EMISSION-SPECTROSCOPY OF LOW-CURRENT VACUUM ARCS

Spectra of copper vapour arcs in a vacuum (20 A-1 kA) are compared with the previously reported spectrum of a copper vapour spark in vacuum. In contrast to the latter, the spectra of the arcs consist mainly of Cu I and Cu II lines but only of a few lines of Cu III. The energy balance of a vacuum arc is discussed in a simplified manner taking a 100 A copper arc as an example. The results are: (i) the input energy is mainly transferred into heating of the cathode, kinetic energy of ions, enthalpy of electrons, ionization energy of ions, and energy invested in electron emission (latent work function). All other contributions are negligibly small. (ii) The electron temperature relatively far away from the spot centre (> 100-mu-m) can hardly exceed 2 eV. (iii) Since the number of photons emitted by the arc spot plasma is relatively small, it is not possible to perform plasma diagnostics by emission spectroscopy with simultaneous spectral, spatial and temporal resolution.