A novel microwave frequency sweep interferometer for ECR plasma electron density measurements

The paper presents the measurement strategy and the experimental results collected by a novel microwave interferometer for plasma density measurements in compact Electron Cyclotron Resonance Ion Sources (ECRIS). Microwave interferometry represents the best candidate for non-intrusive plasma diagnostics in these plasma devices, being also a challenging task because the plasma under test is contained in a microwave resonant cavity. The developed K-band microwave interferometry system is based on the so-called “frequency sweep” or FMCW method. This method is very versatile, easy to integrate and robust with respect to the uncontrolled disturbances of the spurious reflections introduced by the cavity boundaries, which give rise to additional frequency components in the beating frequency spectrum. The direct signal component has been extracted by bandpass filtering, i.e. by filtering out the multipath contributions. The paper includes the description of the calibration setup, performed by a holed paraffin block where the paraffin-air index of refraction variation imitates the air-plasma one, and the first measurements on a real plasma obtained at the Italian National Institute for Nuclear Physics (INFN-LNS).