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The effect of sawteeth on plasma performance and transport in the plasmas of tokamak is an important issue in the fusion field. Sawtooth oscillations can trigger heat and turbulence pulses that propagate into the edge plasmas, and thus enhance the edge shear flow and induce a transition from low confinement mode to high confinement mode. The influences of turbulence spreading and symmetry breaking on edge shear flow with sawtooth crashes are observed in the J-TEXT tokamak. The edge plasma turbulence and shear flow were measured using a fast reciprocating electrostatic probe array. The experimental data were analyzed using methods such as conditional average and probability distribution function. After sawtooth crashes, the heat and turbulence pulses in the core propagate to the edge, with the turbulence pulse being faster than the heat pulse. Figures 1 (a)-(e) show the core electron temperature, and the edge electron temperature, turbulence intensity, turbulence drive and spreading rates, Reynolds stress and its gradient, and shearing rates, respectively. Following sawtooth crashes, the edge electron temperature increases and the edge turbulence is enhanced, with turbulence preceding temperature. The enhanced edge turbulence is mainly composed of two parts: turbulence driven by local gradient and turbulence spreading from core to edge. The development of the estimated turbulence spreading rates is prior to that of the turbulence driving rates. The increase in the turbulence intensity can cause the enhancements of the turbulent Reynold stresses and its gradient, thereby enhancing shear flows and radial electric fields. Turbulence spreading leads to the development of edge Reynolds stresses and shear flow faster than edge electron temperature. The Reynolds stress arises from the symmetry breaking of the turbulence wave number spectrum. After sawtooth collapse, the joint probability density function of radial and poloidal wave numbers of turbulence intensity became highly skewed and anisotropic, exhibiting strong asymmetry, as seen in the figures 1 (f) and (g). The development of turbulence spreading flux at the edge is also prior to the particle flux driven by turbulence, indicating that turbulent energy transport is not simply accompanied by turbulent particle transport. These results show that turbulence spreading and symmetry breaking can enhance turbulent Reynolds stress, thereby driving shear flows, after sawtooth crashes.
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Keywords:
- tokamak /
- sawtooth /
- edge shear flow /
- Reynolds stress
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