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.

Magnetized plasma sheath in the presence of negative ions

“…On the other hand, in the presence of the magnetic field, adjacent to the Debye sheath, another layer forms known as the magnetic presheath or Chodura sheath. If plasma contains negative ions as one of its components, then the sheath structure has been observed to be modified, 7,8 and such plasmas are termed as electronegative plasmas. For such plasma, the sheath potential was studied by Thompson and Boyd in 1950s.…”

Sheath structure in plasmas with nonextensively distributed electrons and thermal ions

“…[2][3][4][5][6][7][8] However, in most of these theoretical investigations, the electron distribution has been assumed to be Maxwellian. But it is well known that the Maxwell distribution is valid only for the macroscopic ergodic equilibrium state.…”

An analytic expression for the sheath criterion in magnetized plasmas with multi-charged ion species

“…In this case,Ẑ 1 ¼ 1;f 1 ¼ 0;d 1 ¼ 0; a 1 ¼ 0, and d 1 ¼ 1 (or n 0e ¼n 01 ) in relations (25) and (26). Therefore, the Bohm criterion is…”

“…24,25 In addition, the wall potential is specified by a given value u w which can be the floating potential or a more negative value for which Boltzmann distribution holds, i.e., ju w j > 3k B T e =e. 26 Therefore, the electron and negative ion density distributions are written as follows: 15,26,27 n e ¼ n 0e exp eu T e ;…”