Ion Energy Distribution in Thermionic Vacuum Arc Plasma

The purpose of this paper is to investigate the ion energy distribution in thermionic-vacuum-arc plasma. Using emissive probe technique, the spatial plasma potential distribution has been measured. In some conditions, the anode plasma potential is constant and close to the anode potential. Also, a sharp potential drop characterizes the potential distribution between the anode plasma and the vessel walls. Moreover, we have analyzed the equivalent electric circuit of the plasma during arc discharge operation. The total anode discharge current is the sum of the thermoelectric current flowing from the filament and the current flowing from the anode to the vessel walls. The current intensity, which flows to the vessel, is directly proportional with the arc voltage, and it is a measure of the plasma ionization degree. In some experimental conditions, the value of this current reaches up to 35% of the total anode discharge current. In order to obtain the ion energy distribution, a magnetic deflection ion energy analyzer was used. It was found that ions have quasi-monoenergetic energy, and its value is strongly related to the plasma potential distribution. The optical emission spectroscopy measurements confirm these results denoting that ions are quasi-monoenergetic and highly energetic (up to 1 keV).