Formation of stationary and transient spots on thermionic cathodes and its prevention

Abstract

Spots on cathodes of high-pressure arc discharges induced by a rapid increase in the arc current are studied numerically and experimentally. Appearance of stationary and transient spots is analysed in the context of the general pattern of steady-state modes of current transfer to thermionic cathodes and their stability. Transient spots are studied in experiments with COST-529 standard lamps. Modelling and experimental results are in reasonable agreement. A method to prevent formation of transient spots on cathodes of high-pressure arc discharges by means of short negative rectangular current pulses is proposed and validated both numerically and experimentally. Experimental indications are found that the main mechanism of blackening of burners of HID lamps that accompanies appearance of transient cathode spots is evaporation of the cathode material and not sputtering.

Numerical investigation of the stability of stationary solutions in the theory of cathode spots in arcs in vacuum and ambient gas

“…The thermal and electrical conductivities κ(T ) and σ (T ) of copper and tungsten were assumed to be the same as in [8] and [6], respectively. The specific heat values c p (T ) of copper and tungsten were evaluated with the use of data from [11] and with account of the latent heat of melting (which was introduced along the same lines as is done in the simulation of metal casting [12]; see [13] for details). Functions q(T w , U), and j (T w , U) were determined as described in [14] for the copper cathodes of vacuum arcs and in [15] for the tungsten cathodes of arcs burning in argon.…”

Understanding and modelling plasma–electrode interaction in high-pressure arc discharges: a review

“…Hence, the answer to the above-stated question is positive: yes, experimental data obtained in experiments with arcs in inert gases at the pressure of a few bars are representative of cathodes of HID lamps. [57,60,71,72,96] concern modelling of non-stationary spots induced on arc cathodes by rapid variations of the arc current; a topic important for practice in connection with occurrence of spots on cathodes of ac HID lamps after the change in polarity. (One can also mention the experimental work [97], in which measurements are reported of the electrode sheath voltage in arcs operated in noble gases with sinusoidal and switched dc currents.)…”

“…(Note that a similar effect is produced by an enhanced thermal conductivity of the cathode material [108].) A transient spot on a rounded cathode is formed considerably later, is considerably cooler and lives much less than the spot on a cathode with the flat front surface [72].…”

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

“…It follows that all abrupt transitions are monotonic in time. Indeed, no oscillations of arc voltage and luminosity of the cathode surface is observed in the experiments on transitions between diffuse and spot modes on arc cathodes; e.g., [14,[21][22][23]. Hence, all abrupt transitions are related to bifurcations of steady-state solutions.…”