Liu Jin-Yuan

Dynamics of Dust in a Plasma Sheath with Magnetic Field

Dynamics of dust in a plasma sheath with a magnetic field was investigated using a single particle model. The result shows that the radius, initial position, initial velocity of the dust particles and the magnetic field do effect their movement and equilibrium position in the plasma sheath. Generally, the dust particles with the same size, whatever original velocity and position they have, will locate at the same position in the end under the net actions of electrostatic, gravitational, neutral collisional, and Lorentz forces. But the dust particles will not locate in the plasma sheath if their radius is beyond a certain value.

Sheath Structure of an Electronegative Plasma

We investigate the sheath structure of an electronegative plasma at steady state with the assumptions of cold positive ions and hot negative ions. The modified Bohm criterion is obtained with the Sagdeev potential by introducing a modified ion sound velocity. At the same time the electric potential, net space charge and particles densities in the sheath are analysed in several cases of different temperature ratios of electrons to negative ions and different density ratios of negative ions to positive ions.

Electronegative Plasma Sheath Structure in a Magnetic Field

The structure of an electronegative plasma sheath in an oblique magnetic field is investigated with a fluid model. We assume the system consists of hot electrons and negative ions as well as cold positive ions. Densities of particles and distributions of the spacious potential in various states of magnetic field are studied. The result shows that the existence of magnetic field and negative ions has great effects on the plasma sheath structures. In addition, the effects of negative ion density and temperature on the structure of the electronegative plasma sheath are discussed.

Two-dimensional numerical research on effects of titanium target bombarded by TEMP II accelerator

Two-dimensional numerical research has been carried out on the ablation effects of titanium target irradiated by intense pulsed ion beam (IPIB) generated by TEMP II accelerator. Temporal and spatial evolution of the ablation process of the target during a pulse time has been simulated. We have come to the conclusion that the melting and evaporating process begin from the surface and the target is ablated layer by layer when the target is irradiated by the IPIB. Meanwhile, we also obtained the result that the average ablation velocity in target central region is about 10 m/s, which is far less than the ejection velocity of the plume plasma formed by irradiation. Different effects have been compared to the different ratio of the ions and different energy density of IPIB while the target is irradiated by pulsed beams.

Dust Charging in the Sheath of an Electronegative Plasma

We theoretically investigate the dust charging in the sheath of an electronegative plasma, by using a single dust grain model based on a previous sheath structure [Chin. Phys. Lett 20 (2003) 1537] in which cold positive ions and hot negative ions have been assumed. It is found that dust grains are first charged negatively at the sheath edge and then begin to be charged positively in the sheath. Moreover, both the temperature ratio of electrons to negative ions and the density ratio of negative ions to positive ions have effects on the neutral point of the dust charge.

Monte Carlo simulation of ion transport process in ECR microwave plasma with negative bias

In the presence of the external magnetic field, Monte Carlo method has been used to simulate a complete course, in which the ions pass through the neutral region, the sheath region and finally are absorbed by the workpiece surface with negative bias in Electron Cyclotron Resonance microwave plasma. The charge exchange and elastic scattering collisions between ions and neutrals are taken into account. The distributions of the ion energy and angle at the workpiece surface are obtained. It is found that under the same neutral gas pressure the lesser the distance from the target, the more the high-energy ions and smaller the scattering angle, and most of the ions are almost vertically incident on the target. As the neutral gas pressure increases, the number of high-energy ions at the target decreases and the number of low-energy ions increases. At the same time, numerical results show that with increasing magnetic mirror ratio, the number of high-energy ions striking the target increases and the scattering angle decreases obviously. The effect of secondary electrons on the sheath potential cannot be neglected.

Effect of the Plasma Pressure on Magnetohydrodynamic Kink Instability in a Cylindrical Geometry

A semi-analytical method is introduced to study kink instability in cylindrical plasma with line-tied boundary conditions. The method is based on an expansion for magnetohydrodynamics (MHD) equations in one-dimensional (1D) radial eigenvalue problems by using Fourier transforms. The MHD equations then become an ordinary differential equation. This method is applicable to both ideal and non-ideal MHD problem. The effect of plasma pressure (P0) on kink instability is studied in a cylindrical geometry. Complex discrete spectra are presented. Two-dimensional (2D) eigenfunctions with the line-tied boundary conditions are obtained. The growth rate and radial eigenfunctions are different in the two cases of P0 = 0 and P0 ≠ 0, which indicate that the effect of plasma pressure can not be ignored if it is large enough. This method allows us to understand the role of individual radial eigenfunctions, and is also computationally efficient compared to direct solutions of the MHD equations by the finite difference method.

Effect of Secondary Electron Emission on the Sheath in SPT Chamber

A one-dimensional slab model of the plasma sheath in the stationary plasma thruster (SPT) chamber is developed in this study. It is considered that secondary electrons emitted from ceramic walls are partially trapped by the bulk plasma in the SPT chamber; some secondary electrons drift across the sheath where they are generated and the bulk and move towards the opposite sheath. Thus both the secondary electron emission (SEE) from one sheath and the partially trapped secondary electrons from the opposite sheath contribute to this sheath. The results indicate that both the SEE coefficient and trapping coefficient have a significant impact not only on the distributions of both electrons and ions of the SPT sheath but also on the energy flux loss to the SPT wall. When the trapping coefficient increases, the energy flux of electrons deposited to the walls will increase whereas that of ions will decrease. Besides, the critical electron temperature will decrease greatly with the increase of the trapping coefficient.

Sheath Structures of Strongly Electronegative Plasmas

The sheath structures of strongly electronegative plasmas are investigated on basis of the accurate Bohm criterion obtained by Sagdeev potential. It is found that the presheath transition between the bulk plasma and the sheath almost does not exist there, and that distributions of electrons, negative and positive ions in the sheath form a pure positive ion sheath near the boundary of the electrode. Furthermore, the density distribution of space net charge has a peak near the sheath edge, the spatial potential within the sheath falls faster, and the sheath thickness becomes thinner.

Energy Distribution of Ions Incident on Radio-Frequency Biased Electrodes in External Magnetic Field

A radio-frequency (rf) plasma sheath model in an oblique magnetic field is established and the energy distribution of ions (IED) incident on the rf sheath biased electrodes is numerically investigated. The simulation results reveal that the external magnetic field can have a decisive impact on the ion flux and energy distribution of the sheath. The ion energy can be transferred between the perpendicular and parallel components under the action of a magnetic field.