TIME RESOLVED RECORDING OF ARC SPOT FORMATION ON COLD CATHODES

摘要

The electrodes of cold HID-lamps are heavily stressed during lamp ignition by a high voltage pulse. For a reduction of this load by an improved electrode design, a better understanding of the ignition is desired. The ignition may be decomposed into two different processes. One process is the formation of charge carriers and of a bulk plasma in the lamp volume. A broad knowledge is available concerning these processes. The other process is the establishment of an electron emission at the cathode, which is sufficiently high to transfer an arc current. The understanding of this process resulting in the formation of an arc spot is rather incomplete. If the arc is extinguished immediately after ignition the electrode surface shows the formation of craters by melting and vaporization of the electrode material. One explanation for the emission of electrons is that microscopic structures on the electrode surface are heated by the high energy of the incoming ions. To investigate the influence of the surface structure on the ignition of arc spots independently of the processes in the discharge volume, a special experimental setup was developed. An arc ignited between two electrodes in a pure argon atmosphere (0.106 MPa) is blown magnetically against a third so-called commutation electrode CE. The ignition is recorded by electrical measurements, high-speed photography and time resolved spectroscopy. The electrodes are prepared in different ways for reproducing fine microscopic surface structures. First measurements are made with polished electrodes of copper and graphite. After that, electrodes will be prepared with pulverized tungsten.