Importance of high local cathode spot pressure on the attachment ofthermal arcs on cold cathodes

The importance of having high local cathode spot pressures for the self-sustaining operation of a thermal arc plasma on a cold cathode is theoretically investigated. Applying a cathode sheath model to a Cu cathode, it is shown that cathode spot plasma pressures ranging 7.4-9.2 atm and 34.2-50 atm for electron temperatures of ~1 eV are needed to account for current densities of 10⁹ and 10¹⁰ A·m^-2, respectively. The study of the different contributions from the ions, the emission electrons, and the back-diffusing plasma electrons to the total current and heat transfer to the cathode spot has allowed us to show the following. 1) Due to the high metallic plasma densities, a strong heating of the cathode occurs and an important surface electric field is established at the cathode surface causing strong thermo-field emission of electrons. 2) Due to the presence of a high density of ions in the cathode vicinity, an important fraction of the total current is carried by the ions and the electron emission is enhanced. 3) The total current is only slightly reduced by the presence of back-diffusing plasma electrons in the cathode sheath. For a current density j_tot=10⁹ A·m^-2 , the current to the cathode surface is mainly transported by the ions (76-91% of j_tot while for a current density j_tot = 10¹⁰ A·m^-2, the thermo-field electrons become the main current carriers (61-72% of j_tot). It is shown that the cathode spot plasma parameters are those of a high pressure metallic gas where deviations from the ideal gas law and important lowering of the ionization potentials are observed