Working Principle of the Hollow-Anode Plasma Source

The hollow-anode discharge is a special form of glow discharge. It is shown that a drastically reduced anode area is responsible for a positive anode voltage drop of 30-40 V and an increased anode sheath thickness. This leads to ignition of a relatively dense plasma in front of the anode hole. Langmuir probe measurements inside a specially designed hollow anode plasma source give an electron density and temperature of n_e=10⁹-10¹¹ cm^-3 and T_e=1-3 eV, respectively (nitrogen, current 100 mA, flow rate 5-50 scc min^-1). Driven by a pressure gradient, the 'anode' plasma is blown through the anode hole and forms a bright plasma jet streaming with supersonic velocity (Mach number 1.2). The plasma-stream can be used, for instance, in plasma-assisted deposition of thin films.