[en] Systematic studies of first mirrors (FMs) contamination, deposition mitigation and cleaning have been performed in EAST and laboratory. It turned out that the auxiliary systems such as gas puffing, wall conditioning and limiters could lead to a serious deposition on FMs and result in at least 60% degradation of the reflectivity. Besides, the exposure environment and depth–diameter ratios (DDRs) of the protective ducts will influence the degree of deposition mitigation and homogeneity of deposition. The reflectivity of the FM protected with duct at the wavelength of 240–400 nm could sharply decrease at the beginning of the exposure, which differs from that in the wavelength range more than 400 nm. In addition, the total and specular reflectivity of FM cleaned by radio frequency magnetron sputtering plasma (RFMSP) could be recovered by up to 90% and 80 % respectively in the wavelength range of 300–800 nm

No abstract available

[en] Highlights: • Epitaxial (1 − x)Ba(Zr_0_._2Ti_0_._8)O_3–x(Ba_0_._7Ca_0_._3)TiO_3 (BZT–xBCT) thin films were successfully grown on (0 0 1)-oriented SrTiO_3 (STO) substrates by pulsed laser deposition (PLD). • Both structural and physical properties of BZT–xBCT thin films exhibited strong composition dependence. • An MPB-like behaviour was found at around x = 0.5 with highest dielectric, ferroelectric and piezoelectric properties. • The piezoelectric coefficients of our BZT–0.5BCT thin films were in upper range of the existing piezoelectric thin film materials. - Abstract: Lead-free (1 − x)Ba(Zr_0_._2Ti_0_._8)O_3–x(Ba_0_._7Ca_0_._3)TiO_3 (BZT–xBCT) (x = 0.3, 0.4, 0.5, 0.6, 0.7) thin films were deposited on SrRuO_3 (SRO)-electroded (0 0 1)-oriented SrTiO_3 (STO) single crystal substrates by pulsed laser deposition (PLD). X-ray diffraction and selected area electron diffraction (SAED) patterns verified epitaxial growth for all the thin films. High ferroelectric activities were evidenced by the well-defined polarization-electric field hysteresis loops. Both structural and physical properties of the thin films demonstrated a strong composition dependence and an MPB-like behaviour was found at around x = 0.5, which is in agreement with that of its bulk counterparts. The superior ferroelectric and piezoelectric responses were attained at x = 0.5 with remanent polarization P_r of 17.8 μC/cm"2 and piezoelectric coefficient d_3_3 of 100 ± 5 pm/V

[en] The plasma facing components (PFCs) will experience strong plasma surface interactions (PSI) especially in long pulse and high preference plasma. The lifetime of the wall components can be seriously reduced by the substantial particle and heat fluxes, and the impurities eroded from the wall can be transported long distance to the plasma and badly influence the plasma performance. Therefore, study the physical mechanism of plasma wall interaction (PWI) as well as the edge plasma behavior is quite necessary. The Experimental Advanced Superconducting Tokamak (EAST) was built to achieve long pulse and high performance plasma to study the physics and engineering issues relevant to the next-step long pulse fusion devices such as ITER and DEMO. There are many PSI issues capable to be studied in EAST such as: practical test of W divertor and experiment for ITER, excessive heat load control, particle exhaust and impurity control as well as edge recycling and density control. In the following, more detailed information about EAST capabilities in PSI studies and some main recent research activities will be introduced. (author)

[en] Plasma wall interaction (PWI) in tokamaks will have a serious impact on the lifetime of plasma facing components, the performance of fusion plasmas and the tritium retention in devices. Tungsten (W) is the most promising candidate material for the divertor region of next step fusion reactors, such as ITER. The Experimental Advanced Superconducting Tokamak (EAST), which can operate with long pulse, high performance plasma and W divertor, provides a good platform for the W related PWI studies under steady state plasma. In this paper, recent progress on W-related PWI studies on EAST are summarized. According to the edge spectroscopic observations, W erosion rates and impurities content in the plasma are characterized and analyzed. Correspondingly, W erosion and re-deposition at the divertor region was simulated by the Monte Carlo code ERO. Furthermore, control of W erosion via different means was studied. (author)

[en] A ZE41 alloy was processed by equal channel angular pressing (ECAP) at 320⁰C. The microstructure and tensile properties were investigated. It has been observed that ECAP refines both the grains and precipitates, thus modifies the strength and ductility of the alloy. The sample after 6-passes of ECAP processing has a yield stress of 230MPa and elongation of 20%, compared with 160 MPa and 8% prior to the ECAP. The deformation of the alloy is predominated via dislocation slip with mechanical twinning.

