This article deals with the experimental results of the research on specific features of prebreakdown states origins and conditions of high-vacuum breakdown development of the inter-electrode gap, in particular when the cathode is a high emission material, which allows you to have the density of thermionic emission current up to several hundred amperes per square centimeter. It is shown that in the presence of thermionic current values of the current density up to 80 A/cm2
pre-breakdown states take place on the leading edge of the pulse, and then on the trailing edge and pulse plateau. Pre-breakdown state on pulse plateau subsequently develop in the breakdown of the electrode gap. When increasing current density up to 440 A/cm2 and above, high-voltage the breakdowns through microsecond intervals occur.
An empirical relationship between the breakdown voltage and the current density for different values of the inter-electrode gap is established. Breakdowns lead to significant erosion of the cathode surface (craters formation).