New perspectives in vacuum high voltage insulation. II. Gas desorption

An examination has been made of gas desorption from unbaked electrodes of copper, niobium, aluminum, and titanium subjected to high voltage in vacuum. It has been shown that the gas is composed of water vapor, carbon monoxide. and carbon dioxide, the usual components of vacuum outgassing, plus an increased yield of hydrogen and light hydrocarbons. The gas desorption was driven by anode conditioning as the voltage was increased between the electrodes. The gas is often desorbed as microdischarges--pulses of a few to hundreds of microseconds--and less frequently in a more continuous manner without the obvious pulsed structure characteristic of microdischarge activity. The quantity of gas released was equivalent to many monolayers and consisted mostly of neutral molecules with an ionic component of a few percent. A very significant observation was that the gas desorption was more dependent on the total voltage between the electrodes than on the electric field. It was not triggered by field-emitted electrons but often led to field emission, especially at larger gaps. The study of gas desorption led to some important new observations about the initiation of high-voltage breakdown and the underlying processes of vacuum outgassing. The physical processes that lead to voltage-induced desorption are complex, but there is strong evidence that the microdischarges are the result of an avalanche discharge in a small volume of high-density vapor desorbed from the anode. The source of the vapor may be water or alcohol stared as a fluid in the many small imperfections of a polished metal surface. Microdischarges can then trigger field-emitted electrons which, in turn, heats a small area of the anode. As the temperature of this region of the anode reaches about 500 degrees C, some fraction of the desorption products are ionized positively and accelerated to the cathode, producing secondary electrons with a yield pl eater than unity per incident ion. The positive ions appear to originate from the bulk of the metal rather than from surface ionization and the yield increases exponentially with temperature, rapidly producing a runaway condition, i.e., electrical breakdown. These observations support a new perspective on vacuum-high-voltage insulation and produce new insight into vacuum outgassing of metals. (C) 1998 American Vacuum Society.