Pressure ionization: its role in metal vapour vacuum arc plasmas and ion sources

In most plasmas ionization is brought about by classical ionizing collisions between panicles. In metal vapour vacuum arc plasmas and ion sources, however, pressure ionization plays an important role in determining the final ion composition. In this paper we describe the basic mechanism of pressure ionization as a collective phenomenon in non-ideal plasmas, induced mainly by the many-panicle Coulomb interaction and quantum-mechanical exchange. Then we outline the properties of the cathode spot plasmas of vacuum arc discharges. The path of the cathode material in the electron density-temperature phase diagram is considered; starting from the solid, the matter is heated, forms a liquid and then a supercritical fluid which expands explosively, driven by a very high pressure gradient. The final ion charge state distribution is produced and frozen in the transition from the dense. non-ideal plasma (where pressure ionization is dominant) to the expanded, ideal, non-equilibrium plasma. We conclude that the final ion charge state distribution depends on the thermophysical properties of the solid and liquid cathode material as well as on the electronic structure of its atoms and ions. Some consequences for the performance of the metal vapour vacuum arc ion source are discussed.