J. Mentel

The anode region of electric arcs: a survey

Electric arc anodes frequently determine functional performance and lifetime of a number of arcing devices, ranging from discharge lamps to plasma spray torches. While there have been numerous studies of the anode region of electric arcs, our understanding of the detailed physical processes is still limited. The reason for this lack of detailed knowledge is that numerous factors influence the arc–anode interaction, and that the plasma–solid interface in high intensity arcs is in general not accessible to diagnostics and one has to rely on indirect measurements. Throughout this survey, the emphasis will be on high intensity arc anodes, i.e. on plasmas with temperatures of more than 10 000 K and electron densities exceeding 1022 m−3 outside the boundary layer, and heat fluxes exceeding 107 W m−2. The plasma parameters in the boundary layer as obtained with different techniques by a number of investigators for a variety of conditions are presented, and the effect of macroscopic flow conditions is discussed. Experimental and modelling results are presented. A brief comparison with low current arcs is also given, and the areas that need further research are highlighted.

Determination of HID electrode falls in a model lamp I: Pyrometric measurements

To verify models describing the near-electrode regions electrodes of pure and doped tungsten for high intensity discharge lamps are investigated in a special model lamp. It can be operated with arc currents of 1 A to 10 A, DC or AC with arbitrary waveforms up to a few kHz. Argon and xenon, at pressures from 0.1 MPa to 1 MPa, are used as fill gases. A large variety of electrodes can be inserted. To perform spatially resolved measurements they are displaced reproducibly within the discharge tube during lamp operation. Spatially resolved pyrometric measurements of the electrode surface temperature in the case of DC operation are presented. From the temperature distribution the power loss of the electrodes by thermal radiation and heat conduction is determined. It increases almost linearly with the arc current at the anode and less than linear at the cathode. A relation is deduced between the cathode fall and the power fed into the cathode setting up the power balance of the cathodic current transfer zone. The resulting cathode falls show a strong dependence on the electrode diameter. Electrical measurements of separate cathode and anode falls are given in a subsequent paper. The outcomes of both methods and of modelling are compared in a third paper.

The anode region of low current arcs in high intensity discharge lamps

A survey is given of the behaviour of arc anodes which are operated under conditions being existent in high intensity discharge lamps. Experimental investigations are mainly performed at a special model lamp. They comprise optical observations of the anodic arc attachment, spatially resolved pyrometric measurements of the electrode temperature, electrical measurements of the anode fall and spatially resolved spectroscopic measurements of the electron temperature and density in front of the anode. A model of the anodic boundary layer is worked out composed of a constriction zone with an enhanced electrical power input and an anode sheath which may be an electrical generator. It is confirmed by comparing with the measured data that the power input into the anode is composed of a term linearly dependent on the arc current with a proportionality constant formed by the sum of the electrode work function and the electron enthalpy within the arc column and a small correction term being reduced with increasing anode temperature. A reduction in the work function by an anodic emitter effect is demonstrated for a high pressure sodium discharge, but in contrast to the cathode it is absent if thoriated electrodes are used. If HID electrodes are operated with switched dc or ac currents interferences between the anodic and cathodic phases occur.

Birefringent filter with arbitrary orientation of the optic axis: an analysis of improved accuracy.

The transmission characteristics of a birefringent quartz filter inserted under Brewsters angle into a linear laser resonator is investigated theoretically and experimentally for arbitrary orientations of the optic axis. Exact expressions for filter curves of zero losses as functions of wavelength and orientation of the optic axis are given and confirmed by measurements. The Jones matrix of birefringent filter, which describes the propagation of plane waves through the filter with improved accuracy, is derived. Transmission curves are calculated, showing that the optimum selectivity strongly depends on the orientation of the optic axis. For a special filter the results are verified by measurements.

Ignition of mercury-free high intensity discharge lamps

To achieve a better understanding of the ignition behaviour of D4 lamps for automotive headlights the ignition of mercury-free metal iodide test lamps characterized by a high xenon pressure, a small electrode distance and small electrode–wall distances is investigated. The ignition of these lamps is dominated by a high voltage requirement. Nevertheless lamps are found that show a surprisingly low ignition voltage. Electrical measurements and simultaneous optical observations of the ultra-fast streamer processes show that the breakdown takes place in two different modes. One of the ignition modes which requires a high ignition voltage is characterized by a breakdown in the volume between the electrode tips. The other mode is characterized by streamer discharges along the wall. In this case the cathode, its base and the wall around is involved in the ignition process and the lamp breaks down at low voltages.

