Properties of a differentially pumped constricted hollow anode plasma source

Abstract

Uniform high density plasma sources are important for homogeneous deposition of thin films on material surfaces. For plasma heating by energetic neutral beams in magnetically confined fusion devices, uniform plasma density in front of the extraction grids of negative ion beam source is highly desirable. In this paper we investigate the properties of a direct current plasma source based on electrostatic confinement of secondary electrons between adjacent parallel plate cathodes while the constricted anode is a differentially pumped narrow cylindrical tube whose outer surface is insulated from the plasma. The discharge is operated using a dc power supply which provides maximum rated current of 1.5 A at 3.0 kV peak voltage. Uniform plasma density is obtained outside the parallel plates which can be extended over arbitrary area by simply increasing the size and number of the cathode plates. The plasma near the constricted anode is characterized by an intense luminous glow whose brightness depends inversely on the surface area of the anode. The bulk plasma has spatially distinct electron density and electron temperature regions. The electron temperature between the cathode plates is almost 2–3 times higher than outside the plates. The region around the anode shows a steep rise in electron density and electron temperature. The discharge current is found to be oscillating in the range 250–400 kHz which is attributed due to the instability of the anode glow. The frequency of the oscillation depends on the applied power, pressure and the separation between the cathode plates. A qualitative discussion is presented which explains the various characteristic properties of the discharge.