A sheath correction method for electron density measurements with the microwave resonant curling probe

Abstract

A correction method accounting for plasma sheath effects that appear when performing an electron density measurement with a microwave resonant probe immersed in plasma is described. The diagnostic is the novel curling probe that has already shown promising capabilities in various plasma sources. The correction method is based on the evaluation of two effects relative to the resonant probe operation and its interface with the plasma. First, the characteristic decay length of the electromagnetic field emitted by the curling probe, which defines the probed volume, has been characterized for the different harmonic resonance modes. Second, a semi-analytical model has been adapted to describe the plasma structure forming near the probe, which quantitively describes the electron density profiles across the sheath structure. The correction method is then developed uniquely from numerical simulations and is independent of other diagnostics. Experimental results inside two plasma sources, an inductively coupled plasma and an ECR plasma thruster, are presented and discussed. The validity of the method is assessed (i) by comparing curling probe corrected densities with Langmuir probe results, (ii) by varying the probe orientation to the expanding plasma flow, and (iii) by using two harmonics of the probe. The method is shown to significantly improve the accuracy of electron density measurements. Possible improvements to the method are also discussed.