Grid Near The Ion Wave Mode Excitation Mechanism And Dust Ion Acoustic Wave Nonlinear Evolution

In plasma physics linear and nonlinear ion waves are a popular subject which is not only fundamental to understand collective behavior of plasma, but also shows great importance on applications, such as plasma diagnostics and wave heating. In double plasma device(DPD) ion waves can be excited by grid. But systematic research about excitation mechanism of ion waves by grid is not convincing both theoretically and experimentally. So to establish an appropriate model and interpret how ion waves are excited and propagate is important and fundamental in understanding collective behavior of basic plasmas.In this dissertation we mainly concentrated experimentally on the evolution and propagation of ion waves excited by grid in a modified double plasma device. Also how dust-acoustic shock waves and nonlinear dust-ion acoustic waves propagate in an inhomogeneous plasma has been investigated analytically and numerically.Using an emissive probe a complete potential distribution around an inserting grid into the target chamber is achieved, which provides a fundamental comprehension about the sheath and presheath of the negative or positive biased grid. The experimental measurement of ion-rich sheath of a negative biased grid accords with the theoretical result of Child-Langmuir sheath qualitatively.In our experiments grid excitation mode is always operated instead of DP mode. Usually three types of grids with different diameter as well as a metal plate are tried to excite ion waves by applying time varying ramp signals. It is showed that the metal plate can only excite simple compressional or rarefactive ion-acoustic wave which always attenuates along the distance after its formation.For grid excitation part ions can cross the grid and enter into the presheath with Bohm speed to form local beam ions. Simultaneously a plenty of plasma ions diffuse into the presheath. Thus the local region of presheath can be considered as a beam-plasma system, in which two normal waves, one stable fast ion beam wave(FIBW) and one unstable wave, can exist. For a 90mm diameter grid beam ion density in the presheath is relatively small, so the unstable mode can't grow evidently. Once a ramp signal with short rise time and large amplitude is applied to the grid, ions will get across the sheath with a directed speed higher than Bohm speed and form a group of free-streaming burst ions. If the collective speed of the burst ions is higher enough than ion acoustic speed, they will be collected by a probe in bulk plasma and called as ion burst mode(IBM). While the burst ions move faster slightly than the normal FIBW, FIBW shall gain energy from the burst ions and grow up because of inverse Landau damping. In bulk plasma beam ions are decelerated to form plasma ions, so FIBW shall evolve into the normal ion-acoustic wave which can be identified as a soliton.Inserting a 220mm diameter grid into the plasma the unstable wave can grow up dramatically because of the much greater beam ion density in the presheath. The grown unstable wave can trap the beam ions which move with a close speed as the phase speed of the unstable wave and evolve into a nonlinear ion hole which is an ion-phase space vortex structure. If a 440mm diameter grid which is almost as large as the chamber is used, stable fast ion beam wave and slow ion beam wave can be excited.Inhomogeneity in our DPD system can significantly modify the propagation of ion waves. The phase speed of any type of ion waves which are excited along the plasma flow can travel faster than that is excited against the plasma flow. The later shall damp more quickly.We also investigate analytically the evolution of dust-acoustic shock waves(DASW) in an inhomogeneous plasma as well as the nonlinear dust-ion-acoustic waves (DIAW). Neglecting the dispersion the propagation of DASW can be described with Burgers' type equation using the reductive perturbation method. It is shown that against the density gradient the percentage of negative charges residing on dust grains will increase though total plasma density decreases. So the phase speed of DASW would increase first and then decrease, and the amplitude of DASW exhibit a complex variation due to the interaction of inhomogeneity and collisions between dust grains and neutral particles. The governing KdV-Burgers equation describing the propagation of nonlinear DIAW has been obtained too. If we don't consider the variation of dust charges, the coefficient of nonlinear term will transit from negative value to positive value. So its evolution will change much after crossing the transition point. If the variation of dust charges is taken into account a new dissipation appears and the nonlinear term would not change its sign. We also study the evolution of the governing equation of DIAW by numerical method. The results confirm the importance of the dust charge variation on the wave propagation.