Numerical Simulation And Theoretical Studies Of Two-Plasmon Decay Instability In Magnetized Plasma

Two-plasmon decay (TPD) instability has been widely studied in direct-drive inertial confinement fusion (ICF) plasma. Howerver, neither theoretical analysis nor experimental research has been drawn much attention on TPD in a tokamak magnetized plasma. The reason is that the power of radio-frequency (rf) wave in tokamak plasma is much lower than the laser of ICF. But in TEXTOR tokamak second harmonic electron cyclotron resonance heating (ECRH) and electron cyclotron current drive (ECCD) experiment, evidences of the backscattering signal, ion heating and acceleration are observed when extraordinary (X) wave pass through the magnetic island in the 200-600kW power level. Even though the backscattering is considered to be directly related to TPD, it still can't be explained by the present analytical theory. Thus, it is necessary to study the mechnism of TPD in a magnetized plasma with lower rf power.In this paper, the methods of numerical simulation and theoretical analysis are combined to study the TPD in magnetized plasma. On one hand, two-dimensions slab model and plasma fluid equations are used to develop a full space-time parallel code. On the other hand, weak mode coupling approximation is adopted to get the analytic expressions of TPD growth rate and intensity threshold in magnetized plasma. Finally, the TPD is calculated by the analytic solution and parallel code according to the real parameters of TEXTOR tokamak ECRH experiment, respectively.Perfectly matched layer absorbing boundary condition is first used to the two-fluid parallel code. The well-posedness and the feasibility of the PML method are discussed, and the governing equation is also derived. Meanwhile, the numerical results and the stability analysis of the linear system show that the PML technology perfectly matched. Furthermore, the numerical results show that the absorption of PML is directly related to the thickness of PML and the absorption coefficient, PML does not affect the physical processes within the computational region, the absorption of PML is not affected by the incident wave power and electron temperature, and the maximum reflection error is lower than 1%.In the theoretical work, the effects of upper hybrid sub-wavenumber, electron density, and magnetic field on TPD absolute and convective instabilities are investigated. The results show that the influence of magnetic field on absolute instability depends on the perpendicular wavenumber ky and parallel wavenumber kx of upper hybrid wave (to pump wave ko), the effects of different magnetic field and ky on absolute instability growth rate and convective amplification are distrinct, the density and magnetic field will take obvious effect on the convective amplification factor with small kx, the linear damping and electron density inhomogeneity will restrain the convection amplification, and the effect of electron density inhomogeneity on TPD is larger than the ones of static magnetic field.The threshold intensity of TPD on TEXTOR tokamak ECRH experiment is calculated by the analytic solution and parallel code, respectively. Both of their results show that the power of electron cyclotron X-wave is more than ten mW with the linear or quasi-linear plasma profile. The simulation result with non-monotonicity density profile displays that the TPD threshold can be reduced by one order of magnitude, but it still can't interpret the experiment. However, if the correction of the kinetic effect is considered within non-monotonicity density profile, numerical result reveals that the TPD threshold (about 200kW) will decrease nearly two orders of magnitude compared to a linear or quasi-linear case. In this case, the phenomenon of backscattering of X-wave in the experiment can be explained reasonably.In this paper, the theoretical results have shown that the trapping of the upper hybrid wave and non-monotonic density profile can significantly reduce the threshold power of TPD. This research will be useful for the further study of the nonlinear interaction of rf-wave and magnetized plasma.