Effect of vessel diameter on ignition and electron emission characteristics in radio frequency plasma cathodes

To reduce the power consumption at a radio frequency plasma (RFP) cathode for electric rocket propulsion in space, we experimentally investigated the ignition and electron emission characteristics of the RFP cathode as a function of the discharge vessel diameter. Owing to the increase in the ratio of the electrons loss to the electron production, the ignition perfonnance degrades as the vessel diameter becomes smaller. The anode current voltage characteristics are not affected by the vessel diameter and the anode current emitted from the cathode is saturated at higher anode voltages. In contrast to the effect of the vessel diameter on the ignition performance, the RF power required to achieve an electron current of 1 A for the 2-cm cathode is lower than that for the 4-cm cathode. This improvement of the electron emission performance seems to be as a result of increase in the ratio of the ion collection area to the vessel surface area. Therefore, these results suggest that the discharge vessel diameter is determined by the balance between the plasma ignition performance and electron emission performance.