The influence of both positive and negative bias on global and plasma-edge parameters has been comparatively studied with a newly designed electrode biasing system in J-TEXT tokamak. Comparing to the 0V bias case, the global particle confinement of plasma is enhanced under bias with both polarities, with the increments of central line-averaged density and soft-X ray emission as well as the reduction of edge H α radiation level. The suppression of plasma-edge fluctuations and turbulent particle transport are obviously observed under bias, in different degrees with different polarities. The potential fluctuation amplitude is observed to be increased at the vicinity of limiter under positive bias, with the existence of a peaked low frequency mode (LFM) characterized as high coherence and near-zero cross-phase poloidally in the edge region, which is not found in the negative bias case. The poloidal correlation length of turbulence is greatly enhanced under bias with both polarities, and shows a positive correlation with the amplitude of poloidal phase velocity, which is mainly driven by the local J r × B torque at plasma edge under bias. The characteristic parameters of intermittent events (i.e. blobs), including amplitude, radial velocity, related particle flux and radial size, are decreased dramatically under bias in the edge region.
The structure of the ␥ / ␥Ј phase interface in a Ni-based single-crystal superalloy is simulated by molecular dynamics ͑MD͒ using an embedded atom method potential. From the calculated results we find that three dislocation network patterns, namely square, rectangle, and equilateral triangle, appear on ͕100͖, ͕110͖, and ͕111͖ interphase interface, respectively. The dislocation networks consist of four edge dislocations ͑͗011͘ ͕100͖, ͗110͘ ͕110͖, ͗001͘ ͕110͖, and ͗112͘ ͕111͖͒. The energy of the ␥ / ␥Ј phase interface for ͕100͖, ͕110͖, and ͕111͖ plane is 271 mJ/ m 2 , 240 mJ/ m 2 , and 32 mJ/ m 2 . The side length of network is 166.8 Å for the square, 166.8 Å and 235.8 Å for the rectangle and 166.8 Å for the equilateral triangle. The relationship between the size of network and mismatch is presented quantitatively. The calculated results can be supported by very recent experiments. Based on the MD simulation and the energy analysis we have revealed the basic characteristic of structure on ␥ / ␥Ј phase interface. The related mechanism of the stability of the interphase interface is also discussed.
A two-dimensional and stationary magnetohydrodynamic model of a plasma spray torch operated with argon is developed to predict the plasma properties in a steady operating mode. The model couples a submodel of a refractory cathode and its non-equilibrium boundary layer to a submodel of the plasma in local thermodynamic equilibrium in a self-consistent manner. The Navier–Stokes equations for a laminar and compressible flow are solved in terms of low and high Mach number numerical approaches. The results show that the Mach number can reach values close to one. Simulations are performed for electric currents of 600 A and 800 A, and gas flow rates of 40, 60, and 80 NLPM. The plasma parameters obtained by the two approaches differ, and the differences become more pronounced for higher currents and gas flow rates. The arc voltage, the electric power, and the thermal efficiency from both the low and high Mach number models of the plasma agree well with experimental findings for a current of 600 A and a flow rate of 40 NLPM. For higher currents and gas flow rates, the results of the low and high Mach number models gradually differ and underline the greater appropriateness of the high Mach number model.
A newly designed electrode biasing system has been constructed for driving plasma rotation in J-TEXT tokamak. To reduce the influence to the plasma, the system contains a pneumatic driving system so that it can reciprocate in a single discharge, with a stroke of about 5 cm in 100 ms. The power supply of the system can provide stable and adjustable dc voltage in the range of 0-700 V, with adjustable duration of 10-200 ms; its instantaneous power output can reach up to more than 200 kW. In addition, the power supply can also provide a multi-cycle voltage waveform, with adjustable pulse width and voltage amplitude. When applying a positive bias to the plasma, both an improvement of plasma confinement and the speed-up of plasma-edge toroidal rotation in the same direction of plasma current are observed in the experiments.
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