Effect of stress ratio on fatigue properties of a titanium alloy (TC-17) in the high cycle fatigue (HCF) and very high cycle fatigue (VHCF) were investigated by electromagnetic and ultrasonic fatigue testing. The S-N curves at R = À1, 0.1, 0.5 and 0.7 at 110 Hz and 20 kHz were obtained and discussed. The effects of frequency on fatigue strength was also investigated. It was concluded that the fatigue strength with 50% fatigue failure probability at R = 0.1, 0.5 and 0.7 is lower to the Goodman line and shows a bilinear decreasing trend. Cleavage fracture of primary grains in the surface and interior initiation zone were observed. The formation of the facets induced by the basal or prismatic slips of the H.C.P grains decreased the fatigue strength with variation in mean stress.

A new set of actively cooled toroidal double-ring graphite limiters has been developed in the Hefei Tokamak-7 ͑HT-7͒ ͓X. Gao et al., Phys. Plasmas 7, 2933 ͑2000͔͒ for long pulse operation. The extension of operational region and density behavior with graphite ͑C͒ limiters have been studied in this paper. Extended high-density region at the high plasma current low-q a was obtained. The density profile with the C limiter was studied to compare with the previous molybdenum ͑Mo͒ limiter. The critical density of multifaceted asymmetric radiation from the edge ͑MARFE͒ onset is observed in the region of Z ef f 1/2 f GW = 0.9ϳ 1.2, where f GW = n e / n GW . ͑Here n e is the maximum line average electron density and n GW is the Greenwald density.͒ Under the same injected power, the critical density of MARFE onset with the new C limiter is much higher than the previous Mo limiter.

Multiple electromagnetic coherent modes with frequencies f ∼ 20–300 kHz and toroidal mode numbers n = 1 and n = 2 have been observed and investigated in radio-frequency heated H-mode plasmas of the EAST tokamak. The experimental results show that the two main branches of these coherent modes are driven by energetic electrons (EEs), which are produced in the processes of radio-frequency current drive and heating. Bicoherence analysis indicates that there are strong nonlinear mode interactions between the two branches (mother waves), i.e. one is in the low-frequency range of f ∼ 20–50 kHz and the other one is in the high-frequency range of f ∼ 120–250 kHz, and their nonlinear couplings can generate many harmonics (daughter waves). Both coherent modes propagate poloidally along the electron diamagnetic drift direction. The gyrokinetic eigenvalue simulations support the view that both the low-frequency and the high-frequency coherent modes observed in EAST are Alfvén eigenmode (AE) type, and the kinetic effects of background plasmas and EEs are responsible for the formation and excitation of AEs, respectively. The low-frequency coherent mode is identified as the kinetic beta-induced Alfvén eigenmode located in the edge, and the high-frequency coherent mode is radially global, which is characterized by a toroidal Alfvén eigenmode (TAE) in the core and also has the components of a kinetic TAE and ellipticity-induced Alfvén eigenmode in the outer region due to the large downshift of the Alfvén continuum gap from the core to the edge in H-mode discharges.

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