An apparatus includes an ionization chamber and an electron source device at least partially disposed inside the ionization chamber. The ionization chamber is configured to receive at least one chemical and provide plasma having ionized chemicals. The electron source device includes at least one filament configured to generate electrons, and a cathode configured to emit secondary electrons from the front surface when the electrons from the at least one filament hit the back surface of the cathode. The front surface of the cathode is shaped convex facing inside the ionization chamber.