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CLUMP MECHANISM

The vacuum breakdown mechanism based on this theory makes following assumption:

(i) A loosely bound particle known as clump exists on one of the electrode surfaces.

(ii) When a high voltage is applied between the two electrodes, this clump gets charged and subsequently gets detached from the mother electrode and is attracted by the other electrode.

(iii) The breakdown occurs due to a discharge in the vapor or gas released by the impact to the particle at the opposite electrode.

It has been observed that for a certain vacuum gap if frequent recurrent electric breakdowns are carried out, the withstand voltage of the gap increases and after certain number of breakdown, it reaches an optimum maximum value. This is known as conditioning of electrodes and is of paramount importance from practical reasons. In this electrode conditioning, the micro-emission sites are supposed to have been destroyed.

Various methods for conditioning the electrodes have been suggested. Some of these are

(i) To treat the electrodes by means of hydrogen glow discharge. This method gives more consistent results.

(ii) Allowing the pre-breakdown currents in the gap to flow for some time or to heat the electrodes in vacuum to high temperature.

(iii) Treating the electrodes with repeated spark breakdown. This method is however quite time consuming.

The area of electrodes for breakdown of gases, liquids, solids or vacuum plays an important role. It has been observed that if the area of electrodes is increased for the same gap distance in uniform field, the breakdown voltages are reduced.

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