What determines current density at the cathode of a thermionic arc?

The physical processes responsible for determining the current density at the cathode of a thermionic arc are considered. It is shown that depending on the arc current and gas pressure, the arc could operate in one of the two modes. In the case of relatively low arc current and/or relatively low gas pressure, the arc current density is determined by a limited ability of the near-cathode plasma to provide the necessary ion current density. In this case, current density is proportional to the pressure and independent of the arc current. The second mode is realized when the arc current and/or gas pressure are relatively high. In this mode, the current density is determined by variations in the voltage drop along the cathode surface. In the second mode, current density is independent of pressure and inversely proportional to the arc current in this mode. Another factor that should be taken into account when considering current distribution is non-local relationships between electron current, ion current and heat flux at the cathode. It is shown that local approximations fail at distances shorter than ▴r = 2(ation)1/2, where a is the plasma thermal diffusivity, and tion is the ionization time in the plasma. Non-locality is important when ▴r is comparable to the emitter dimension. Estimations for a high-current arc in 1 atm argon show that ▴r is in the range from 0.2 to 1 mm. Calculations showed that current and temperature distributions at the cathode obtained in the non-local approach could be quite different from those obtained in a local approximation. [ABSTRACT FROM AUTHOR]