Investigation of the flickering of La{sub 2}O{sub 3} and ThO{sub 2} doped tungsten cathodes
- Ruhr University Bochum, Electrical Engineering and Plasma Technology, 44780 Bochum (Germany)
- Plansee SE, Metallwerk-Plansee-Str. 71, 6600 Reutte (Austria)
Short-arc lamps are equipped with tungsten electrodes due to their ability to withstand a high thermal load during operation. Nominal currents of more than one hundred amperes lead to a cathode tip temperature near the melting point of tungsten. To reduce the electrode temperature and, thereby, to increase the maintenance of such lamps, ThO{sub 2} or tentatively La{sub 2}O{sub 3} are added to the electrode material. They generate a reduced work function by establishing a monolayer of emitter atoms on the tungsten surface. Emitter enrichments on the lateral surface of doped cathodes are formed. They are traced back to transport mechanisms of emitter oxides in the interior of the electrode and on the electrode surface in dependence of the electrode temperature and to the redeposition of vaporized and ionized emitter atoms onto the cathode tip by the electric field in front. The investigation is undertaken by means of glow discharge mass spectrometry, scanning electron microscope images, energy dispersive x-ray spectroscopy, and through measurements of the optical surface emissivity. The effect of emitter enrichments on the stability of the arc attachment is presented by means of temporally resolved electrode temperature measurements and by measurements of the luminous flux from the cathode-near plasma. They show that the emitter enrichments on the lateral surface of the cathode are attractive for the arc attachment if the emitter at the cathode tip is depleted. In this case, it moves along the lateral surface from the cathode tip to sections of the cathode with a reduced work function. It induces a temporary variation of the cathode tip temperature and of the light intensity from the cathode-near plasma, a so-called flickering. In particular, in case of lanthanated cathodes, strong flickering is observed.
- OSTI ID:
- 22490773
- Journal Information:
- Journal of Applied Physics, Vol. 118, Issue 2; Other Information: (c) 2015 AIP Publishing LLC; Country of input: International Atomic Energy Agency (IAEA); ISSN 0021-8979
- Country of Publication:
- United States
- Language:
- English
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