The hairpin resonator: A plasma density measuring technique revisited

Abstract

A microwave resonator probe is a resonant structure from which the relative permittivity of the surrounding medium can be determined. Two types of microwave resonator probes (referred to here as hairpin probes) have been designed and built to determine the electron density in a low-pressure gas discharge. One type, a transmission probe, is a functional equivalent of the original microwave resonator probe introduced by R. L. Stenzel [Rev. Sci. Instrum. 47, 603 (1976)], modified to increase coupling to the hairpin structure and to minimize plasma perturbation. The second type, a reflection probe, differs from the transmission probe in that it requires only one coaxial feeder cable. A sheath correction, based on the fluid equations for collisionless ions in a cylindrical electron-free sheath, is presented here to account for the sheath that naturally forms about the hairpin structure immersed in plasma. The sheath correction extends the range of electron density that can be accurately measured with a particular wire separation of the hairpin structure. Experimental measurements using the hairpin probe appear to be highly reproducible. Comparisons with Langmuir probes show that the Langmuir probe determines an electron density that is 20–30% lower than the hairpin. Further comparisons, with both an interferometer and a Langmuir probe, show hairpin measurements to be in good agreement with the interferometer while Langmuir probe measurements again result in a lower electron density.

Plasma power measurement and hysteresis in the E–H transition of a rf inductively coupled plasma system

“…The electron density was also measured using a hairpin probe. 10 In the same plasma region a laser induced fluorescence technique was used to make relative measurements of argon metastable densities in the plasma. For these measurements, the 394.9 nm wavelength was used to pump metastables from 1s 5 to 3p 2 level and the fluorescence of the 3p 2 to 1s 2 at 433 nm was recorded using an Andor intensified charge coupled device behind a narrow band filter.…”

Performance of a Floating Hairpin Probe in Strongly Magnetized Plasma

“…The technique is based on measuring the plasma dielectric constant using a resonant structure resembling a hairpin [7][8][9]. As compared to LP, the hairpin is electrically floating hence perturbation caused to the plasma is significantly reduced.…”

Experiments on Plasma Turbulence Created by Supersonic Plasma Flows with Shear

“…i.e., on the electron density, which has earlier and elsewhere been used for density diagnostics [26,27,28].…”