Different modes of arc attachment at HID cathodes: simulation and comparison with measurements

Abstract

Based on a model for the plasma boundary layer of high intensity discharge cathodes, simulations are performed and compared with experimental results. To solve the power balance of the cathode body different methods are used, namely a 1D integral solution as well as 1D, 2D and 3D finite-element calculations. The simulations are done for cylindrical tungsten cathodes operated in different pure noble gas discharges (0.1–1.0 MPa) and with currents between 0.5 and 10 A. Under these conditions different modes of arc attachment are found, both in simulations and experiments.For the diffuse mode of arc attachment an excellent quantitative agreement between measurements and the simulations is obtained, reflecting an improved accuracy of measurements and simulation. In addition, different spot modes are found. At least one of these modes is also observed in the experiment. Also for this spot mode the agreement between measurements and simulation for the integral quantities is good. There are still some open questions concerning the spot mode of cathodic arc attachment.Varying the geometric dimensions of the cathode, the proper simulation of the heat conduction problem of the cathode body is shown. Variations of the plasma properties, like gas type and pressure, prove the conceptional capability of the boundary layer model for the simulation of different modes of arc attachment. Evaluating the cathode fall characteristics, regions of existence for the different modes are found, which are similar to the experiments.

Investigating the dependence of the temperature of high-intensity discharge (HID) lamp electrodes on the operating frequency by pyrometric measurements

“…The variation of T tip in dependence on ϕ disappears to a large extent at higher frequencies, but a weak temperature increase is observed if the frequency is raised. For f = 10 kHz the average value of the electrode tip temperature approaches T tip = 3140 K. It may be compared with T tip ≈ 3000 K of the cathode [11] and T tip ≈ 2780 K of the anode [16], if an electrode with the same parameters is operated with a dc current of I = 3 A. The courses of T tip (ϕ) for i = 4 A, f = 10, 25, 500 Hz and 10 kHz in figure 3b are qualitatively the same.…”

“…with noble gases or mercury. It is on one hand an outcome of intense theoretical studies and on the other hand of experimental investigations, mainly at a model lamp, within the last years [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15]. The description of the dc operation of anodes in HID lamps is less advanced, but at least a qualitative understanding has been achieved for cold and to some extent also for hot HID anodes [16,17,18,19].…”

“…The layer decouples the cathode from the bulk plasma. Accordingly, its properties are completely determined by the cathode fall U c and the temperature of the cathode surface T c [10,11]. The power balance of the system plasma boundary layer -cathode has different solutions beneath a distinct arc current corresponding to the experimentally observed diffuse and spot mode of cathodic arc attachment.…”

Investigating the gas phase emitter effect of caesium and cerium in ceramic metal halide lamps in dependence on the operating frequency

“…The temperature of the electrode tip T tip was then determined by an extrapolation of the integrated 1D heat balance. Reliable results were obtained with this method for electrodes operated with a dc-current [1,6,11]. But if the electrodes are operated with an ac-current the modulation of the electrode temperature is limited to a small area behind the electrode tip [15].…”

“…If pure tungsten electrodes are used the temperature of the electrode tip is raised up to its melting point, T m = 3695 K, to supply a sufficiently high thermionic electron emission for a diffuse attachment of the lamp arc on the electrodes [9,10,11,12,13,14,15]. But the work function and with it the electrode temperature can be substantially reduced if a dipole layer consisting of a monolayer of atoms being electropositive with respect to tungsten is deposited on the tungsten surface.…”