on the laser-treated specimen, which served as an effective barrier for reducing anodic dissolution.

Edge localized mode (ELM) is successfully mitigated by helium and deuterium supersonic molecular beam injections (SMBIs) on HL-2A. During the ELM mitigation by SMBIs, gradients of ion temperature (T i) and electron density are softened in the pedestal. It has been observed that the averaged gradient of the T i decreases around 44% and the well depth of radial electric field (E r) is reduced by the SMBI during ELM mitigation. Furthermore, at least 20% decrements of T i have to be attained to achieve a noticeable increase (decrease) of the ELM frequency (amplitude). In addition, the duration of ELM mitigation with helium SMBI is much longer than that with deuterium, likely due to the higher level of recycling neutral gas compared to that of deuterium; in the case of ELM mitigation by helium SMBI, the recovery duration of the density gradient is much shorter (10–20 ms) than that of T i (up to 40 ms or longer), indicating the importance of the T i in the ELM mitigation by SMBI. Finally, it has been observed that the T i is reduced before the beginning of the ELM mitigation, suggesting that the mechanism of the ELM mitigation by SMBI is closely related to the cooling effect.

Impurity transports in two neighboring discharges with and without electron cyclotron resonance heating (ECRH) are studied in the HL-2A tokamak by laser blow-off (LBO) technique. The progression of aluminium ions as the trace impurity is monitored by soft x-ray (SXR) and bolometer detector arrays with good temporal and spatial resolutions. Obvious difference in the time trace of the signal between the Ohmic and ECRH L-mode discharges is observed. Based on the numerical simulation with one-dimensional (1D) impurity transport code STRAHL, the radial profiles of impurity diffusion coefficient D and convective velocity V are obtained for each shot. The result shows that the diffusion coefficient D significantly increases throughout the plasma minor radius for the ECRH case with respect to the Ohmic case, and that the convection velocity V changes from negative (inward) for the Ohmic case to partially positive (outward) for the ECRH case. The result on HL-2A confirms the pump out effect of ECRH on impurity profile as reported on various other devices.

Impurity transport in the scrape-off layer (SOL) has been studied in ohmically heated discharges of the HL-2A tokamak based on space-resolved vacuum ultraviolet spectroscopy. The vertical profile (from the plasma center to the lower X-point) of carbon emissions of CIII (977 A: 2s2 1S0-2s2p ) and CIV (1548 A: 2s 2S-2p 2P) as well as the ratio of CIV to CIII were measured to investigate the edge impurity transport with relation to impurity source locations and sputtering characteristics. The experimental result shows that the impurity profile in the SOL has been clearly changed against different source locations. The emission of CIII and CIV from the mid-plane is stronger than that from the X-point when the impurity source is located at the divertor plate. The profile becomes flat as a result. When the impurity source changes to the dome source, the profile clearly changes to a slightly peaked one, indicating the edge carbon emission at the X-point is stronger than the mid-plane. The change to the limiter source makes the profile further peaked by increasing the carbon emission at the X-point. In the case of the dome impurity source, the intensity of CIII/ne and CIV/ne, normalized to line-averaged electron density, ne, decreases with ne at low ne (ne ⩽ 2.6 × 1019 m−3) and becomes saturated at high ne (ne > 2.6 × 1019 m−3). In contrast, the ratio of CIV to CIII increases with ne at low ne and starts to decrease at high ne. A numerical simulation with 3D edge plasma transport code, EMC3-EIRENE, suggests that a poloidal asymmetry in the impurity flow profile and an enhanced physical sputtering play an important role in the edge impurity distribution, particularly in the screening efficiency of C2+ and C3+ ions in the SOL region of the HL-2A tokamak.

Edge impurity transport has been studied in the stochastic magnetic field layer called the ergodic layer of the Large Helical Device. Chord-integrated full vertical profiles of C IV (312.4 A: 1s23p–1s22s) near edge X-points are observed at horizontally elongated plasma cross sections. Measured C IV profiles are almost flat at low density (≤2 × 1013 cm−3), while two peaks called the 'X-point peak' newly begin to appear near edge separatrix X-points with increase in density in addition to ordinary edge peaks. The X-point peaks become very clear at high-density range (≥6 × 1013 cm−3). The C3+ ions analyzed with three-dimensional edge plasma transport code, EMC3–EIRENE, move upstream at low density due to large thermal force. The widely expanded C3+ distribution makes the C IV profile flat. With increasing the density, the friction force becomes dominant and the impurity ions start to move downstream. The C3+ ions stay in the vicinity of the edge X-point where magnetic field lines are very short. The X-point peaks are then clearly formed with increase in the C3+ density. Two-dimensional C IV distribution is also observed for different plasma axis positions. It is found that the C IV emission becomes strong along the poloidal trajectory of X-points and the C IV poloidal trace moves from inboard to outboard X-point trajectories when the plasma axis is changed from 3.60 to 3.75 m. The three-dimensional simulation can well reproduce the enhancement of C IV trajectory along the X-point. However, a discrepancy with the experiment is still seen in the C IV intensity between inboard and outboard X-points.

It is found that resonant magnetic perturbation (RMP) fields have a stabilizing effect on the radiating edge plasma, realizing stable sustainment of radiative divertor (RD) operation in the Large Helical Device (LHD). Without RMP, thermal instability leads to radiative collapse. Divertor power load is reduced by a factor of 3–10 during the RMP-assisted RD phase, while maintaining relatively good core plasma confinement with confinement enhancement factor . It has also been demonstrated that the RMP field itself can initiate transition to RD operation by increasing perturbation strength, while keeping constant density and injection power. The results show a possibility of a new control knob for divertor power load in a 3D magnetic field configuration. It is also found that after the transition to RD, the energy confinement enhancement factor based on ISS04 scaling increases by a factor of 1.4 compared with the attached phase. The operation range of the RMP-assisted RD is identified in terms of RMP strength and radial location of the resonance layer of the RMP. A 3D edge radiation structure is analysed using the edge transport code EMC3-EIRENE and the results are compared with experiments. The comparison indicates that the application of RMP modulates the 3D edge radiation structure such that an intense radiation appears around the X-point of the m/n = 1/1 island in the case with RMP, while it is located at the inboard side without RMP.

In the Large Helical Device (LHD) operated with a metallic (stainless steel) first wall, it is found that the iron density, nFe, at the plasma core is fairly low (nFe ⩽ 108 cm−3) in general neutral beam (NB)-heated discharges, while the iron quickly increases with the appearance of impurity accumulation when a multi-hydrogen ice pellet is injected or the NB input power is largely reduced. Although the highest iron density (nFe ⩽ 1010 cm−3) at the plasma centre in the LHD is observed from such discharges, it suggests a still low iron concentration (nFe/ne