Numerical Simulation Research On The Lower Hybrid Wave In Magnetized Plasma

Lower hybrid wave(LHW) heating and lower hybrid current drive(LHCD) are unavoidable for tokamak magnetic confinement fusion. Although the accessibility condition is satisfied, LHW can't propagate to the high density area as it is predicted by linear theory on the tokamak experiments. Experimental results even show a sharp falling of current drive efficiency. Theoretical and experimental results reveal that the nonlinear wave-plasma interaction—parametric decay instability(PDI) is closely correlated with this phenomenon. Currently research of theory is limited to the linear stability analysis which depends on the assumption of uniform plasma distribution. Full-wave numerical simulation based on the finite-difference time-domain(FDTD) and Mac Cormack methods is considered to be an effective way to study the physical processes of the coupling and propagation of LHW in inhomogeneous plasma. The full-wave numerical simulation not only can provide the linear and nonlinear interaction processes, but also can provide coincident physical pictures with the theories and experiments in the whole time and space.According to the two-fluid physical model and slab computational model, the numerical algorithms based on the combination of the FDTD and Mac Cormack methods are put forward. Then, the parallel arithmetic code is developed. The linear full-wave numerical results have reproduced the physical processes of the propagation and the fast-slow mode conversion of LHW. Numerical results agree well with the analytical solutions, which show the numerical code is correct and reliable. The nonlinear numerical results show that the PDI of LHW occurs significantly at scrape-off layer plasma parameters. The pump LHW decays into a low- frequency ion sound wave and a sideband daughter LHW. Meanwhile, the occurrence of PDI leads to the significant spectral broadening of pump wave. Besides, the increasing of the plasma temperature can suppress the PDI at scrape-off layer. The nonlinear numerical results of PDI agree well with R. Cesario's experimental and linear theoretical results. If different parameters are adopted, nonlinear numerical results show that the two-plasmon decay(TPD) instability occurs when the resonant decay conditions are matched. The pump LHW decays into two lower frequency daughter LHWs, and the frequencies of both daughter LHWs are nearly half of the pump wave frequency. All these numerical simulation results will provide valuable references for the researches of density threshold problems occurring in the LHCD experiments.