The plasma boundary layer of HID-cathodes: modelling and numerical results

From the experimental finding that the cathodic plasma boundary layer in front of a thermionically emitting cathode is independent of the bulk plasma, the conclusion is drawn that the power flux density and the current density from the boundary layer to the cathode can be reduced to functions which depend only on the cathode temperature and the cathode fall. To advance the calculation of these so called transfer functions an already existing model of the cathodic plasma boundary layer consisting of a space charge sheath and a pre-sheath is reconsidered. The latter is split into a zone in which the ion current is formed and into an ion acceleration zone. A closed expression is deduced for the ion current density with regard to the back diffusion of neutralized particles from the cathode but with disregard of mass inertia. The electron temperature in the boundary layer is related to the cathode temperature and the cathode fall by the power balance of the electrons in the boundary layer. Special properties of the cathodic boundary layer of an argon arc are given. They are calculated with rate coefficients which are evaluated with cross sections from the literature. Numerical results are presented showing the dependence of the transfer functions on the cathode fall, gas pressure, work function of the electrode material and on the properties of the rare gases neon, argon, krypton and xenon.

Spectroscopic investigation of the plasma boundary layer in front of HID-electrodes

To investigate the properties of the plasma boundary layers of an argon arc in front of high intensity discharge (HID) electrodes a special spectroscopic measuring system was set-up in front of a model lamp. It is characterized by a high spatial resolution and a high degree of automation of the measuring procedure. Measurements were performed at the boundary layers in front of anodes and cathodes made of pure and thoriated tungsten with diameters from 0.6 to 2 mm operated with currents from 0.5 to 10 A at pressures from 0.1 to 0.3 MPa. The electron temperatures Te were deduced from Boltzmann-plots of population densities of a couple of excited states just below the ionization limit, which were obtained from measured line intensities. Electron densities ne were determined from measured continuum intensities. In front of the anodes, maxima of Te and ne were found with Te of the order of 12 000 K and ne of the order of 5 × 1021 m−3, indicating strong deviations from local thermal equilibrium. In the diffuse mode of cathodic arc attachment, steep gradients are measured with values of Te of the order of 12 000 K and of ne of the order of 5 × 1022 m−3. The measured temperatures in the boundary of the spot mode of cathodic arc attachment exceed 22 000 K. The electron temperatures in front of the cathode are in accordance with the results of modelling.

Der Einsatz der Wendelinstabilität in wandstabilisierten Lichtbögen

The onset current of the helical instability of wall stabilized arcs can be determined by a theoretical model from a dimensionless function which is only dependent on the power input and a factor containing the gas properties and the arc conductance. The model describes the onset current of the instability in hydrogen and nitrogen as function of the tube diameter and of the pressure. Questions of restabilization are discussed.

Arc attachment at HID anodes: measurements and interpretation

Anodes for high intensity discharge lamps made of cylindrical tungsten rods and the plasma in front of them are investigated in a special lamp filled with argon and other noble gases at pressures of 0.1–1 MPa. The arc attachment on these anodes takes place in a constricted mode. The temperature is measured pyrometrically along the electrode axis and the anode fall electrically. The electron temperature, Te, and the electron density, ne, within the anodic boundary layer are determined spectroscopically with high spatial resolution. It is found that the power input into the anode increases nearly linearly with the arc current. The proportionality constant is mainly determined by the work function of the electrode material and Te but is independent of the electrically measured anode fall and scarcely dependent on the electrode dimensions. The constriction is more pronounced in cold anodes, with maxima of Te and ne in front of the electrode surface, than on hot anodes with thermionic electron emission and vaporization of the electrode material. The distances of the Te- and ne-maxima from the anode surface are increased and Te is reduced in front of the anode with increasing anode temperature. The experimental findings may be explained by a model of the anodic boundary layer consisting of a thin sheath in front of the surface and a more extended constriction zone. The current and voltage are anti-parallel within the sheath. The power which is needed to sustain the sheath is supplied by an enhanced electrical power input into the constriction zone.

Experimental investigation of electrodes for high pressure discharge lamps

To support modelling of the electrodes in high intensity discharge lamps a model lamp was realized for precise caloric and electrical measurements at the electrodes. In this model lamp the properties of electrodes made of pure and modified tungsten were investigated in a current range between 1-10 A in dependence on the electrode geometry. The electrodes were operated in Ar or Xe in a pressure range between 0.1-1 MPa. By a determination of the radiation power emitted by the electrode from spatially resolved temperature measurements and by caloric measurements of the power removed by heat conduction the power fed from the arc into the electrode was determined. From these measurements the cathode fall was evaluated assuming that the electrical power invested into the cathode sheath is mainly transfered to the cathode by the ion current. The cathode tails are found to he in agreement with values which are determined by probe measurements. It confirms the power transfer to the cathode by the ion current. The outcomes are separate voltage current characteristics of the cathode and anode falls in dependence on the electrode properties and operating conditions. Complications result from two modes of current transfer at the cathode: a spot mode of high current density and a diffuse mode of low current density but with a higher cathode fall. It is shown that the spot mode is favoured by a low current, a strong cooling of the cathode and a high gas pressure, the diffuse mode by the opposite conditions. In a transition region the cathode changes statistically under the influence of the surface structure between the two modes.