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.

The propagation of the first ionization wave in a compact fluorescent lamp (T4 tube with standard electrodes) during ignition was investigated for various initial dc-voltages (both polarities measured against ground) and gas compositions (with and without mercury). In addition the effect of the presence of a fluorescent powder coating was studied. The propagation velocity of the initial wave was measured by an assembly of photomultipliers installed along the tube, which detected the light emitted by the wave head. The propagation was found to be faster for positive than for negative polarity. This effect is explained involving processes in the electrode region as well as in the wave head. Waves propagate faster in the presence of a fluorescent powder coating than without it and gases of lighter mass show a faster propagation than gases with higher mass.

A time dependent model of a low pressure, mercury–rare gas discharge with thermionic electrode is presented. The model is applicable to ac-operated fluorescent lamps, which is the focus of this work. The model describes a one-dimensional negative glow plasma that is bounded on one side by a thermionic electrode and a sheath, and on the other by a positive column plasma. The electrode/sheath component of the model, together with the mutually interacting negative glow plasma, allows for self-consistent calculation of the electrode sheath potential. The model describes a smooth transition in the plasma parameters from electrode to positive column and thus reveals the spatial extent of the influence of the electrode and sheath processes. A detailed description of the model is presented, as well as results of calculations pertaining to a standard fluorescent lamp. Also shown are measurements from a 2 mm interferometer and an internal floating probe, both of which compare favourably with the calculations.

The warm-electron-driven (2 keV) whistler electron microinstability [Phys. Rev. Lett. 59, 1821 (1987)] of the Constance B electron-cyclotron-resonance-heated (ECRH), quadrupole mirror-confined plasma experiment has been studied. Experiments show (i) that the instability comes in fairly regular bursts on axis and continuously in time off axis due to the minimum-B geometry, (ii) a frequency spectrum that is insensitive to changes in the plasma parameters, and (iii) instability-induced power losses which are not greater than 10% of the ECRH power input for the regimes studied. A linear perturbation analysis of the relativistic Vlasov equation together with Maxwell’s equations has been made. Using the ECRH distribution function, a new distribution function well suited for describing ECRH, mirror-confined plasmas, the analysis shows the instability frequency spectrum to be insensitive to changes in cyclotron frequency, temperature, and density, in agreement with experimental results, and only sensitive to changes in ECRH frequency.

Using a full-potential band structure approach, we have investigated the work function of barium oxide coated tungsten cathodes in low pressure discharge lamps. The main objective of this work is to understand why the work function for such cathodes is lower than that of the uncoated tungsten. The model studied in this work is based on a well known supposition that the source of thermionic electrons is the barium atoms released from the barium oxide coating due to a chemical reaction with the underlying metallic tungsten. For the unrelaxed seven-layer model of (100) surface of barium on barium oxide, the work function is calculated to be 2.22 eV, which is lower than that of BaO, Ba, and W metals separately. For a fully relaxed nine-layer surface, it becomes 1.36 eV. Although this value of the work function is lower than those estimated for the fluorescent cathodes by electrical measurements, which averages contributions from surfaces in all possible random orientations, this model provides a satisfactory explanation of the lowering of the work function of tungsten based cathodes in low pressure fluorescent lamps.