The development of parametric instabilities in a large scale inhomogeneous plasma with an incident laser beam composed of multiple-frequency components is studied theoretically and numerically. Firstly, theoretical analyses of the coupling between two laser beamlets with certain frequency difference $deltaomega_0$ for parametric instabilities is presented. It suggests that the two beamlets will be decoupled when $deltaomega_0$ is larger than certain thresholds, which are derived for stimulated Raman scattering (SRS), stimulated Brillouin scattering (SBS), and two plasmon decay (TPD), respectively. In this case, the parametric instabilities for the two beamlets develop independently and can be controlled at a low level provided the laser intensity for individual beamlet is low enough. Secondly, numerical simulations of parametric instabilities with two or more beamlets ($Nsim20$) have been carried out and the above theory model is validated. Simulations confirm that the development of parametric instabilities with multiple beamlets can be controlled at a low level, provided the threshold conditions for $deltaomega_0$ is satisfied, even though the total laser intensity is as high as $sim10^{15}$W/cm$^2$. With such a laser beam structure of multiple frequency components ($Ngtrsim20$) and total bandwidth of a few percentages ($gtrsim4%$), the parametric instabilities can be well-controlled.
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