[en] Equal channel angular pressing (ECAP) was applied to an extruded AZ80 Mg alloy in order to refine microstructure and enhance mechanical properties. The results show that the initial grain size of 20 μm of the as-extruded samples has been reduced by 75% after the first pass of ECAP, down to about 5 μm. Also as a result of the severe plastic deformation, the intermetallic compound Mg₁₇Al₁₂, which was distributed along grain boundaries and forms networks before the ECAP, has been re-arranged into a more homogeneous distribution and with a more spherical shape after the ECAP. Although no strengthening effect has been observed, the grain refinement and precipitate fragmentation caused by the ECAP has resulted in a 28% increase in the elongation to failure in tensile tests. Fractography observation and texture measurement have also been carried out.

[en] Full text: The 3D Monte Carlo code ERO taking into account a material mixing surface model has been used to simulate tungsten (W) erosion and migration on EAST with an upper full W divertor and DIII-D with toroidally continuous W rings embedded in the divertor target. Modelling shows that the transport of carbon (C) impurities not only dominates the W sputtering source but also determines the overall erosion and deposition balance in the mixed materials surface. With a self-consistent calculation of C impurity transport and taking into account the re-erosion of W by returned eroded particles,W gross erosion rates measured by WI spectroscopy can be well reproduced by the modelling for both devices. The E x B drift and lower electron temperature at the radial outboard side lead to a net deposition zone where W and C are accumulated, which is consistent with the measurements with several changeable inserts in a specially designed collector probe at the DiMES system in DIII-D. In the net erosion zone closer to the outer strike point, the W coverage on C is very low and saturated independent of exposure time, agreeing with the measurements by collector probes. Strong sheath effects on material erosion rates have also been observed using external biasing samples. The particle flux and material erosion as a function of biasing voltage have been analyzed by SPICE2 and ERO codes. Both the PIC simulation and the D emission measured by a fast camera reveal that with increasing biasing voltage the ion flux decreases at the biased area while increases at the adjacent downstream tile, although the biased sample potential is far below the plasma potential. Detailed modelling shows that the ion flux variation at different area is due to the strong gradient of the electric field in the sheath, which results in different magnitude of the polarization drift above the biased and nonbiased surface. The reduced ion flux and incident energy are responsible for more than an order of magnitude reduction of erosion with slight positive voltage biasing in the experiments. The critical role of C impurities and the sheath in determining high-Z material erosion and migration have been revealed. This understanding indicates promising methods for erosion control, which is critical for material lifetime, plasma impurity content, and tritium retention in future fusion reactors. (author)

[en] An as-cast AZ91 magnesium alloy was processed by Equal Channel Angular Pressing (ECAP) at 320⁰C. The microstructure and mechanical properties were studied. It has been found that ECAP refines both the grains and precipitates, thus modifies the strength and ductility of the processed alloy. After the first pass of ECAP, the yield stress improves significantly from 71 MPa to 140 MPa.

[en] An as-cast AZ91 magnesium alloy was processed by equal channel angular pressing (ECAP) at 593K and its microstructure and mechanical properties were studied using electron microscopy and room temperature tensile tests, respectively. It has been found that after the first pass of ECAP, the grain size of the alloy shows a bi-modal distribution, containing fine grains of about 14 μm and large dendrite structure. The dendritic structure completely disappeared after two passes of ECAP. The average grain size of the alloy after six passes of ECAP becomes less than 10 μm. The yield stress of the alloy has significantly increased from 65 MPa of the as-cast alloy to 135 MPa after the first pass of ECAP, but does not show much change with further ECAP. However, the elongation to failure measured from the alloy processed by the first pass of ECAP is similar to that measured from the as-cast alloy. A noticeable increase of the elongation to failure has been observed after the second pass of ECAP, which then remains at the similar level with further ECAP process. The fractography of the tensile tested samples have been studied using scanning electron microscope (SEM) and focused ion beam (FIB) microscope. The facture surface of the as-cast alloy is predominated by cleavages. Although not predominantly, cleavage has also been frequently observed in the alloy processed by one pass of ECAP. With further ECAP process, the facture surface becomes profuse in dimples, characteristic of ductile facture, consistent with the ductility change observed. FIB observation suggests that the cracking is mainly initiated at the blocky particles.