Criterion of sheath formation in magnetized low pressure plasma

Abstract

A criterion of sheath formation is obtained for magnetized low pressure plasmas. The criterion includes the effect of both collision and the magnetic field. The collision parameter is defined by the ratio of the ionization length to the ion mean free path. The ionization frequency is assumed to be constant. The condition obtained is consistent with the electrostatic case. In the absence of the magnetic field and the collision, it retrieves the Bohm criterion at the sheath edge. For an electrostatic case in the absence of ion neutral collision, the Bohm criterion determines the ion entry speed within the sheath. However, the presence of collision limits the validity of the criterion to a threshold value of the collision parameter. In the magnetized scenario, the validity is found to be dependent on the magnetic field angle besides the collision parameter. Even in a collisionless scenario, the validity is in question depending on the field angle. A critical collision parameter is found for a definite magnetic field strength beyond which there is no more angle dependency on the validity criterion. The effect of the magnetic field on the space charge deposition is highlighted.

Magnetized plasma sheath in the presence of negative ions

“…This, in turn, changes the overall ion dynamics in the presence of a magnetic field. The resulting velocity sharing and sheath formation has been discussed in detail by Moulick et al 1,29 In order to get the appropriate initial conditions for all the components of ion velocity, the Taylor series expansion must be done at the midplane of the plasma, and the sum of the first few terms should be considered. It is not only important for the magnetized plasma but also for the electrostatic plasma (most importantly if the solution is sought from the midpoint or bulk of the plasma).…”

Secondary electron emission and collisional effects in a two‐electron temperature plasma sheath

“…The results obtained in this case tallies well with the previous study. [ 38 ] Now, the electron emission from the wall is introduced by putting $$$ \gamma =0.9 $$$ without changing the electron distribution, and the results are shown in Figure 3b. The space charge distribution drastically changes when secondary electrons are emitted from the wall.…”

Effect of collisions on the plasma sheath in the presence of an inhomogeneous magnetic field

“…Bohm's criterion in a collisional magnetized plasma is explored in the Ref. [9,10]. Such studies examined the possibility of a modified Bohm's criterion, which limits both maximum and minimum allowable sheath entrance velocity of the positive ion at the sheath edge.…”