Design of a Microwave Frequency Sweep Interferometer for Plasma Density Measurements in ECR Ion Sources

Abstract

Electron Cyclotron Resonance Ion Sources (ECRIS) are among the candidates to support the growing request of intense beams of multicharged ions. Their further development is related to the availability of new diagnostic tools, nowadays consisting of few types only of devices designed on purpose for such compact machines. Microwave Interferometry is a non-invasive method for plasma diagnostics and represents the best candidate for the whole plasma density measurements. Interferometry in ECR Ion Sources is a challenging task due to their compact size. The typical density range of ECR plasmas ranging from 10 11 to about 10 12 cm 3 , causes the probing beam wavelength to be in the order of few centimetres which is comparable to the chamber radius. The paper describes the design of a new microwave interferometer based on the so-called “frequency sweep” method: the density is here derived by the frequency shift of a beating signal obtained during the fast sweep of both probing and reference microwave signals; inner cavity multipaths contributions can thereby be suppressed by cleaning the spurious frequencies from the beating signal spectrum.