Ion charge state distributions of vacuum arc plasmas: The origin of species

TLDR

Vacuum arc plasmas arise from micrometer‑sized, nonstationary cathode spots, and although ion charge state distributions have been measured for about 50 elements, a satisfactory theoretical description remains lacking. This study aims to compute ion charge state distributions by assuming the spot plasma undergoes an instantaneous transition from equilibrium to nonequilibrium during expansion. The authors model the frozen charge state distributions using Saha equations within a Debye‑Hückel nonideal plasma approximation for all metals and selected elements, treating the observable CSDs as the result of freezing at the transition point. The resulting periodic table of CSDs lists mean charge, neutral vapor fraction, and effective temperature and density at freezing, and the analysis shows that the instantaneous freezing concept and effective parameters hold for various currents and magnetic fields while temperature fluctuations broaden the distributions.

Abstract

Vacuum arc plasmas are produced at micrometer-size, nonstationary cathode spots. Ion charge state distributions (CSD's) are experimentally known for 50 elements, but the theoretical understanding is unsatisfactory. In this paper, CSD's of vacuum arc plasmas are calculated under the assumption that the spot plasma experiences an instantaneous transition from equilibrium to nonequilibrium while expanding. Observable charge state distributions are the result of a freezing process at this transition. ``Frozen'' CSD's have been calculated using Saha equations in the Debye-H\"uckel approximation of the nonideal plasma for all metals of the Periodic Table and for boron, carbon, silicon, and germanium. The results are presented in a ``periodic table of CSD.'' The table contains also the mean ion charge state, the neutral vapor fraction, and the effective plasma temperature and density at the freezing point for each element. The validity of the concepts of ``instantaneous freezing'' and ``effective temperature and density'' is discussed for low and high currents and for the presence of a magnetic field. Temperature fluctuations have been identified to cause broadening of CSD's.