High accuracy plasma density measurement using hybrid Langmuir probe and microwave interferometer method

Abstract

High spatial resolution plasma density measurements have been taken as part of an investigation into magnetic nozzle physics at the NASA/MSFC Propulsion Research Center. These measurements utilized a Langmuir triple probe scanned across the measurement chord of either of two stationary rf interferometers. By normalizing the scanned profile to the microwave interferometer line-integrated density measurement for each electrostatic probe measurement, the effect of shot-to-shot variation of the line-integrated density can be removed. In addition, by summing the voltage readings at each radial position in a transverse scan, the line density can be reconstituted, allowing the absolute density to be determined, assuming that the shape of the profile is constant from shot to shot. The spatial and temporal resolutions of this measurement technique depend on the resolutions of the scanned electrostatic probe and the interferometer. The measurement accuracy is 9%-15%, which is on the order of the accuracy of the rf interferometer. The measurement technique was compared directly with both scanning rf interferometer and standard Langmuir probe theory. The hybrid technique compares favorably with the scanning rf interferometer, and appears more accurate than probe theory alone. Additionally, our measurement technique is generally applicable even for nonaxisymmetric plasmas.

Dynamics and Electronegativity of Oxygen RF Plasmas

“…In comparison with the directly measured line integrated electron density of about 9.5 × 10 14 m −2 from MWI, the probe measurement provides a value which is by a factor of 2.7 lower. Such differences in the electron density determination, comparing interferometry and electric probe diagnostics, were also reported in the literature [30][31][32][33]. Moreover, the Druyvesteyn method assumes an isotropic electron energy distribution function and the transition of the electrons towards the probe as collision-free.…”

Plume detachment from a magnetic nozzle

“…Assuming a radially uniform velocity profile, a high accuracy density profile is obtained over multiple shots. 22 Density profiles obtained with these two different methods compared favorably. Triple-probe measurements 15 of electron temperature gave T e = 1.2 eVϮ 20% in hydrogen and T e = 1.4Ϯ 20% in helium with no evidence of spatial or temporal variation, as is shown in Fig.…”

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

“…Lobbia and Beal [23] give an insight of the recommended practices when applying LP technique to electric propulsion testing. Microwave techniques may also be employed: interferometry, allowing non-invasive measurements [24], and resonant probes, such as the hairpin. Electric thruster performances are quite sensitive to the local perturbation induced by a diagnostic probe: current collection and secondary electron emission from a bare metallic surface.…”