Numerical investigation of the magnetized plasma sheath characteristics in the presence of negative ions

Abstract

In this work, a planar and low-pressure discharge in a mixture of a noble gas (argon) and an electronegative gas (oxygen) is considered. It is assumed that the produced plasma consists of electrons, two species of positive ions, and one species of negative ion. The behavior of the density distribution and kinetic energy of these charged particles in the sheath region are studied. Also, it is assumed that a weak external magnetic field which is nearly perpendicular to the wall is exerted to the sheath region. The positive ion species are considered as a cold, collisionless fluid while both electron and negative ion densities obey the Boltzmann distribution. By using a hydrodynamic approach and ignoring ionization and recombination, it is shown numerically that by increasing the density of the negative ions in the plasma the density distribution of both positive ion species and the kinetic energy of these ion species decreases and increases, respectively. Also, it is shown that in the presence of the negative ions the normalized electrostatic potential of the sheath region changes and by increasing the negative ion densities the normalized electrostatic potential in the sheath increases. In addition, the effect of the density ratio of both positive ion species, density and temperature of the negative ions, and the magnitude of the external magnetic field are studied on the net density distribution of the charged particles in the sheath region. The obtained numerical results show that by decreasing the temperature of the negative ions the amplitude of the fluctuations of the net density distribution of the charged particles in the sheath region increases and the position of these fluctuations shifts toward the sheath edge. Finally, it is seen that in the absence of negative ions the net density distribution of charged particles in the sheath region is monotonic while in the presence of negative ions it is nonmonotonic.