A coaxial arrangement of a ring-shaped ceramic magnet and a disk-shaped rare-earth magnet is used to trap plasma electrons in circular orbits near the surface of the planar cathode. This configuration results in the formation of a ring-shaped plasma which is stable at all pressures in the range 0.0007 Torr to over 10 Torr. A model is presented which describes the sensitivities of plasma emission intensities to the discharge current for atoms and ions of both the plasma gas and the sputtered flux of cathodic material. The model is based on the number of electron collisions needed to generate the radiating species. Experimental data for the species Ar, Ar<sup>+</sup>, Cu, and Cu<sup>+</sup> are compared to the model.
Lori McCaig and Richard Sacks, "Current Sensitivity of Plasma Voltage and Emission Line Intensities in a Planar Magnetron Glow Discharge Device," Appl. Spectrosc. 46, 18-24 (1992)