The curling probe: A numerical and experimental study. Application to the electron density measurements in an ECR plasma thruster.

The measurement of electron density is a key issue in understanding and controlling plasma applications. To date, plasma density in electric thrusters has been mainly evaluated with electrostatic techniques, such as the Langmuir probe, which could be quite invasive. In this paper, we propose the application of a microwave resonant probe, the curling probe, to the diagnostic of an electrodeless plasma thruster. The measurable electron density range and the probe accuracy are found to be limited by the probe natural frequency. We present the numerical study and the experimental characterization of three curling probes with different natural frequencies (700, 1400, and 3000 MHz, approximately). First, an analytical equation of the natural frequency as a function of geometrical parameters is drawn from 2D numerical simulations. Then, a procedure based on solid dielectric etalons is proposed for the absolute calibration of the probe. Finally, measurements are performed in the plume of an electron cyclotron resonance plasma thruster. Electron densities from 10 to 10 cm have been obtained in agreement with hairpin and Langmuir probes results. A wall-embedded probe has enabled measurements inside the thruster source with minimal plasma perturbation. A possible curling probe configuration, embedded in a reactor wall, is proposed as a fully non-invasive diagnostic for plasma sources.