93_ashokan_spectral_nature - 1 92 IEEE !Zkansactions on...

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192 IEEE !Zkansactions on Electrical Insulation Vol. 28 No. 2, April 1993 Spectral Nature of Luminosity Associated with the Surface Flashover Process T. Asokan and T. S. Sudarshan Department of Electrical and Computer Engineering, University of South Carolina, Columbia, SC ABSTRACT Investigations of the luminous events associated with the break- down along the surfaces of large band gap insulators such as polycrystalline alumina and monocrystalline quartz in vacu- um were made by employing a phototube and a very sensitive photomultiplier tube (PMT). The onset of luminous signals, in general, was found to start much before the onset of voltage collapse during breakdown. The early onset of luminosity may be due to the presence of various defect or trapping centers within the forbidden gap of the insulator. The spectral nature of the light emitted during breakdown was resolved (in terms of the wavelength) by interfacing a monochromator with the PMT. In the case of the monocrystalline quartz specimen, the breakdown luminosity was observed to be associated with the defects located at 2.76 1.91 eV below the conduction band edge. The breakdown luminosity of polycrystalline alumina was found to be associated with the defects corresponding to energy levels at - 1.91, 2.25, 2.45 2.76 eV. The formation of these defects is discussed in terms of the nonstoichiomet- ric nature of the lattice structure at the surface. Besides, the samples were found to emit light when no breakdown occurs. This no-breakdown light emission was found to be attributed to the deep level defects. The observed results suggest that the surface flashover process is primarily controlled by the de- fect or trapping centers located within the forbidden gap of the insulators. 1. INTRODUCTION MISSION of electrons from the cathode due to electric Efi eld enhancement at the cathode-insulator-vacuum triple junction is known to be the primary cause (or initi- ation process) for the failure of insulator-bridged vacuum gaps at fields much lower than that of the plain vacuum gap [l-51. As the failure of the insulator-bridged vacuum gap was realized to occur along the surface of the insula- tor (i.e., surface flashover), factors such as (1) interaction dynamics of the field emitted electrons with the surface of the insulator and (2) surface condition or properties of the insulator, have received considerable attention in the past [6-131. The interaction dynamics in relation to surface flashover were established by studying insulators of different (e.g., conical shape) shapes [6-81. The inves- tigations of various samples possessing different surface roughness [9-111 and a thin layer of Crz03 or CuaO on the surface [12-131, revealed that the surface properties of the insulator are very important in the context of surface flashover. Despite the above investigations, the physical mechanism responsible for the surface flashover is still under debate.
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This note was uploaded on 06/11/2011 for the course ELECTRICAL 124 taught by Professor Ghjk during the Spring '11 term at Institute of Technology.

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93_ashokan_spectral_nature - 1 92 IEEE !Zkansactions on...

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