  This paper considers nonlinear dynamics of plasma oscillations modeled by a
forced modified Van der Pol-Duffing oscillator. These plasma oscillations are
described by a nonlinear differential equation of the form $ \ddot{x}+ \epsilon
(1 +{x}^{2}){\dot{x}} + x+ \alpha \epsilon{x}{\dot{x}} + {\beta}x^{2}+\gamma
x^{3}= F\cos{\Omega t}.$ The amplitudes of the forced harmonic, superharmonic
and subharmonic oscillatory states are obtained using the harmonic balance
technique and the multiple time scales methods. Bifurcation sequences displayed
by the model for each type of oscillatory states are performed numerically
through the fourth order Runge- Kutta scheme. The influences of the differents
parameters and of amplitude of external forced have been found.
