It is shown that, as the gas pressure in a forevacuum plasma electron source increases, electric breakdown in the accelerating gap is caused by the reverse flow of ions from plasma that is generated by both the electron beam and high-voltage glow discharge (HGD). By modifying the electrode system geometry in the accelerating gap of the electron source, it is possible to provide for a two-to threefold decrease in the HGD current. This allows the upper pressure in the electron source to be increased up to about 100 Pa when air is used as the working gas and up to 160 Pa in the source filled with helium.