The cataphoretic emitter effect exhibited in high intensity discharge lamp electrodes

Abstract

A mono-layer of atoms, electropositive with respect to the substrate atoms, forms a dipole layer, reducing its work function. Such a layer is generated by diffusion of emitter material from the interior of the substrate, by vapour deposition or by deposition of emitter material onto arc electrodes by cataphoresis. This cataphoretic emitter effect is investigated within metal halide lamps with transparent YAG ceramic burners, and within model lamps. Within the YAG lamps, arcs are operated with switched-dc current between rod shaped tungsten electrodes in high pressure Hg vapour seeded with metal iodides. Within the model lamps, dc arcs are operated between rod-shaped tungsten electrodes—one doped—in atmospheric pressure Ar. Electrode temperatures are determined by -pyrometry, combined with simulation of the electrode heat balance. Plasma temperatures, atom and ion densities of emitter material are determined by emission and absorption spectroscopy. Phase resolved measurements in YAG lamps seeded with CeI3, CsI, DyI3, TmI3 and LaI3 show, within the cathodic half period, a reduction of the electrode temperature and an enhanced metal ion density in front of the electrode, and an opposite behavior after phase reversal. With increasing operating frequency, the state of the cathode overlaps onto the anodic phase—except for Cs, being low in adsorption energy. Generally, the phase averaged electrode tip temperature is reduced by seeding a lamp with emitter material; its height depends on admixtures. Measurements at tungsten electrodes doped with ThO2, La2O3 and Ce2O3 within the model lamp show that evaporated emitter material is redeposited by an emitter ion current onto the electrode surface. It reduces the work function of tungsten cathodes above the evaporation temperature of the emitter material, too; and also of cold anodes, indicating a field reversal in front of them. The formation of an emitter spot at low cathode temperature and high emitter material density is traced back to a locally reduced work function generated by a locally enhanced emitter ion current density.

Investigation of the interaction of dense noble gas plasmas with cold cathodes: II Arc spot ignition on Au, Pd and Pt cathodes

“…The effect of ionized vapour has some similarity with the gas phase emitter effect being attributed to a reduction of the work function by a dipole layer, which is formed on hot cathode surfaces by its bombardment with ions provided by heated up metal vapour. [ 14 ] The influence of the boiling temperature on arc commutation will become clearer by investigations at cold tungsten cathodes because of quite different thermal properties of W than of Al, Cu, Ti, graphite, Au and Pd. In the third paper, [ 13 ] the arc commutation on cathodes made of pure and doped tungsten, the end faces of which are prepared by several pre‐treatments, is investigated with the same experimental set‐up used in the two preceding papers.…”

Foundations of plasma photonics: lamps, lasers, and electromagnetic devices

“…Finally, for lamps with a filling that is too aggressive to permit a long lifetime using any of these solutions, a so-called 'gas-phase emitter' is required. In this case, some of the gas-phase components, such as CeI 3 or DyI 3 can form a mono-layer on the electrode surface, thereby reducing the effective work function and hence the surface temperature [9,19,21].…”

“…In low-frequency or DC-driven lamps, cataphoresis can occur but generally it is only significant locally around the electrodes, where it is an important factor in gas-phase emitter effects like discussed in section 2.4 [9].…”

Bifurcations in the theory of current transfer to cathodes of DC discharges and observations of transitions between different modes

“…In the case of refractory cathodes of high-pressure arc discharges, the theory based on the concept of multiple solutions has gone through a detailed experimental validation by means of different methods, such as spectroscopic measurements, electrostatic probe measurements, electrical and pyrometric measurements, and calorimetry; see, e.g., [12][13][14][15], review [16] and references therein, and also the recent review [17] The theory of current transfer to cathodes of arc discharges is simpler from the theoretical point of view than the theory for the case of glow discharge. The eigenvalue problem governing the stability of steady-state solutions against small perturbations is self-adjoint (Hermitian) in this case [18].…”