The boundary layers of ac-arcs at HID-electrodes: phase resolved electrical measurements and optical observations

Abstract

Arcs are operated in noble gases with sinusoidal and switched dc currents of different frequencies in a model lamp between tungsten electrodes with properties appropriate for high intensity discharge lamps. The sum of the cathode and anode fall, called electrode sheath voltage ESV(t) = uc(t) + ua(t), is determined by a variation of the arc length and the anode fall ua(t) by probe measurements along the arc axis and an extrapolation procedure. It is found that ua(t) adopts low, approximately constant values within a half cycle. Therefore the time variation of the ESV reflects that of uc. It was observed by high speed photography that a high value of uc after current zero crossing (CZC) may initiate a transition of the initially diffuse mode of cathodic arc attachment into a spot mode. It is accompanied by a breakdown of uc causing the formation of a voltage peak starting at current zero crossing. It is a prominent example of the so-called commutation peak (CP). By an adjustment of the operation conditions to a low uc the cathodic arc attachment remains diffuse during the whole half-cycle. It is also indicated by high speed photography that the arc constriction in front of the anode is relieved with increasing operation frequency. It is illustrated by measurements that commutation peaks may be removed by an increase in the arc current, a decrease in the electrode cooling, a reduction of the filling gas pressure and an increase in the operation frequency. Moreover it is demonstrated, that the formation of a CP is sensitively dependent on the electrode surface structure.

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

“…Quite simple but reliable phase resolved measurements of the ESV can be made at the model lamp using the possibility of a controlled relocation of the electrodes during lamp operation [21]. The arc voltage U arc can be separated into a cathode fall U c , an anode fall U a and a voltage drop along the arc column.…”

“…Therefore it may be helpful to consult ESV measurements for a better interpretation of the pyrometric measurements. Comparing both it has to be considered that the variation of the ESV in dependence on ϕ is mainly determined by the variation of the cathode fall with ϕ at least if the lamp is operated with a switched dc current [21]:…”

“…But at a definite current, which is determined by the properties of the electrode and of the gas atmosphere, the power input into the anode and with it the anode tip temperature exceed those of the cathode [16,19]. However, it was shown recently operating a model lamp with a sinusoidal or switched dc current [21,22] that the results obtained in the case of dc operation of HID electrodes can not simply be transferred to the cathodic and anodic phase in the case of ac operation. Interferences are observed between the cathodic and anodic arc attachment, increasing with rising operating frequency.…”

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

“…It is accomplished by trigger signals for the cameras, which are shifted with an adjustable time delay with respect to a well defined reference phase angle. A reproducible reference phase angle can be deduced from zero crossing of the lamp voltage as it was shown by Langenscheidt [13]. To trigger the cameras only the rising edge of the lamp voltage is used.…”

“…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.…”