Ion flux from vacuum arc cathode spots in the absence and presence of a magnetic field

Because plasma production at vacuum cathode spots is approximately proportional to the arc current, arc current modulation can be used to generate ion current modulation that can be detected far from the spot using a negatively biased ion collector. The drift time to the ion detector can used to determine kinetic ion energies. A very wide range of cathode materials have been used. It has been found that the kinetic ion energy is higher at the beginning of each discharge and approximately constant after 150 μs. The kinetic energy is correlated with the arc voltage and the cohesive energy of the cathode material. The ion erosion rate is in inverse relation to the cohesive energy, enhancing the effect that the power input per plasma particle correlates with the cohesive energy of the cathode material. The influence of three magnetic field configurations on the kinetic energy has been investigated. Generally, a magnetic field increases the plasma impedance, arc burning voltage, and kinetic ion energy. However, if the plasma is produced in a region of low field strength and streaming into a region of higher field strength, the velocity may decrease due to the magnetic mirror effect. A magnetic field can increase the plasma temperature but may reduce the density gradients by preventing free expansion into the vacuum. Therefore, depending on the configuration, a magnetic field may increase or decrease the kinetic energy of ions.