Plasma of Vacuum Discharges: The Pursuit of Elevating Metal Ion Charge States, Including a Recent Record of Producing Bi13+

Metal ions in the plasma of vacuum discharges are commonly multiply charged with ion charge states from 1+ to 3+, reaching 4+ and 5+ for some metals. The elevation of metal ion charge states in vacuum discharge plasma is an interesting challenge for plasma physics because it requires a deeper understanding of the processes leading to a more intense ionization of the electrode material. It also has practical implications, for example, for metal ion sources: elevation of ion charge state leads to a proportional increase in ion beam energy for a given accelerating voltage. During the last two decades, various techniques have been used to increase the ion charge states, including: 1) application of a strong magnetic field to the cathode region of the vacuum arc; 2) application of supplemental microwave power to the discharge plasma; 3) injection of an electron beam into the discharge area; and 4) application of a short current pulse to the discharge as to transiently increase the discharge voltage and power, emulating the conditions of a high-current vacuum spark. In this paper, we briefly survey the different techniques of metal ion charge state elevation and then present new experimental results by utilizing the spark regime and combining it with a strong pulsed magnetic field applied to the cathode region. Beams of ions with high charge state, up to a record Bi¹³⁺, were extracted from vacuum spark plasma. It is argued that the addition of a magnetic field to the spark plasma magnetizes the electrons and limits plasma expansion, which leads to an increase in the electron temperature relative to the free expansion case and to an increase in the likelihood of electrons to cause ionizing collisions.