Experiments have been conducted to identify the key parameters in the increase of the breakdown strength of a vacuum gap through repetitive breakdowns. The current-voltage waveforms were observed, the residual stress in electrodes was measured and the chemical composition of the electrode surfaces and the energy of the valance-band electrons were analyzed. These experiments revealed that prebreakdown currents (field emission currents) are sometimes observed and sometimes not. The residual stress before and after 500 breakdowns changed significantly. X-ray photoelectron spectroscopy analysis before and after 500 breakdowns showed that the spectral lines of oxygen and carbon due to contaminants on the electrode surface were completely removed, and the copper ones appeared clearly. The energy spectrum of anodes after 500 breakdowns shifted towards the lower binding energy to almost coincide with the theoretical spectrum, while the energy spectrum of the cathodes shifted towards the higher binding energy.
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