Overdense plasma production in a low-power microwave discharge electron source

Plasma characterization of a low-power microwave discharge electron source was conducted. The electron source, which was developed for the neutralizat ion of the 150 mA-class ion beam exhausted from an ion thruster, consists o f a small discharge chamber of 18 mm diameter, into which an L-shape antenn a is directly inserted into the magnetic circuit comprised of permanent mag nets and iron yokes. An overdense. plasma production for the 4.2 GHz microw ave was observed for an input power range from 3 to 26 W and for the mass f low rate of 0.5-2.0 sccm. In such a wide range, the plasma density inside t he discharge chamber can be proportionally increased as the microwave input power. This is because the direct insertion of the microwave antenna into the ECR magnetic field removes the accessibility difficulty of the microwav e, and enables energy transmission from the antenna to the plasma even in t he overdense mode. In addition, high-energy electrons above the ionization energy were observed for the large microwave input power above 10 W, and th ese electrons from the antenna also contribute to plasma production.