| Plasma sheath is the region between plasma and the wall.Much atten-tions has been paid to the magnetized plasma sheath in recent years.The study of magnetized plasma sheath has significant applications in magnet-ic fusion plasmas(e.g.,near divertors)and low temperature plasma-wall interactions.A number of results have been achieved,but many problems still remain unsolved.The electron density is usually assumed to obey the Boltzmann distri-bution when the magnetic field lines are oblique to the wall.However,the electrons travelling along the field lines toward the wall,will be reflected by the electric field in the sheath.Hence the electron density deviates from the Boltzmann relation.In the studies of the magnetized plasma sheath,the fluid model of one spatial dimension and three velocity coor-dinates is usually used.In this case,a large velocity shear arise in the sheath.Therefore,the shear viscosity will have a significant effect on the sheath structure.In addition,the case when the magnetic field is parallel to the wall was seldom investigated,and the results of a few fluid models contradicts the physical speculations.This dissertation deals with these unsolved problem.In the second chapter,the effect of electron reflection on the magne-tized sheath with magnetic field lines oblique to the wall is studied using fluid model with one-dimensional space and three-dimensional velocity.It is shown that,the ion entrance velocity normal to the wall(i.e.the critical Mach number,or the generalized Bohm criterion)depends on the wall potential,which differs from the previous results(i.e.,the critical Mach number is independent of the wall potential),and it increases with the increase of the negative bias on the wall.The wall floating poten-tial is lower and the sheath thickness is smaller than the model with the Boltzmannian electrons.The increase of the magnetic field strength and the ion-to-electron temperature ratio causes the compression of the sheath and reduces the difference in the sheath profiles between the models with and without the electron reflection.The effect of the shear viscosity and bulk viscosity on the magnetized sheath with magnetic field lines oblique to the wall is studied in the third and the fourth chapters,respectively.It is shown that the effect of the viscosity is to compress the sheath.The difference in the sheath pro-files between the models with and without the viscosity reduces with the increase of the magnetic field but enlarges with the increase of the ion temperature.The boundary conditions become more complex when con-sidering the viscosity.The reasonable ion velocity shear should be given at the sheath edge in addition to the discussion of the ion critical Mach number.The critical Mach number is unaffected by the viscosity in the present model,but is reduced by the E x B and diamagnetic drifts at the sheath edge.In the fifth chapter,the two-fluid model is used to study the situation with magnetic field lines parallel to the wall.It is shown that the electron sheath is formed when the electron inertia is neglected,but the plasma density increases in the sheath toward the wall.With the inclusion of the electron inertia,the ion sheath with decreasing densities is obtained.These results(including that in some literatures)do not agree with phys-ical speculations that the sheath across the magnetic field should be an electron sheath with decreasing densities,which indicates that the flu-id models describing the case of magnetic field parallel to the wall are questionable. |
