Numerical Study Of The Interaction Between GAM And Drift Wave Turbulence

Energy issues have been closely related with human life. Since the oil, coal and other non-renewable resources are being depleted, the development of nuclear energy is important. And the hope to achieve controlled nuclear fusion is the magnetic confinement fusion and the inertial confinement fusion. While in magnetic confinement fusion, the mechanism of anomalous trans-port has been one of the key points of fusion research. It is generally considered that anomalous transport is driven by drift wave turbulence, so how to suppress the drift wave turbulence is sig-nificant for improving the constraint performance of Tokamak. Geodesic acoustic mode (GAM) is an electrostatic oscillation mode which is unique to the toroidal plasmas, and the mode structure is symmetrical in toroid, approximately symmetrical in poloidal field. Theoretical studies show that GAM can be used for adjusting the drift wave turbulence via the nonlinear interaction, which provides a modulation method to improve the transport in plasmas. According to the research by Rosenbluth, because of the kinetic dispersion effect, when GAM and the drift wave sideband have the same radial group velocity, they will couple together and spread out of the pump wave range in a very short period of time, showing the convective instability. And when GAM and drift wave sideband have the group velocity in opposite direction, GAM will continue to grow in the area of pump wave, which suppresses the drift wave turbulence.In this paper, we study the excitation of GAM by drift wave turbulence. According to the coupled equations derived from Z.Qiu, L.Chen et al, which describes the nonlinear interactions between drift waves and geodesic acoustic modes, we program a MATLAB code using the finite difference method, and give the numerical results. We simulate the spatial and temporal evolutions of interactions between drift wave sideband and GAM under different parameters.Topics include:We take the direction of the group velocity of drift wave sideband positive, by changing the direction of the GAM group velocity, to study the corresponding convective/absolute instability. First, under conditions of convective instability, we take a uniform drift wave pump, and find that drift wave sideband and GAM couple together and spread out, while in the. absolute unstable conditions, both GAM and DW sideband grow in the range of pump wave. Then, as a contrast, we take the Gaussian pump wave, at the condition of the same or opposite direction of group velocity, we observe the corresponding convective instability and absolute instability, and plot the GAM amplitude at the origin point. In the case of uniform pump, by taking convective instability, we measure the wave number of the wave envelop of coupled drift wave sideband and GAM, then calculate the group velocity and linear growth rate. Finally, still in the case of the uniform pump and also the conditions of convective instability, and when the non-uniformity of diamagnetic frequency is taken into account, we find that the drift wave sideband propagate much faster than GAM and it will bounce back at the inflection point of the potential well and spread back, so the parametric decay process becomes an absolute instability, which can suppress drift wave turbulence. Since the Tokamak is usually the case of convective instability, so the result is meaningful.Finally, conclusions and discussions for future works are given.