Near-anode Region Research Articles

Plasmadynamics model for nonequilibrium processes in N2/H2 arcjets

A two-temperature chemical nonequilibrium model is developed for nitrogen/hydrogen (N 2/H2) arcjet thrusters. All viscous flow properties are considered assuming steady, laminar, continuum, and axisymmetric flow. A seven-species N2/H2 plasma composition of molecules, atoms, ions, and electrons is assumed, and a finite rate chemistry model is employed to model collisional processes among the species. Separate energy equations are formulated for the electrons and heavy species. The anode temperature distribution is included, and propellant electrical conductivity is coupled to the plasma properties, allowing for a selfconsistent current distribution. The numerical solution employs the compressible form of the pressureimplicit with splitting of operators algorithm to solve the continuity and momentum equations. Numerical results are presented for a low-power simulated hydrazine thruster. The centerline constrictor region of the arcjet flowfield is predicted to be near thermal equilibrium, whereas a high degree of thermal nonequilibrium is predicted in the near-anode region of the arcjet nozzle. Strong electric fields near the anode produce elevated electron temperatures that enhance ionization levels and electrical conduction through the arcjet boundary layer. Radial diffusion of electrons from the arc core also enhances the near-anode ionization levels. Thus, the nonequilibrium approach is required to accurately model the plasma current distribution.

The origin of current chopping in vacuum arcs

A semiempirical model is presented which relates the phenomenon of current chopping of vacuum arcs to rapid microscopic processes on the cathode surface. First, for a resistive circuit the approximate chopping current as a function of power-current amplitude was calculated using two DC arc lifetime parameters which were obtained experimentally. Secondly, it is shown that spontaneous arc extinction is the result of an accumulation of characteristic arc instabilities, each of which can be described as a randomly occurring transient arc resistance. Qualitatively, the 'severity' of these instabilities is demonstrated to depend on arc current, arc length, and circuit parameters. Thirdly, the presence of instabilities seems to be a manifestation of an ion deficiency in the near-anode region. This deficiency may be due to a discontinuity in the ionized mass flow into the plasma, occurring at a failure to initiate a new cathodic emission site in time. Based on these findings, the nature of the current-chopping phenomenon, hitherto explained by a minimum cathode spot operation current, can be coupled to the dynamics of the spot. >

Oscillations in high-current arc discharges and processes in the near-anode region

Oscillations in high-current arc discharges and processes in the near-anode region

Built-in getter-ion pumps

It is shown that theory of the Penning discharge essentially involves taking into account the collective interaction of the electrons in the near-anode region. Following investigations of the Penning discharge in a strong magnetic field (density up to 12 kOe) empirical formulae are proposed for calculating the pumping speed and the minimum operating pressures of getter-ion pumps. Methods of calculation for built-in getter-ion pump parameters and design peculiarities are considered.

Field screening in insulating crystals: The quasistatic case

A study is made of a metal-dielectric-semiconductor-dielectric-metal structure subjected to a linearly increasing external voltage. On the assumption of quasistatic screening conditions, the carrier-density and field distribution in the near-anode and nearcathode regions of the semiconductor are determined. This distribution proves to be nonuniform not only in the region of the surface space charge induced by the external field, but also within the interior of the semiconductor to a depth ∼τμE0·tM/τ0, where τ is the lifetime and μ the mobility of the carriers, E0/τ0 is the rise rate of the external field, and tM is the Maxwellian relaxation time. The character of the distribution for opposite faces of the semiconductor is different, since in the near-cathode region for n-type semiconductors allowance must be made for the effect of the uncompensated space charge of the current carriers on the conductivity.