We have developed a method to measure plasma parameters in a discharge by analyzing the impedance of a search coil wound around a discharge tube. This technique is based on the diamagnetic characteristic of the plasma. When a high-frequency current passes through the search coil, a diamagnetic current is induced inside the plasma, and the energy to the search coil is dissipated by Joule loss due to the resistivity of the plasma. As a result, the impedance of the search coil varies depending on plasma properties. The relationship among plasma parameters, magnetic field and impedance of the search coil is derived from Maxwell's equations and generalized Ohm's law. Thus, plasma parameters can be determined from the variation of the impedance. The present diagnostic method was applied to measure electron density and electron temperature in a mercury-argon plasma at a pressure of 2.5 Torr.

https://iopscience.iop.org/article/10.1143/JJAP.38.231/pdf

Plasma parameter measurement by the impedance of a coil inductively coupled with a plasma

Summary form only given. Using atomic absorption we measure the densities of neutral and ionic barium in the vicinity of the barium oxide coated electrodes in low pressure Hg-rare gas discharges. The technique is sensitive to the small amount of barium that evaporates from the electrode during a steady state discharge. It uses a current modulated barium hollow cathode lamp light source and a phase sensitive detector. In the present work we concentrate on comparing barium densities with electrode temperature. Spatially resolved electrode temperature is measured with a thermal emission based diagnostic developed previously. Comparisons are made with respect to variations of key discharge parameters, such as current and rare gas pressure. As such, we investigate in detail the connection between the electrode temperature and the presence of free barium in the discharge.

Measurement of barium emitted from thermionic electrodes in steady-state Hg-rare gas discharges

A scanning Fabry–Perot interferometer (FPI) is used to determine the temperature of barium atoms that are liberated from the electrodes of fluorescent lamps during their steady-state operation. Barium, a constituent of the work function lowering emitter material that is placed on the tungsten coil that forms the electrode, is liberated primarily by evaporation from the hot (~1300 K) thermionic electrode. However, there may be situations or modes of operation in which barium is, in addition, sputtered, a condition which may lead to increased end-darkening, shortened life and increased mercury consumption in the lamp. Using the FPI diagnostic, the occurrence of sputtering is inferred when barium temperatures are much greater than the electrode temperature. The FPI diagnostic senses resonance radiation (λ = 553 nm) emitted by barium atoms excited in the low pressure discharge environment, and infers temperature from the Doppler broadened linewidth. The diagnostic has proven to be successful in a number of situations. Measurements have been made on rare gas discharges and on Hg–argon discharges for different discharge currents, gas pressures and auxiliary coil currents. Measurements are phase resolved for ac-driven discharges.

/pdf/fabry-perot-measurements-of-barium-temperature-in-29y29pvcmd.pdf

Fabry-Perot measurements of barium temperature in fluorescent lamps

Evaporation of emitter material from electrodes of fluorescent lamps has been measured by means of atomic absorption of Ba. A hollow-cathode discharge lamp served as background radiator. The arrangement is briefly described. Spot resolved measurements has been done in preheating and in discharge regimes. Essential results are dependence on evaporation rate and filament structure, evaporation and arising of cathode spot in discharge, and diminished evaporation regarding preheating.

Atomabsorption von Barium in Leuchtstofflampen

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Atomabsorption von Barium in Leuchtstofflampen