Experimental scaling laws for multipolar electron cyclotron resonance plasma sources

Abstract

The performance of five different diameter multipolar electron cyclotron resonance (ECR) plasma sources is evaluated by comparing (1) ion production energy costs, (2) microwave coupling efficiency, (3) electron temperatures and ion energies, and (4) discharge absorbed power density versus input power and pressure. The experimental measurements demonstrate that ECR plasma source performance is a function of operating pressure, gas type, input power, discharge size, and applicator size. When using 2.45 GHz excitation the most efficient and optimum plasma source sizes have diffusion lengths of 1.5–2.5 cm and associated discharge diameters of 12.5–25 cm. Microwave coupling efficiencies are in excess of 90% and ion production energy costs are 600–700 eV/ion or less and absorbed power densities are less than 1 W/cm3. The 25 cm diam discharges are the most efficient with ion energy production costs of 400–200 eV/ion and absorbed power densities are less than 0.2 W/cm3. Small sources have lower microwave coupling efficiencies and much higher ion production energy costs and operate with very high absorbed power densities and electron temperatures. When the applicator size is increased to 30 cm or more, performance of a source excited with 2.45 GHz is limited by the ability to control the exciting electromagnetic modes.