Fatigue properties of rolled AZ31B magnesium alloy plate

Fatigue strength,crack initiation and propagation behavior of rolled AZ31B magnesium alloy plate were investigated. Axial tension-compression fatigue tests were carried out with cylindrical smooth specimens.Two types of specimens were machined with the loading axis parallel(L-specimen)and perpendicular(T-specimen)to rolling direction.Monotonic compressive 0.2%proof stress,tensile strength and tensile elongation were similar for both specimens.On the other hand,monotonic tensile 0.2%proof stress of the L-specimen was slightly higher than that of the T-specimen.Moreover,monotonic compressive 0.2%proof stresses were lower than tensile ones for both specimens.The fatigue strengths of 107cycles of the L-and T-specimens were 95 and 85 MPa,respectively. Compared with the monotonic compressive 0.2%proof stresses,the fatigue strengths were higher for both specimens.In other words, the fatigue crack did not initiate and propagate even though deformation twins were formed in compressive stress under the cyclic tension-compression loading.The fatigue crack initiated at early stage of the fatigue life in low cycle regime regardless of specimen direction.The crack growth rate of the L-specimen was slightly lower than of the T-specimen.Consequently,the fatigue lives of the L-specimen were longer than those of the T-specimen in low cycle regime.

MHD Instabilities and Their Effects on Plasma Confinement in Large Helical Device Plasmas with Intense Neutral Beam Injection

MHD stability of the Large Helical Device (LHD) plasmas produced with intense neutral beam injection is experimentally studied. When the steep pressure gradient near the edge is produced through L-H transition or linear density ramp experiment, interchange-like MHD modes whose rational surface is located very close to the last closed flux surface are strongly excited in a certain discharge condition and affect the plasma transport appreciably. In NBI-heated plasmas produced at low toroidal field, various Alfvén eigenmodes are often excited. Bursting toroidal Alfvén egenmodes excited by the presence of energetic ions induce appreciable amount of energetic ion loss, but also trigger the formation of internal and edge transport barriers.

Progress of LHCD and IBW Heating Experiments on HT-7 Tokamak

Experimental progress of lower hybrid current drive(LHCD)and ion Bernsteinwave(IBW)heating was achieved significantly in HT-7.Long pulse plasmas,up to 400 s,wereobtained by LHCD.High power lower hybrid wave(LHW),with P_(lhw)of 800 kW at 2.45 GHz,was launched and the current drive efficiency was studied for different wave powers.Electrontemperature and Z_(eff)were measured in LHCD discharges.Heating experiment and recent resultsof high power ion Bernstein wave,with P_(ibw)of 600 kW at 27 MHz,were summarized.

Observation of the Two-Dimensional Distribution of Impurity Line Emissions Using a Space-Resolved EUV Spectrometer in LHD

A space-resolved EUV spectrometer for measuring the one-dimensional distribution of impurity line emissions in Large Helical Device (LHD) has been upgraded to measure two- dimensional distributions of impurity line emissions with an extension of working wavelength range to 30～650 A. The two-dimensional measurement is performed by scanning the observation chord horizontally. A rectangular plasma region of 520 × 700 mm 2 in vertical and horizontal sizes can be observed during a single horizontal scan. The horizontal scan requires a time duration of 5 s at least. The spatial resolution is 10 mm in the vertical direction when a spatial-resolution slit of 0.2 mm in width is adopted. Although a spatial resolution in the toroidal direction is 75 mm, it is a function of CCD exposure time and horizontal scanning speed. Two-dimensional distribution of EUV line emissions from several impurities has been successfully observed for the first time from steady discharges in LHD. In this paper two-dimensional distributions of He II (303.78 A), C V (40.27 A), C VI (33.73 A) and Fe XX (132.85 A) located at different radial positions are presented with simple analysis on the magnetic field structure of LHD.

Space-Resolved VUV and EUV Spectrometers in HL-2A

A normal incidence vacuum ultraviolet (VUV) and a grazing incidence extreme ultraviolet (EUV) spectrometers have been developed for the edge and core impurity measure- ments in HL-2A tokamak. The VUV and the EUV spectrometers cover wavelength ranges of 300～3200 A and 50～500 A, respectively. A spatial resolution of 2 mm has been achieved for the VUV spectrometer when a space-resolved slit 50 μm in width is used. The space-resolved slit is placed between the entrance slit and the grating of the spectrometer. The spectral resolutions of 0.15 A for the VUV spectrometer in the wavelength coverage of the concave 1200 grooves/mm grating and of 0.22 A for the EUV spectrometer at λ=200 A with a flat-field laminar-type holo- graphic grating are obtained. The sensitivity of the VUV spectrometer was calibrated in situ with the plasma bremsstrahlung radiation. The experimental results from both spectrometers are presented, especially the line intensity radial profiles measured by the VUV spectrometer.

Effect of Ne Glow Discharge on Ion Density Control in LHD

Neon glow discharge cleaning was firstly attempted in Large Helical Device (LHD) instead of He glow discharge to remove hydrogen neutrals and to control the ion density, hi. The Ne glow discharge continued for 8 hours overnight after a three-day experiment. At the second night Ha emission became weaker than the emission usually observed in the He glow discharge. A clear reduction of the hydrogen influx was also observed in neutral beam injection (NBI) dischargeswith Ne puff， whereas the neon recycling was strongly enhanced with appearance of a flat density profile. As a result， the lowest density limit was further reduced down to 0.2×10^13 cm^-3. Theuse of Ar puff formed a peaked density profile with a high Ti of 7 keV.

Refueling for Steady-State Plasma by Repetitive Pellet Injection in Large Helical Device

A repetitive pellet injector has been developed for investigation of refueling issues towards the steady-state operation in Large Helical Device (LHD). Continuous operation of more than 10000 pellet launching at 10 Hz has been demonstrated. The maximum repeating rate is 11 Hz. No technical constraint for longer operation has been found. The reliability of pellet launch has exceeded 99.9%. The initial application to the NBI-heated plasmas has been successful in the last experimental campaign of LHD. Although the pulse length is limited by the operational constraint of NBI, the plasma with a density of 8×10^19 m^-3 has been sustained for 2 s by the pellet injection at 10 Hz. A prospect for the future experiment is discussed on the basis of the initial result.