In this paper a model of the quasineutral plasma and the transition between the plasma and the dielectric wall in a Hall thruster channel is developed. The plasma is considered using a two-dimensional hydrodynamic approximation while the sheath in front of the dielectric surface is considered to be one dimensional and collisionless. The dielectric wall effect is taken into account by introducing an effective coefficient of the secondary electron emission ͑SEE͒, s. In order to develop a self-consistent model, the boundary parameters at the sheath edge ͑ion velocity and electric field͒ are obtained from the two-dimensional plasma bulk model. In the considered condition, i.e., ion temperature much smaller than that of electrons and significant ion acceleration in the axial direction, the presheath scale length becomes comparable to the channel width so that the plasma channel becomes an effective presheath. It is found that the radial ion velocity component at the plasma-sheath interface varies along the thruster channel from about 0.5C s ͑C s is the Bohm velocity͒ near the anode up to the Bohm velocity near the exit plane dependent on the SEE coefficient. In addition, the secondary electron emission significantly affects the electron temperature distribution along the channel. For instance in the case of sϭ0.95, the electron temperature peaks at about 16 eV, while in the case of sϭ0.8 it peaks at about 30 eV. The predicted electron temperature is close to that measured experimentally. The model predictions of the dependence of the current-voltage characteristic of the EϫB discharge on the SEE coefficient are found to be consistent with experiment.

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A kinetic model is developed of Teflon ablation caused by a plasma. The model takes into account the returned atom flux that forms in the non-equilibrium layer during the ablation. This approach makes it possible to calculate the ablation rate for the case when the Teflon surface temperature and the density and temperature in the plasma bulk are known.

The two-dimensional expansion of a current carrying plasma jet in the interelectrode gap of a vacuum arc with an axial magnetic field is analysed by finding the steady state solution of the fully ionized plasma in the hydrodynamic approximation. Two models are presented: (1) expansion into a duct with known geometry and (2) free jet expansion. The first approach models the plasma jet expansion with a conical shape. In the second model the geometric position of the free boundary was determined by the free hydrodynamic jet expansion into vacuum without and with the influence of a magnetic field. In the case of plasma expanding into a conical guide, it was found that the flow field in the near-axis region does not depend on the cone angle for cone angles . The radial velocity becomes comparable to the axial velocity due to the expansion, depending on the cone angle and the initial axial velocity. A model of the free boundary plasma expansion was developed, based on the jet-like (i.e. axial velocity larger than the radial velocity) plasma flow in the vacuum arc near the cathode spot. The free jet boundary was calculated by solving the equations for the normal and tangential velocity components at the free boundary. It was found that the plasma jet had a conical shape, and for axial distances 3 - 4 times greater than the initial jet radius, the radial velocity becomes comparable with the axial velocity if no magnetic field is imposed. Imposition of a magnetic field reduces the radial component of the plasma velocity. The streamline angle is about for a 0.001 T magnetic field and about for a 0.01 T magnetic field. The plasma remains quasi-neutral in all regions except in the space charge boundary layer, where an outward directed electric field appears for low magnetic fields, and an inward directed field is present for strong magnetic fields.

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The low-density plasma flow in an axial magnetic field to a disk-shaped anode in a vacuum arc was studied theoretically using a two-dimensional model. The plasma expansion was modeled using the sourceless steady-state hydrodynamic equations, where the free boundary of the plasma was determined by a self-consistent solution of the gas-dynamic and electrical current equations. The anode was modeled as a current and plasma collector, which does not influence the plasma flow field. Magnetic forces from both the azimuthal self-magnetic field, and the imposed axial magnetic field were taken into account. It was found that the self-magnetic field does not substantially influence either the plasma jet shape, density, velocity, or the current density distribution for arc currents I⩽200 A. On the other hand, the plasma jet angle (α0) at the starting plane and the radial plasma density gradient force in the expansion region do have a strong influence on the plasma and current flow. The mass and current flow in a 500 A arc are compressed in the near axis region, leading to an increase in the plasma and axial current density by a factor of 1.5 at a distance of about two plasma jet radii from the starting plane. The calculated arc current–voltage characteristics agree qualitatively with experiments on arc behavior in an axial magnetic field.

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