Plasma source

A plasma source comprises a thermionic emitter (2) heated by an induction coil (7), which also provides radiofrequency energy within an electrically insulated cylindrical former (1). A cylindrical anode (10) is concentric with emitter (2) and axially displaced therefrom, generating a potential difference between anode (10) and emitter (2). The potential difference between anode (2) and ground and axial magnetic fields causes the plasma to be extracted from the source. Emitter (2) is held at negative potential via a conductive support (5). Process gas is introduced near emitter (2) and a secondary gas injected in the anode space. Radiofrequency excitation of emitter (2) generates electrons via thermionic and field effects, resulting in efficient plasma generation. Both electron generation effects contribute to a broad energy spectrum of electrons, providing effective neutralisation of the plasma. Moreover, the time varying axial magnetic field induced by coil (7) in the vicinity of emitter (2) provides enhancement of plasma generation and confinement of plasma to minimise erosion of emitter (2).