Cathodic Vacuum Arc Plasma of Thallium

Thallium arc plasma was investigated in a vacuum arc ion source. As expected from previous considerations of cathode materials in the Periodic Table of Elements, thallium plasma shows leadlike behavior. Its mean ion charge state exceeds 2.0 immediately after arc triggering and diminishes to the predicted 1.60 and 1.45 after about 100 and 150 mus, respectively. The most likely ion velocity is initially 8000 m/s and decays to 6500 and 6200 m/s after 100 and 150 mus, respectively. Both ion charge states and ion velocities decay further toward steady-state values, which are not reached within the 300-mus pulses used here. It is argued that the exceptionally high vapor pressure and charge exchange reactions are associated with the establishment of steady-state ion values